Insects, Their Ways and Means of Living/Chapter IX

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==CHAPTER IX: THE CATERPILLAR AND THE MOTH ==

"I'HE 1.1FE OF A CATERP1LLAR

la- is one of those bleak davs of early spring that so offen follow a period of warmth and sunshine, when living things seem led to believe the fine weather bas come to stay. Out in the woods a hand of little caterpillars is clinging to the surtce of something that appears to be an oral swelling near the end of a twig on a wild cherry tree (Fig. 143). The tinv. creatures, scarce a tenth of an inch in length, sit motionless, benumbed by the cold, many with bodies bent into hall circles as if too nearly ffozen to straighten out. Probably, however, they are all un- conscious and suffering nothing, l'et, if they were ca- l»able of it, they would be wondering what rate brought them into such a forbidding «orld. But rate in this case was disguised most likely in the warmth of yesterdav, which induced the caterpillars to leave the eggs in whi'ch they had safely passed the winter. The empty eggshells are inside the spindle-shaped thing that looks so like a swelling of the twig, for in fact this is merely a protective covering over a mass of eggs glued fast to the bark. The surface of the covering is perfor- ated by many little holes from which the caterpillars emerged, and "is swathed in a network of fine silk threads which the caterpillars spun over it to give themselves a surer footing and one they might cling to unconsciously in the event of adverse weather, such as that which makes them helpless now. A'hen nature designs any creature

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PL.\ TE 14

C

F

The tent caterl3illar ([alacosonm americana) A, an egg mass on an apple twig (about natural size). B, young caterpillar feeding on an opening leaf bud. C, branches of an apple tree with a tent in a (ork, (rom which trails of silk lead outward to the twigs where the caterpillars are feeding on the leaves. D, a full- grown caterpillar (three-fourths natural size). E, cocoon. F, 13u13a, taken l'roIll a cocoon. G, male moth. H, female moth laying eggs CATERPILI.AR AND THE MOTH

to live under trying circumstances she grants it some safeguard against destruction. The web-spinning habit is one which, as we shall see, these caterpillars will develop to a much greater extent later in their lives, for our little acquaintances are young tent caterpillars. They are round most often among woodland trees, on the chokecherry and the wild black cherry. But they commonly infest apple trees in the orchards, and for this reason {heir species has been named the apple-tree tent caterpillar, to distinguish it from

related forms.that do hOt com- monly inhabit cultivated fruit trees. The scientific name is ,lla/acosoma americana. The egg masses of the tent caterpillar moths are not hard to find at this season. They are generally placed near the t"lps of the twigs, which they appear to surround, and being of the same brownish color as the bark, they look like swollen parts of the twigs themselves /Plate 4 A, Fig. 244A). Most of them are five-eighths to seven-eighths of an inch in length and ahnost hall of this in width, but they vary in thickness with the diameter of

FIG. I43. Young tent cater- pillars on the egg mass from which they have just hatched. (I_- rimes natural size)

the twig. A closer inspection shows that the mass really clasps the twig, or incloses it like a thick jacket lapped clear around. In fcrm the masses are usually sym- metrical, tapering at each end, but some are of irregular shapes, and those that have been placed at a forking or against a bud have one end enlarged. The greater part of an egg mass consists of the cover- ing material, which is a brittle, fihny substance like dry mucilage. Some of it is often broken away, and some-

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times the tops of the eggs are entirely bare. The eggs are placed in a single layer next the bark (Fig. 144 B), and there are usually 300 or 400 of them. They look like little, pale-gray porcelain jars packed closely together and glued to the twig by their rounded and somewhat compressed lower ends. The tops are fiat or a little convex. Each egg is the twenty-fourth of an inch in height, about two-thirds of this in width, and bas a capacity of one caterpillar. The covering is usually hall again as deep as the height of the eggs, but varies in thick- ness in different specimens. The outer surface is smooth and polished, but the interior is full of irregular, many- sided ar spaces, separated from one another by rhin partitions (B). V'herever the covering of an egg mass bas been broken away, the bases of the partition walls leave brown lines that look like cords strapped and tied into an irregular net over the eggs (B), as if for double security against insurrection on the part of the inmates. But neither shells nor fastenings will offer effective resistance to the little caterpillars when they are taken with the urge for ffeedom. Each is provided with efficient cutting in- struments in the form of sharp-toothed jaws that will enable it to open a round hole through the roof of its cell (Fig. I44 C). The superstructure is then easily pene- trated, and the emerging caterpillar finds itself on the surface of its former prison, along with several hundred brothers and sisters when all are out. All this time the members of that unfortunate brood we noted first have been clinging benumbed and motion- less to the silk network on the covering of their deserted eggs. The cold continues, the clouds are threatening, and during the afternoon the hapless creatures are drenched by hard and chilling tains. Through the night foliowing they are tossed in a northwest gale, while the temperature drops below freezing. The next day the wind continues, and frost comes again at night. For three

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THE CATERPILLAR AND THE MOTH

days the caterpillars endure the hostility of the elements, without food, without shelter. But already the buds on the cherry tree are sending out long green points, and when the temperature moderates on the fourth day and

from which the young caterpillars have emerged.  D, newly-hatched caterpillars (enlarged about __------ nine times) 1

the sun shines again for a brief period, the revived outcasts are able to find a few fresh tips on which to nibble. In another day the young leaves are unfolding, of- fering an abundance of tender for- age, and the season of adversity for these infant caterpillars is over. This family of tent caterpillars was hatched near Washington on the 25th of March. The newly-hatched caterpiilars (Fig. 144 D) are about one-tenth of an inch in length. The body is widest through the first segment

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and tapers somewhat toward the other end. The general color is blackish, but there is a pale gray collar on the first segment back of the head and a grayish line along the sides of the bodv. Most of the segments have pale rear margins above which are often bright yellow or orange on the fourth to the seventh segments. There is usually a darker line along the middle of the back. The body s covered with long gray hairs, those on the sides spre'ading outward, those on the back curving forward. After a few days of feeding the caterpillars increase to nearly twice their length at hatching. Wlen the weather continues fair after the time of hatching, the caterpillars begin their lives with happier days, and their early history is different from that of those unfortunates described above. Three other broods, which were round hatching on March _2, before the period of bad weather had begun, were brought indoors and reared under more favorable circumstances. These caterpillars spent but little time on the egg masses and wasted only a few strands of silk upon them. They were soon off on exploring expeditions, small processions going outward on the twigs leading from the eggs or their vicinity, while some individuals dropped at the ends of threads to see what might be below. Most, however, at first went upward, as if they knew the opening leaf buds should lie in that direction. If this course, though, hap- pened to lead them up a barren spur, a squirming, furry mob would collect on the summit, apparently bewildered by the trick their instinct had played upon them. On tle other hand, many followed those that first dropped down on threads, these in turn adding other strands till soon a silken stairway was constructed on which indi- viduals or masses of little woolly bodies dangled and twisted, as if either enjoying the sport or too fearful to go farther. For several davs the young caterpillars led this happy, irresponsible lire, expl¢ring twigs, feeding wherever an

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THE CATERPILLAR AND THE MOTH

open leaf bud was encountered, dangling in loose webs, but spinning threads everywhere. Yet, in each brood, the individuals kept within reach of one another, and the trails of silk leading back to the main branch alwavs insured the possibility of a family reunion whenever tl{is should be desired. One morning, the 27th , one family had gathered in its scattered members and these had already spun a little tentli.ke web in the crotch between the --- main stem of the sup- \- porting twig and two small branches (Fig. I45). Some members were crawling on the surface of the tent, others were resting within, still others were traveling back and forth on the silk trails  leading outward on the branches, and the rest were massed about the buds devouring the young leaves. The es- tablishment of the tent marks the beginning of a change in the cater- pillars' lives; it entails responsibilities that de- F,« 45- First t«nt made by young t«nt mand a fixed course of caterpillars. (About halfnatural size) daily living. In the lires of the tent caterpillars this poirt is what the beginning of school days is to us--the end of irresponsible ffeedom, and the beginning of sub- jection to conventional routine. Every tent caterpillar family that survives infancy eventually reaches the point where it begins the con-

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struction of a tent, but the early days are not always spent alike, even under similar circumstances, nor is the tent always begun in the same manner. In the State of Çonnecticut, where the season for both plants and insects is much later than in the latitude of Washington, three broods of tent caterpillars were ob- served hatching on April 8 of the same year. These caterpillars also met with dull and chilly weather that kept them huddled on their egg coverings for several days. After four days the temperature moderated suffi- ciently to allow the caterpillars to move about a lit(le on

the twigs, but none was seen

Fro. 146. Young tent caterpillars matted on a fiat sheet of web spun in the crotch between two branches. (About natural size)

feeding (iii the I4th--six days af ter the hatching. Yet they had increased in slze to about one- eighth of an inch in length. Wherever these cater- pillars camped in their wanderings over the small apple trees they inhabit- ed, they spun a carpet of silk to rest upon, and there the whole family collected in such a crowded mass that it looked like a round, furry mat (Fig. I46). The car- pers afforded the cater- pillars a much saler bed than the bare, wet bark of the tree, for if the sleepers shouid become

stupefied by cold the claws of their feet would mechanically hold them fast to the silk during the period of their help- lessness. The test came on the 16th and the night fol- lowing, when the campers were soaked by hard, cold rains

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THE CATERPILLAR AND THE MOTH

till they became so inert they seemed reduced to lifeless masses of soggy wool. On the afternoon of the 7th the temperature moderated, the sun came out a few rimes, the wetness evaporated from the trees, and most of the caterpillars revived sufficiently to more about a little and dry their fur. Though a few had been washed off the carpets by the violence of the storm and had perished on the ground, and in one camp about twenty dead were left behind on the web, the majority had survived. For several days after this, during better weather, the caterpillars of these families continued their free exist- ence, feeding at large on the opening buds, but returning during resting periods to the webs, or constructing new ones at more convenient places. Often each family split into several bands, each with its own retreat, yet all re- mained in communication by means of the silk trails the members left wherever they went. The camping sites were Cther against the surface of a branch or in the hollow of a crotch. Though the carpet- like webs stretched over these places were spun appar- ently only to give secure footing, those at the crotches often roofed over a space well protected beneath, and fre- quently many of the caterpillars crawled into these spaces to avail themselves of their shelter. Yet for twelve days none of the broods constructed webs designed for cover- ings. Then, on the morning of the oth, one family was round to have spun several sheets of silk above the carpet on which its members had rested for a week, and all were now inside their first tent. These caterpillars were near- ing the end of their first stage, and two days later the first molted skins were round in the tent, fourteen davs after the date of hatching. In Stage II the caterpillars have a new color pattern and one which begins to suggest that characteristic of the species in its more mature stages (Fig. 48). On the upper part of the sides the dark color is broken into a series of quadrate spots each spot partially split lengthwise by a

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light streak, and the whole series on each side is bordered above and below by distinct pale lines, the upper line often yellowish. Below the lower line there is a dark hand, and below this another pale line just above the bases of the legs. The back of the first body segment bas a brown transverse shield, and the last three segments are continuously brown, without spots or lines. From now on the tents increase rapid!y in size by suc- cessive additions of web spun over the tops and sides, each new sheet covering a fiat space between itself and the last. The old roofs thus become successively the floors of the new stories. The latter, of course, lap over on the sides, and many continue clear around and beneath the original structure; but since the tent was started in a crotch, the principal growth is upward with a continual expansion at the top. During the building period a symmetrical tent is really a beautiful object (Plate 4 C). Hall hidden among the leaves, its silvery whiteness pleas- ingly contrasts with the green of the foliage; its smooth silk walls glisten where the sun falls upon them and reflect warm grays and purples from their shadows. The caterpillars have adopted now a community form of living; all feed together, all rest and digest at the same time, all work at the same time, and their days are divided into definite periods for each of their several duties. There is, however, no visible system of government or regulation, but with caterpillars acts are probably func- tions; that is, the urge probably comes from some physio- logical process going on within them, which may be in- fluenced somewhat by the weather. The activities of the day begin with breakfast. Early in the morning the family assembles on the tent roof, and about six-thirty, proceeds outward in one or several orderly columns on the branches. The leaves on the terminal twigs furnish the material for the meal. After two hours or more of feeding, appetites are appeased, and

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THE CATERPIII.AR AND THE MOTH

the caterpillars go back to the surface of the tent, usually by eight-thirty or nine o'clock. Here they do a little spin- ning on its walls, but no strenuous work is attempted at

FIG. 147. Mature tent caterpillars feeding at night this rime, and generally within hall an hour the entire fam]ly is reassembled inside the tent. Most frequently the crowd collects firs't in the shady side of the outermost story, but as the morning advances the caterpillars seek [71 ]


INSECTS

the cooler inner chambers, where they remain hidden from view. In the early part of the afternoon a light lunch is taken. The usual hour is one o'clock, but there is no set time. Occasionally the participants appear shortly after eleven, sometimes at noon, and again not until two or three o'clock, and rarely as late as four. As they assemble on the roof of the tent they spin and weave again until all are ready to proceed to the feeding grounds. This meal lasts ai3out an bout. When the caterpillars return to the tent they do a little more spinning before they retire for the afternoon siesta. Luncheon is not always fully attended and is more popular with caterpillars in the .vounger stages, being dispensed with entirely, as we shall see, in the last stage. Dinner, in the evening, is the principal meal of the day, and again there is much variation in the time of service. Daily observations made on rive Connecticut colonies from the 8th to the 26th of May gave six-thirty p.m. as the earliest record for the start of the evening feeding, and nine o'clock as the latest; but the dinner hour is preceded by a great activity of the prospective diners assembled on the outsides of the tents. Though the energy of the tent caterpillars is never excessive, it appears to reach its highest expression at this time. The tent roofs are covered with restless throngs, most of the individuals busily occupied with the weaving of new web, working apparently in desperate haste as if a certain task had been set for them to finish before they should be allowed to eat. Possibly, though, the stimulus comes merely from a congestion of the silk reservoirs in their bodies, and the spinning of the thread affords relief. The tent caterpillar does hOt weave its web in regular loops of thread laid on by a methodical swinging of the head from side to side, which is the method of most caterpillars. It bends the entire body to one side, at-

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taches the thread as far back as it tan reach, then runs forward a few paces and repeats the movement, sonletimes on the same side, sometimes on the other. The direction in which the thread is carried, however, is a haphazard one, depending on the obstruction the spinner meets from others working in the same manner. Among the crowd of weavers there are always a few individuals that are not working, though they are just as active as the others.

These are running back and forth over the surface of the tent, like boarders impatiently awaiting the sound of the dinner bell. Perhaps they are in- dividuals that have finished their work by exhausting their supply of silk. At last the signal for dinner is sounded. It is heard by the caterpillars, though it lS not audible to an outsider. A few respond at first and start off on one of the branches leading from the tent. Others follow, and presently a column is marching outward, usually keeping to the well-marked paths of silk till the dis- tant branches are reached. Here the line breaks up into

4;; y

I

FIe. 148. Mature tent cater- pillars. (Naturel size)

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several sections which spread out over the foliage. The tent is soon deserted. For one, two, or three hours the repast continues, the diners often returning home late at night. Observations indicate that this is the regular habit of the tent caterpillar in its earlier stages, and perhaps up to the sixth or last stage of its lire. In at least nine instances the writer noted entire colonies back in the tent for the night at hours ranging from nine to eleven p.m.; but sometimes a part of the crowd was still feeding when last observed. In describing the lire of a community of insects it is seldom possible to make general statements that will apply to all the individuals. The best that a writer can dois to say what he sees most of the insects do, for, as in other communities, there are always those eccentric mem- bers who will not conform with the customs of the major- ity. Occasionally a solitary tent caterpillar may be seen feeding between regular mealtimes. Often one works alone on the tent, spinning and weaving long after its companions have quit and gone below for the midday rest. Such aone appears to be afflicted with an over- developed sense of responsibility. Then, too, there is nearly always one among the group in the tent who can not get to sleep. He flops this way and that, striking his companions on either side and keeping them awake also. These are annoyed, but they. do not retaliate; they eem to realize that their restless comrade bas but a common caterpillar affliction and must be endured. Many of these little traits make the caterpillars seem almost human. But, of course, this is just a popular form of expression; in fact, i t expresses an idea too popular --we take an over amount of satisfaction in referring to our faults as particularly human characteristics. What we really should say is not how much tent caterpillars are like us in their shortcomings, but how much we are still like tent caterpillars. We both revert more or less in our instincts to times before we lived in communities,

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to times when our ancestors lived as individuals irre- sponsible one to another. The tent caterpillars ordinarily shed their skins six times during their lives. At each molt the skin splits along the middle of the back on the first three body seg- ments and around the back of the head. It is then pushed off over the rear end of the body, usually in one piece, though most other caterpillars cast off the head covering separate from the skin of the body in all molts but the last. The moltings take place in the tent, except the molt of the caterpillar to the pupa, and each molt renders the caterpillars inactive for the greater part of two davs. When most of them shed their skins at the same time there results an abrupt cessation of activity in the colony. By the rime the caterpillars reach maturity the discarded skins in a tent outnumber the caterpillars rive to one. The first stage of the caterpillars, as already described (Fig. 144 D), suggests nothing of the color pattern of the later stages, but in Stage II the spots and stripes of the mature caterpillars begin to be formed. In succeeding stages the characters become more and more like those of the sixth or last stage (Plate 14 D, Fig. I48), when the colors are most intensified and their pattern best defined. Particularly striking now are the velvety black head with the gray collar behind; the black shield of the first seg- ment split with a medium zone of brown; the white stripe down the middle of the back; the large black lateral blotches, each inclosing a spot of silvery bluish white; the distinctly bluish color between and below the blotches; and the hump on the eleventh segment, where the median white line is almost obliterated by the crowding of the black ffom the sides. Yet the creatures wearing all this lavishness of decoration make no ostentatious show, for the colors are all nicely subdued beneath the long reddish- brown hairs that clothe the body. In the last stage, the average full-grown caterpillar is about two inches long,

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but sorne reach a length of two and a half inches when fully stretched out. In Connecticut, the tent caterpillars begin to go into their sixth and last stage about the middle of May. They now change their habits in many ways, disregarding the conventionalities and refusing the responsibilities that bound them in their earlier stages. They do little if any spinning on the tent, hOt even keeping it in decent repair. They stay out all night to feed (Fig. 147) , unless adverse weather interferes, thus merging dinner into breakfast in one long nocturnal repast. This is attested by observa- tions made through most of several nights, when the caterpillars of four colonies which went out at the usual time in the evenings were round feeding till at least four o'clock the following mornings, but were always back in the tents at seven-thirty a.m. When the caterpillars begin these all-night banquets, they dispense with the mid- day lunch, their crops being so crammed with food by morning that the entire day is required for its digestion. Sonne writers have described the tent caterpillars as nocturnal feeders, and sonne have said they feed three tirnes a day. Both statements, it appears, are correct, but the observers have hOt noted that the two habits pertain to different periods of the caterpillars' history. At any time during the caterpillars' lires adverse weather conditions may upset their daily routine. For two weeks during May, days and nights had been fair and generally warm, but on the ITth the temperature did not get above 65 ° F., and in the afternoon threatening clouds covered the sky. In the evening light rains fell, but the caterpillars of the rive colonies under observation carne out as usual for dinner and were still feeding when last ob- served at nine p.m. Rains continued through the night, however, and the temperature stood almost stationary between 50 ° and 55 °. The next morning three of the small trees containing the colonies were festooned with water-soaked caterpillars, all

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THE CATERPILLAR AND THE MOTH

hanging nnotionless fronn leaves, petioles, and twigs, be- nunnbed with exposure and incapable of action--nnore nniserable-looking insects could hOt be innagined. No in- stinct of protection, apparently, had prevailed over their appetites; till at last, overconne by wet and cold, they were saved only by sonne innpulse that led them to grasp the support so firnnly with the abdonninal feet that they hung there nnechanically when senses and power of nnove- nnent were gone. Sonne clung by the hindnnost pair of feet only, others grasped the support with all the abdonninal feet. One colony and nnost of another were safely housed in their tents. These had evidently retreated before helplessness overtook thenn. By eight o-'clock in the nnorning nnany of the suspended caterpillars were sufficiently revived to resunne activity. Sonne fed a little, others crawled feebly toward the tents. By 9:45 nnost were on their way home, and at IO:4_ç all were under shelter. Gentle rains fell during nnost of the day, but the tenn- perature gradually rose to a nnaxinnunn of 65 °. Only a few caterpillars fronn the youngest co]ony carne out to feed at noon. In the evening there was a aard, drenching rain, after which several caterpillars fronn two of the tents ap- peared for dinner. The next nnorning, the 9th, the tenn- perature dropped to 49 °, light rains continued, and nota caterpillar from any colony ventured out for breakfast. It looked as if they tiad learned their lesson; but it is more probable they were sinnply too cold and stiff to leave the tents. In the afternoon the sky cleared, the tennperature rose, and the colonies resumed their normal life. The tent caterpillars' mode of feeding is to devour the leaves clear down to the nnidribs (Figs. 48, I49) , and in this fashion they denude whole branches of the trees they inhabit. Since the caterpillars have big appetites, it some- times happens that a large colony in a small tree or several colonies in the same tree may strip the tree bare before they reach maturity. The writer never saw a colony

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reduced to this extremity by its own feeding, but produced similar conditions for one in a small apple tree by removing all the leaves. This was on May 9, and the caterpillars were mostly in their fifth stage. At seven o'clock in the evening the cater- pillars in this col- ony came out as usual, and, after « doing the cus- tomary spinning on the tent, started off to get their dinner, sus- pecting nothing till they came to the cut-off ends of the branches. Then they were clearly bewildered .--they returned and tried the course over again ; they tried another branch, all the other branches; but all ended alike in bare stumps. Yet there were the accus- tomed trails, and their instincts Fla. 149. Twigs of choke cherry and of apple de- c]ear]y said that nuded by tent caterpillars silk paths led to food. So all night the caterpillars hunted for the missing leaves; they went over and over the same courses, but none ventured be]ow the upper part of the trunk. By 3:45 in the morning

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THE CATERPILLAR AND THE MOTH

many had given up and had gone back to the tent, but the rest continued the hopeless search. At seven-thirty a few bold explorers had discovered some remnants ol  water sprouts at the base of the tree and fed there till ten o'clock. At eleven all were back in the tent. At two o'clock in the afternoon the crowd was out again and a mass meeting was being held at the base of the tree. But nobody seemed to have .any idea ofwhat to do, and no leader rose to the occasion. A few cautious scouts were making investigations over the ground to the extent of a foot or a little more from the base of the trunk, but, though there were small apple trees on three sides rive feet away, only one small caterpillar ventured off toward one of these. He, however, missed the mark by twelve inches and continued onward; but probably chance eventually rewarded him. At three p.m. the meeting broke up, and the members went home. They were not seen again that evening or the next morning. During this day, the "_ISt, and the next, an occasional caterpillar came out of the tent but soon returned, and it was not till the evening of the 22nd that a large number appeared. These once more explored the naked branches and traveled up and down the new paths on the trunk, but none was observed to leave the tree. On the -3rd and 24th no caterpillars were seen. On the 2çth the tent was opened and only two small individuals were round wlthin it. Each of th'ese was weak and flabby, its alimentary canal completely empty. But what had become of the rest? Probably they had wandered off unobserved one by one. Certé, inlv there had been no organized migra- tion. Solitary caterpillars were subsequently found on a dozen or more small apple trees in the immediate vicinity. It is likely that most of these had molted and had gone into the lst stage, since their time was ripe, but this was not determined. After the caterpillars go over into their last stage, the tents are neglected and rapidly fall into a state of dilapida-

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tion. Birds often poke holes in them with their bills and rip off sheets of silk which they carry away for nest-build- ,ng purposes. The caterpillars do not even repair these damages. The rooms of the tent become filled with ac- cumulations of frass, molted skins, and the shriveled bodies of dead caterpillars. The walls are discolored by tains which beat into the openings and soak through the refise. Thus, what were shapely objects ofglistening silk are transmuted into formless masses of dirty rags. But the caterpillars, now in their finest dress, are ob- livious of their sordid surroundings and sleep all dav. anaidst these disgusting and apparently insanitary condi- tions. However, the life in the tents will soon be over; so it appears the caterpillars simply think, "What's the use?" But of course caterpillars do not think; they arrive at results by instinct, in this case by the lack of an i'nstinct, for they hve no impulse to keep the tents elean or in

Fro. o. A tent caterpillar in the last stage of its growth, leaving the tree con- taining its nest by jumping from the end of a twig to the ground

repair when doing so would be energy wasted. Nature demands a prac- tical reason for most things. The tent lire continues about a week after the last molt, and then the family begins to break up, t[ae members leaving singly or in bands, but al- wavs as individuals with- Otl[ further COllcerll for one another. Judging from their previous me-

thodical habits, one ould suppose that the caterpillars starting off on their journeys would simply go down the trunks of the trees and walk away. But no; once in their life they must have a dramatic moment. A caterpillar comes rushing out of a tent as if suddenly awakened from

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THE CATERPILI.AR AND THE MOTH

some terrible dream or as if pursued by a demon, hurries outward along a branch, goes to the end of a spur or the tip of a leaf, and without slackening continues into space till the end of the support tickles his stomach, when sud- denly he gives a flip into the air, turns a somersault, and lands on the ground (Fig. 15o). The first performance of this sort was observed on May 15 in the Connecticut colonies. On the afternoon of the 19th, twenty or more caterpillars from two neigh- boring colonies were seen leaving the trees in the same fashion within half an hour. Most of the members of one of these colonies had their last molt on May I2 and 13 . During the next few davs other caterpillrs were ob- served jumping from four trees containing colonies under observation. All of these went off individually at various times, but most of them early in the afternc;on. Many caterpillars simply drop off" when they reach the end of the branch, without the acrobatic touch, but only three were seen to go down the trunk of a tree in commonplace style. The population of the tents gradually decreases during several davs following the time when the first caterpillar departs. ne of the two tents from which the general exodus was noted on May 19 was opened on the 2lSt and was fimnd to contain only one remaining caterpillar. On the evening of the 22nd a solitary individual was out feed- ing from the other tent. The two younger colonies main- tained their numbers until the 22nd, after which they diminished till, within a few days, their tents also were deserted. The members of all these colonies hatched from the eggs on April 8, 9, and Io, so seven weeks is the greatest length of time that any of them spent on the trees of their birth. The caterpillar that left the tent on the çth came flore a colony that began to hatch on April 1% gving an observed minimum of thirty-six days. After the mature caterpillars leave the tents, they wander at large and feed wherever they find suitable


INSECTS

provender, enjoying for a while a new lire free from the domestic routine that bas bound them since the days of their infancy. But even their liberty has an ulterior pur- pose: the time is now approaching when their lives as caterpillars must end and the creatures must go through the mysteries of transformation, which, if successfully accomplished, will convert them into winged moths. I't would clearly be most unwise for the caterpillars of a colony to undergo the period of their metamorphosis huddled in the remains of the tent, where some untoward event might bring destruction to them all. Nature has, therefore, implanted in the tent caterpillar a migratory t,rge, which now becomes active and leads the members of

'I;. |çI. The cocoon of atent cater- pillar. (Natural size)

a family to scatter far and wide. About a week is allowed for the dispersal, and then, as each wan- derer feels within the first warnings of ap- proaching dissolution, it selects a suitable place for inclosing itself in a COCO011.

I t is diflàcult to find many cocoons in the neighborhood where large numbers of caterpillars have dispersed, but such as may be recovered will be round among blades of grass, under ledges of (ences, or in sheds and barns where they are not disturbed. The cocoon is a slender oval or a]most spindle-shaped object, the larger ones being about an inch long and hall an inch in width at the middle I Plate 4 E Fig. l çl). The structureis spun ofwhit¢ silk thread, but its waÏls are stiffened and colored by a yellow- ish substance infiltrated like starch through the meshes of the fabric. In building the cocoon the caterpillar first spins a loose network of threads at the place selected, and then, using this for a support, weaves about itself the walls of the final

[ _8]


THE CATERPILI.AR AND THE MOTH

structure. On account of its large size, as compared with the size of the cocoon, the caterpillar is forced to double on itself to fit its self-imposed cell. Most ofits hairs, how- ever, are brushed off and become interlaced with the threads to forma part of the cocoon fabric. When the spinning is finished, the caterpillar ejects a yellowish, pasty liquid from its intestine, which it smears all over the mner surface of the case; but the substance spreads through the meshes of the silk, where it quickly dries and gives the starchy stiffness to the walls of the finished cocoon. It readi'ly crumbles into a yellow powder, which becomes dusted over the caterpillar within and ftoats off in a small yellow cloud whenever a cocoon is pulled loose from its attachments. The cocoon is the last resting place of the caterpillar. If the insect lires, it will come out of its prison as a moth, leaving the garments of the worm behind. It may, how- ever, be attacked by parasites that will shortly bring about its destruction. But even if it goes through the period of change successfully it nmst remain in the cocoon about three weeks. In the meantime it will be ofinterest to learn something of the structure of a caterpillar, the better to understand some of the details of the process of its trans- formation.

THE STRUCTURE AND PHYSIOLOG" OF THE CATERPILLAR A caterpillar is a young moth that bas carried the idea of the independence of youth to an extreme degree, but which, instead of rising superior toits parents, bas de- generated into the form of a worm. An excellent theme this would furnish to those who at present are bewailing what they believe to be a shocking tendency toward an excess of independence on the part of the young of the human species; but the moral aspect of the lesson some- what loses its force when we learn that this freedom of the caterpillar from parental restraint gives advantages to both young and adults and therefore results in good to [ -83 ]


INSECTS

the species as a whole. Independence entails responsibil- ities. A creature that leaves the beaten paths of its an- cestors must learn to take care ofitse]fin a new way. And

Flc. 15OE. The head of a tent caterpillar A, facial view. B, under surface. C, side view. .dnt, antenna; Clp, clypeus; For, opening of back of head into body; ttphy, hypopharynx; Lk, labium; Lin, labrum; 3Id, mandible; Mx, maxilla; O, eyes; Spt, spinneret

this the caterpillar has learned to do preeminently well, as it bas come up the long road of evolution, till now it possesses both instincts and physical organs that make it

F¢. x$3. The mandibles, or biting jaws, of the tent cater- pillar detached from the head A, front view of right mandi- ble. B, under side of the left mandible, a and p, the an- terior socket and posterior knob by which the jaw is hinged to the head; EAcl, RMd, abductor and adductor muscles that more the jaw in a transverse plane

one of the dominant forms of in- sect lire. The external organs of princi- pal interest in the caterpillar are those of the head (Fig. 152). These include the eyes, the an- tennae, the mouth, the jaws, and the silk-spinning instrument. A facial view of a caterpillar's head shows two large, hemispherical lateral areas separated by a medi- an suture above and a triangular plate (C/p) below. The walls of the lateral hemispheres give at- tachment to the muscles that move the jaws, and their size is no index of the brain-power of

[ -84 ]


THE CATERPILLAR AND THE MOTH

the caterpillar, since the insect's brain occupies but a small part of the interior of the head (Fig. 154 , Br). From the lower edge of the triangular facial plate (Fig. ISZ, Clp) is suspended the broad, notched front lip, or labrum (Lin) that hangs as a protective flap over the bases of the jaws. At the sides of the labrum are the very small antennae (.4rit) of the caterpillar. On the lower part of each lateral hemisphere of the head are six small simple eyes, or ocelli (O), rive in an upper group, and one near the base of the antenna. With all its eyes, however, the caterpillar

Cr Vent Ht /val int Br..Rect Phy -'"'- -  _ Mth / ,    oeGn kG1 Fro. 54- Diagrammatic lengthwise section of a caterpillar, showing the principal internal organs, except the tracheal system d, anus; r, brain; C, crop; I, heart; II, ;ntestine; 11, Malpighian tubule (two others are cut off near their bases); I, mouth; Oe, oesophagus; pharynx; R«et, rectum; çl, silk gland; o«çn, suboesophageal ganglion; stomach, or ventriculus;  ventral nerve cord appears to be very nearsighted and gives little evidence of being able to distinguish more than the presence or ab- sence of an object before it, or the difference between light and darkness. Those tent caterpillars that were starving on the denuded tree failed to perceive other food trees in full leaf only a few feet away. The general external form and structure of the tent caterpillar is shown at A of Figure 159. The body is soft and cylindrical. The head is a small, hard-walled capsule attached to the body by a short flexible neck. Back of the head and neck comes first a body region consisting of three segments that bear each a pair of small, jointed legs (L); and then comes a long region composed of ten segments supported on rive pairs of short, unjointed legs [ 85 ]


INSECTS

(. 4L), the first four pairs being on the third, fourth, fifth and sixth segments, and the last on the tenth segment. The region of the three segrnents in the caterpillar bear- ing the jointed legs corresponds with the thorax of an adult insect (Fig. 63, Th), and that following corresponds with the abdomen (db). The thorax of the adult insect constitutes the locornotor center of the body, but the worrnlike caterpillar has no special locornotor region, and hence its body is not separated into thorax and abdomen. The thoracic legs of the caterpillar terrninate each in a single claw, but the foot of each of the abdorninal legs has a broad sole provided with a series or circlet of claws and with a central vacuum cup. The abdorninal legs of the caterpillar, therefore, are important organs of pro- gression, and are the chier organs of grasping or of cling- ing to hard or fiat surfaces. The jaws of the caterpillar consist of a pair of large, strong rnandibles (Fig. 152 , 3fd) concealed, when closed, behind the labrurn. Each jaw is hinged to the lower edge of the cranium at the side of the rnouth by two ball- and-socket hinges in such a rnanner that, when in action, it swings outward and inward on a lengthwise axis. The cutting edges are provided with a number of strong teeth (Fig. 53), the points of which come together or slide past each other when the jaws are closed. The large complex organ that projects behind or below the rnouth like a thick under lip (Fig. 5 OE C) is a com- bination of three parts that are separate in other insects. These are the second pair of soif jaw appendages, called maxillae (B, C, Mx), and the true under lip, or labium (Lb). The most important part of this cornposite struc- ture in the caterpillar, however, is a hollow spine (A, B, C, Spt) pointed downward and backward from the end of the labiurn. This is the spinneret. Frorn it issues the silk thread with which the caterpillar weaves its tent and its cocoon.

[ OE86 ]


THE CATERPILI.AR AND THE MOTH

The fresh silk is a liquid formed in two long, tubular glands extending far back from the head into the body of the caterpillar (Fig. 154 , 8kGl). The middle part of each tube is enlarged to serve as a reservoir where the silk liquid may accumulate (Fig. 55 A, Res); the anterior narrowed part constitutes the duct (Da), and the ducts

FIG. I. The silk glands and spinning organs of the tent caterpillar A, the silk-forming organs, consisting of a pair of tubular glands (GI, GI), each enlarging into a reservoir (Res), and opening through a long duct (Dct) into the silk press (Pr), with a pair of accessory glands (glands of Filippi, GIF) opening into the ducts B, side view of the hypopharynx (Hphy) with terminal parts of right maxilla (Mx) and labium (Lb) attached, showing the silk press (Pr), its muscles, and the ducts (Dot) opening into it, and the spinneret (,çpt) through which the silk is discharged from the press C, upper view of the silk press (Pr), showing the four sets of muscles (Mcls) inserted on its walls and on the rod-like raphe (Rph) in its roof D, side view of the silk press, spinneret, raphe, and muscles E, cross-section of the silk press, showing its cavity, or lumen (Lum), which is expanded by the contraction of the muscles

[ 87 ]


INSECTS

from the two glands unite in a median thick-walled sac known as the silk press (Pr), which opens to the exterior through the spinneret. Two small accessory glands, which look like bunches of grapes and which are sometimes called the glands of Filippi (Fig. 55 A, B, C, G/F), open into the silk ducts near their front ends. The relation of the silk ducts and the silk press to the spinneret is seen in the side view of the terminal parts of the labium and the left maxilla, given at B of Figure 55- The silk press (Pr) is apparently an organ for regulating the flow of the liquid silk material into the spinneret. It

Flç. 156. The alimentary canal of the tent cater- pillar A, before feeding. B, after feeding. Cr, crop; Int, in- testine; Mal, Malpighian tubules; OE, oesophagus; Rect, rectum; lent, ventri- culus

has been supposed, too, that it gives form and thickness to the thread, but the liquid material has still to pass through the rigid tube of the spinneret. The cut end of the press, given at E of Figure 55, shows the crescent form of the cavity (Lum) in cross- section, and the thickening in its roof (Rph), called the raphe. Mus- cles (Mcls) inserted on the raphe and on the sides of the press serve to enlarge the cavity of the press by lifting the infolded roof. The four sets of these muscles in the tent caterpillar are shown at Ç. The dilation of the press sucks the liquid silk into the cavity through the ducts from the reservoirs, and when the muscles relax, the elastic roof springs back and exerts a pressure on the silk material, which forces the latter through the tube of the spinneret. The continuous passageway from the ducts through

[ 88 ]


THE CATERPILLAR AND THE MOTH

the press and into the spinneret is seen from the side at D. The silk liquid is gummy and adheres tightly to what- ever it touches, while at the same time it hardens rapidly and becomes a tough, inelastic thread as it is drawn out of the spinneret when the caterpillar swings its head away from the point of attachment. The mouth of the caterpillar lies between the jaws and the lips. It opens into a short gullet, or oesophaXus, which, with the pharynx, constitutes the first part of the alimen- tary canal (Fig. 154 ' Pli.v, Oe). The rest of the canal is a wic{e tube occup.ving most of the space within the cater- pillar's body and is divided into the trop (Cr), the stomach, or ventricul'us (l'ent), and the ilttestMe (lt). The crop is a sac for receiving the food and varies in size according to the amount of food it contains (Fig. 56 A, B, Cri. The stomach (Peu) is the largest part of the canal. I ts walls are loose and wrinkled when it is empty, or smooth

and tense when it is full. The in- testine (Int) consists of three divi- sions, a short part just back of the stomach, a larger middle part, and a saclike end part called the rectum (Rect). Six long tubes (Mal) are wrapped in many colis about the in- testine and run iorward and back in long loops over the rear hall of the stomach. The three on each side unite into a short basal tube, which opens into the first part of the intes- tine. The terminal partsofthe tubes are coiled inside the muscular coat

Fro.  57- Crystals from the Malpighian tubules of the tent caterpillar, which are ejected into the walls of the cocoon

of the rectum. These tubes are the Malpighian tubules. When a tent caterpillar goes out to feed, the fore part of its body is sort and flabbv; when it returns to the tent the same part is tight and firm. This is because the tent caterpillar carries its dinner home in its crop, digests it

[ =89 ]


INSECTS

slowly while in the tent, and then goes out for more when the crop is empty. It is quite easy to tell by feeling one of these caterpillars whether it is hungry or hot. The empty, contracted crop is a small bag contained in the first three segments of the body (Fig. 56 A, Cr); but the full crop stretches out to a long cylinder like a sausage, filling the first six segments of the body fB, Cr), its rear end sunken into the stomach, and its front end pressed against the back of the head. The fresh food in the crop consists of a sort, pulpy mass of leaf ffagments. As this is passed into the stomach, the crop contracts and the stomach expands, and the caterpillar's center of gravity is shifted backward with the food burden. As the stomach becomes empty there ac- cunulates in it a dark-brown liquid, and it becomes in- flated with bubbles of gas. When the caterpillar goes to its meals both crop and stomach are sometimes empty, but usua]]y the stomach still contains some food besides an abundance of the brown liquid and numerous gas bubbles. The refuse that accumulates in the middle sec- tion of the intestine is subjected to pressure by the mus- cles of the intestinal wall, and is here molded into a pellet which retains the imprint of the constrictions and pouches of this part of the intestine and looks like a small mulberry when passed on into the rectum and fina!ly extruded from the body. The alimentary canal is a tube made of a single layer of cells extending through the bodv; but its outer surface, that toward the body cavity, is covered by a muscle layer of lengthwise and crosswise fibers, which cause the more- ment of the food through the canal. The gullet and crop and the intestine are lined internally with a thin cuticula continuous with that covering the surface of the body, and these linings are shed with the body cuticula every rime the caterpillar molts. The Malpighian tubules (Figs. 54, 56 A, Mal), being the kidneys of insects, are excretory organs that remove

[ 9 o ]


THE CATERPII.IAR AND THE MOTH

from the blood the waste products containing nitrogen, and discharge them into the intestine along with the waste parts of the food from the stomach. Ordinarily the iXlal- pighian tubules are of a whitish color, but just'before the tent caterpillar is ready to spin its cocoon they become congested with a brig[at vellow substance, l.;nder th'e mcroscope this is seen to consist of masses of square, oblong, and rod-shaped crvstals (Fig. 57)- At this rime the caterpillar has ceased to feed and the alimentary canal contains no food or food refuse. The intestine, however,

FIG. ! ç8. A piece of the fat-body of the rail weborm a, a, globules of fatty oil in the cells; J)«, N«, nuclei of the cells becomes filled with the vellow mass from the Malpighian tubules; and this is the material with which the tent caterpillar plasters the walls of its cocoon, giving them their yellowish color and stiffened texture. The vellow powder of the cocoon, therefore, consists of the cvstals from the Malpighian tubules. We now come to the question of why the caterpillar eats so much. It is ahnost equivalent to asking, "\Vhy is a caterpillar?" The caterpillar is the principal feeding stage in the insect's lire; eating is its business, its reason for being a caterpillar. It eats not only to build up its own organs, many of which are to be broken down to furnish building aterial for those of the moth, but it eats also to store up within its body certain materials in excess of its own needs, which likewise will contribute to the growth of the moth.

[9  ]


INSECTS

The most abundant of the food reserves stored by the caterpillar is fat. With insects, however, fat does not accumulate among the muscles and beneath the skin. sects do not become "fat" in external appearance. Their fatty products are held in a special organ called thefat- bodv. The fat tissue of a caterpillar consists of many small, fiat, irregular masses of fat-containing cells scattered all through the body cavity, some of the masses adhering in chains and sheets forming a loose open network about the alimentary canal, others being distributed against the muscle layers and between the muscles and the body wall. The cells composing the tissue vary much in size and shape, but they are always closely adherent, and in fresh material it is often difficult to distinguish the cell bound- aries. Specimens prepared and stained for microscopic examination, however, show distinctly the cellular struc- ture (Fig. I58). Each cell contains a darkly-stained nucleus (Nu), but the nuclei are seen only where they lie in the plane of the section. The protoplasmic area about the nucleus in each cell appears to be occupied mostly with hollow cavities of various sizes (a), but in life each cavity contains a small globule of fatty oil. The proto- plasmic material between the oil globules contains also glycogen, or animal starch, as can be shown by staining with iodine. Both fat and glycogen are energy-forming compounds, and their presence in the fat cells of the caterpillar shows that the fat-body serves as a storage organ for these materials during the larval life. The stored fat and glycogen will be consumed during the period of metamorphosis, when the insect is deprived of the power of feeding and receives no further nourishment from the alimentary canal. The transformation processes will then depend upon the food materials that the caterpillar has stored in its own body; and the success of the pupal meta- morphosis will depend in large measure on the quantity of these food reserves. A starved caterpillar, therefore,

[ 9  ]


THE CATERPILLAR AND THE MOTH

is likely to be unable to accomplish its transformation, or it will produce a dwarfed or an imperfect adult. How THE CATERPILLAR BECOMES A IOTH A short rime before the caterpillar is ready to spin its cocoon, it ceases feeding. Its body, as we have just learned, contains now an abundance of energy-giving sub- stances stored in the cells of its fat tissue. When the work of constructing the cocoon is started, the alimentary canal is devoid of food material, the crop is contracted to a narrow cylinder, and the stomach is shrunken and flabby. The stomach, however, contains a mass of sort, orange- brown substance which, when examined under the micro- scope, is round to consist, hot of plant tissue, but of animal cdls; it is, in fact, the cellular lining of the caterpillar's stomach which bas already been cast off into the cavity of the stomach. The latter is now provided with a new cell wall. The shedding of the old stomach wall marks the first stage in the dismantling of the caterpillar; it is the beginning of the pupal metamorphosis which will convert the caterpillar into the moth. The new stomach wall will first digest and absorb the débris of the old, in order to conserve its proteid materials for the constructive work of the pupa, and it will then itself become transformed into the stomach of the moth. After the caterpillar bas shut itself into the cocoon, its life as a caterpillar is almost ended. Its external appear- ance is already much altered by the contraction of the body and the "loss of the hairy covering, and during the next three or four days a furter characteristic change of form takes place. As the body continues to shorten, the first three segments become crowded together; but the abdomen swells out, while the abdominal legs are re- tracted until they all but disappear. The creature is now (Fig. I59 B) onl." half the length of the active caterpillar (A), and it would scarcely be recognized as the same in- dividual that so recently spun itself into the cocoon. l "-93 ]


INSECTS

During the progress of change in the external form, the caterpillar gradually loses the power of movement. The resultant inactive period in an insect's lire, immediately preceding the visible change to the pupa, is called the

A."  .g=

Fie,. x9. Transformation of the tent caterpH|ar into the moth [ :94 ]


THE CATERPILLAR AND THE MOTH

prepupal stage of the larva. The insect in the prepupal stage bas suffered no change in external structure, it still wears the larval skin, and its visible difference f?om the active larva is a mere alteration in form. Internally, how- ever, important reconstructive processes are now taking place. The internal activities of reconstruction, which bring about the pupal metamorphosis of the larva to the adult, begin at the head end of the insect and progress poste- riorly. They are preceded by a loosening and subsequent detachment of the larval cuticula from rhe cellular layer of the skin, or epidermis, beneath it. The latter, known also as the h)'podermis, freed now from restraint, enters a period of rapid growth. On the head, the head walls are remodeled and take on a new form, and new antennae and new mouth parts are produced. The new structures have no regard for the forms of the old, though each is pro- duced from a part of the corresponding larval organ. The new antennae, for example, are formed from the larval antennae, but the antennae of the moth are to be much larger than those of the caterpillar. Only the tip, there- fore, of each new organ can be formed within the cuticular sheath of the old; the base pushes inward, and rhe don- gating shaft folds against the face of the newly forming head. The same thing is true of the maxillae and labium, but in the case of the mandibles the procedure is simpler, for the jaws are to be reduced in the moth. The epi- dermal core of each mandible, therefore, simply shrinks within the cuticular sheath of the larval organ, leaving the cavity of the latter almost empty. As the separation of the cuticula from the epidermis progresses over the region of the thorax and a free space is created between the two layers, the wing buds, which heretofore have been turned inside the caterpillar's body, now evert and come to be external appendages of the pupal body though still covered by the curicula of the larva (Fig. 59 C, I/es, I¢"a). The legs of the moth pupa are

[ 295 ]


INSECTS

formed in the same way as are the antennae and the mouth parts, that is, they are developed from the epidermis of the corresponding larval legs; but, by reason of their creased size, they are forced to bend upward against the sides of the body of the pupa, and, when fully formed, each is round to have only its terminal part within the cuticular sheath of the leg of the caterpillar. From the thorax, the loosening of the cuticula spreads backward over the abdomen, until at last the entire insect lies ffee within the cuticular skin of the caterpillar. The so-called prepupal period of the caterpillar, therefore, is scarcely to be regarded as a truly larval stage of the in- sect. it is still clothed in the larval cuticula, and retains externally all the structural characters of the larva; but the creature itself is ira a first growth period of the pupal stage, and may appropriately be designated a propupa. When the cuticula is separated flore the epidermis all over the body, it may be cut open and taken off without injury to the wearer. The latter, now a propupa (Fig. 59 C), is then discovered to be a thing entirely different in appearance flore the caterpillar. It bas a small head bent downward, a thoracic region of three segments, and a large abdomen. The head bears the mouth parts and a pair of large antennae (4nt) ; the thorax carries the wings (IFs, II.') and the legs (L), which latter are much longer than those of the caterpillar, but, being folded beneath the wings, only their ends are visible in side view. The abdomen consists of ten segments and bas lost all ves- tiges of the abdominal legs of the caterpillar (A, .4L). Many important changes have taken place ira the form and structure of the head and in the appendages about the mouth during the change from the caterpillar to the propupa, as may be seen by comparing Figure 59 H, with Figure I5'. Most of the lateral areas of the caterpillar's head (Fig. 5), including the region of the six small eyes on each side, have been converted into the two huge eye areas of the pupa (Fig. I59 H, E), which cover the develop-

[ 9 6 ]


THE CATERPII.LAR AND THE MOTH

ing compound eves of the adult. The antennae (.4nt), as already noted, Jaave increased greatly in size, and they show evidence of their future multiple segmentation. The upper lip, or labrum (Lin), on the other hand, is much smaller in the propupa than in the caterpillar, and the great biting jaws of the caterpillar are reduced to mere rudiments in the propupa (Md), while the spinneret (Fig. 152 , 5'pt) is gone entirely. The labium and the two maxillae are longer and more distinct from each other in the propupa (Fig. 159 H, Lb, Mx) than in the caterpillar, and their parts are somewhat more simplified. The labium bears two prominent palpi (LbPlp). The remodeling in the external form .of the insect pro- ceeds from particular groups of cells in the epidermis, cells that have remained inactive since the time of the embryo, and which, as a consequence, retain an unused vitalitv. These groups of regenerative epidermal cells, which'are the histoblasts, or imaginal discs, of the body wall, have hot been particularly studied in the caterpillar; but in certain other insects they have been round to occur in each segment, typically a pair of them on each side of the back: and a pair on each side of the ventral surface. At the beginning of metamorphosis, as the larval cuticula separates from the epidermis, the cells of the discs multiply and spread from their several centers, and the areas newly formed by them takeon the contour and structure of the pupa ins{ead of that of the larva. The old cells of the larval epidermis, which have reached the limits of their growing powers and are now in a state of senescence, give way before the advancing ranks of invading cells; their tisues go into dissolution and are absorbed into the body. The new epidermal areas finally meet and unite, and to- gether cons-titute the body wallof the pupa. While the new epidermis is giving external form to the pupa beneath the larval cuticula, its cells are generating a new pupal cuticula. As long as the latter is sort and plastic the cell growth may proceed, but when the cutic-

[ =97 ]


INSECTS

ular substance begins to harden; growth ceases, and the external form of the insect will henceforth show no further change in its structural features. The propupa of the nnoth rennains for several days a soft-skinned creat'ure I Fig. 59 C) inside the cuticula of the larva, during which tinne its body contracts in size and its wings, legs, antennae, and nnaxilloe lengthen. The wings are flattened against the sides of the body, and the other appendages are applied close to the under surface. Then a gluelike substance is exuded fronn the body wall, which fixes the nnennbers in their positions and soon dries into a hard coating or glaze over the body and appendages, giving to the whole a shell-like covering. In this way the sort propupa (C beconnes a chrvsalis (D). Finally, the old caterpillar skin splits along the back of the first two body segnnents, over the top of the head, and down the right side of the facial triangle. The p.upa now quickly wriggles out of the enclosing skin and pushes the latter over the rear extrennity of its body into the end of the cocoon, where it rennains as a shriveled mass, the last evidence of the caterpillar. The pupa, or chrysalis, of the tent caterpillar I Fig. 59 D) is nnuch snnaller than the propupa (C), and its length is only about one-third that of the original cater- pillar (A). The color of the chrysalis is at first bright green on the fore parts, yellowish on the abdonnen, and usually nnore or less brown on the back. Soon, however, the color darkens until the front parts and the wings are purplish black, and the abdonnen purplish brown. Though the covering of the chrysalis is hard and rigid, the creature is still capable of a very active wriggling of the abdonnen, for three of its intersegnnental rings rennain flexible. By this provision the pupa is able to divest itself of the larval skin. The pupoe of sonne species of nnoths push thenn- selves partly out of the cocoon just before the tinne of transfornnation to the nnoth, and when the latter ennerges

[ 9 8 ]


THE CATERPII.LAR AND THE MOTH

it leaves the pupal skin projecting from the mouth of the cocoon (Plate I2). Concurrent with the remodeling in the external form of the insect, other changes have been taking place within the body. The first of the complicated metamorphic processes that affect the inner organs occurs in the stomach, where, as we have already observed, the inner wall is cast off at about the time that the caterpillar begins the spinning of its cocoon. This shedding of the stomach lining is quite a different thing from the molting of an external cuticula, for the StOlnach wall is a cellular tissue. Furthermore, wherever other cell lavers are discarded, as in the case of the epidermis, the cells are absorbed Mto the body cat, ity. A new stomach wall is generated usually from groups of small cells that originally lay outside the old wall and were retained when the latter was cast off. These cells, as do the imaginal discs of the epidermis, form a new lining.to the stomach and give a new shape to this organ, which in the adult insect may be quite different from that of the larva. The sheddin of the stomach wall is hot necessarily a part of the metamorphosis, for in some insects and in certain other related animals, it is said, the stomach epithelium as well as the cuticular lining is shed and renewed with each molt of the body wall. The parts of the alimentary canal that lie before and behind the stomach, that is, the oesophagus and crop (Fig. 154, Oe, Cr) and the intestine (It), formed in the embrvo as ingrowths of the bodv. wall, are regenerated from groups of cells in their walls in the same manner as is the epidermis itself, the old cells being absorbed into the bodv. The cuticular linings of these parts are shed with the Cuticula of the body wall at the time of the molt. The complete alimentary canal of the moth is very different from that of the caterpillar, as will be shown in the next section of this chapter/Fig. 164). The walls of the Malpighian tubules are said to be regenerated in some insects, but the tubules do hot change

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much in form in the moth, and they continue their ex- cretory function during the pupal stage. The silk glands of the caterpillar are greatly reduced in size, and their ducts, as a consequence of the suppression of the spinneret, open at the base of the labium within the entrance to the mouth. lnternal organs that have hot been specially modified in their development for the purposes of the larva, in- c]uding usually the nervous system, the heart, the respira- tory tubes, and the reproductive organs, surfer little if any disntegration in their tissues; they simply grow to the mature form, which may be much more elaborate than that of the larva, by a resumption of the ordinary processes of development. The nervous system, and particularly the tracheal system, however, in some insects undergo much reconstruction between the larval and the adult stages. A most important part of the reconstruction between the larva and the adult has to do with the muscle system. Since, in its two active stages, the insect leads usually two very different lires, the mechanism of locomotion is likely to be radicaliy different in the larva and in the adult; and in such cases the transformation of the insect will involve particularly a thorough reorganization of the musculature. Most larvae have acquired an elaborate system of speciai muscles for their own use because they have adopted a wormlike mode of progression. On the other hand, the adults have need of certain muscles, par- ticularly those of the wings, which would be only an en- cumbrance to a larva. Consequently, muscles needed only by the adult are suppressed in the larval stage, and the special muscles of the larva must be cleared away during the pupal stage. The metamorphosis in the muscle sys- rem, therefore, varies much in different insects according to the mechanical difference between the larva and the aduit. The purely larval muscles that are to be discarded when

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THE CATERPILLAR AND THE MOTH

their purpose has been accomplished go into a state of dis- solution during the pupal period. The débris of their tissues is thrown into the blood, from which it is later ab- sorbed as nutriment by the newly forming organs. The caterpillar has a very elaborate system of muscles forming a complicated network of fibers against the inner surface of the body wall, some running longitudinally, others transversely, and still others obliquely. Most of the transverse and oblique fibers are hOt retained in the moth, and if specimens of those muscles are examined during the early part of the pupal period they are seen to have a weak and abnormal appearance; the structure typical of healthy muscle tissue is obscure or indistinctly evident in them, and in places they are covered with groups of free oral cells. These cells are probably p/mgocytes. A phagocyte is a blood corpuscle that destroys foreign proteid bodies in the blood, or any unhealthy tissue of the body. It is hot probable that the insect phagocytes are the active cause of the destruction of the larval tissues, but they do engulf and digest particles of the degenerating tissues. They are present in large numbers in some insects during metamorphosis, and are scarce or lacking in others. The decadent state of the larval tissues that have passed their period of activity lays them open to the attack of the phagocytes, but these tissues will go into dissolution by the solvent powers of the blood alone. Active, healthy tissues are always immune from phagocytes. Some of the larval muscles may go over intact to the adult stage, and others may require only a remodeling or an addition of fibers to make them serviceable for the purposes of the adult. The adult muscles that are com- pletely suppressed during the larval stage appear to be generated anew during the pupal stage. There is a dif- ference of opinion among investigators as to how the new muscles are developed, but it is probable that they take their origin from the same tissues that built up the larval muscles.

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The development of the internal organs proceeds with- out interruption from the beginning of the propupal period until the adult organs are c.ompleted at the end ofthe pupal stage. The external parts, however, do hot make a con- tinuous growth. After reaching a certain stage of de- velopment, the form of the body wall and of the append- ages is fixed by the hardening of the new cuticula on their outer surfaces. In this stage, therefore, they must remain, and the half-mature form attained is that char- acteristic of the pupa. The final development of the body wall and the appendages of the adult is accomplished by a second separation of the epidermis from the cuticula, which allows the cellular layers, now protected by the pupal cuticula, to go through a second period of growth during the pupal stage. This pupal period of growth at last results in the perfection of the external characters of the adult, which are in turn fixed by the formation of the adult cuticula. In the meantime, the new muscles that are to be retained have become anchored at their ends into the new cuticula, and the mechanism of the adult insect is ready for action. The perfect insect, cramped within the pupal shell, has now only to await the proper time for its emergence. Through the whole period of metamorphosis, the insect must depend on its internal resources for food materials. Oxygen it can obtain by the usual method, for its respira- tory system remains functional; but in the matter of food it is in a state of complete blockade. The pupa bas two sources of nourishment: first., the food reserves stored in the cells of the fat-body; second, the materials resulting from the breaking down of the larval tissues, which are scattered in the blood and eventually absorbed. The fat cells, at the beginning of metamorphosis in some insects, give up most of their stored fat and glycogen; and they now become filled with small granules of proteid matter. The proteid granules are probably elaborated in the fat cells from the absorbed detritus of the larval

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THE CATERPII.LAR AND THE MOTH

organs by means of enzymes produced in the nuclei of the cells. The fat cells thus take on the function of a stomach, converting the materials dissolved in the blood into forms that the growing tissues can assimilate. During this time the masses of fat tissue that compose the fat-body of the

"r<>:,.ç::o " :-:oE--: :¢.,' / e I. ,/"   é "... " "/""   O- " .- --.--..:»" :e«::.a«  <çi::.-:: O Fro. 6o. Bodies in the blood of a young pupa of the tent cterpillar a, a free fat cell, containing large oily fat globules, and small proteid granules; b, r, fat cells in dissolution; d, froe proteid granules in the bl?ed, and e, fat globules liberat from the disintegrating fat cells;f, bl?ed corpuscles

larva have broken up into free cells, and these cells, vacuolated with oii globules and later charged with pro- teid granules, now fill the blood. The interior of the moth pupa, or chrysalis, shortly after the larval skin is shed, contains a thick, yellow, creamy liquid. In it there may be discovered, however, the ali'mentary tract, the nervous system, and the tracheal tubes, the latter filled with air; but all these parts are so soft and delicate that they can scarcely be studied by ordinary methods of dissecion. The creamy pulp of the pupa's body, when examined under the microscope, is seen to consist of a clear, pale, amber-vellowish liquid full of small bodies of various sizes (.1Oig. 6o), which give it the opaque appearance and thick consistencv. The liquid medium is the blood, or body lymph. Tle largest bodies in it are free fat cells (a); sma'ller ones are probably blood corpuscles (f); and the

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finest matter consists of great quantities of minute grains (d) floating about separately or adhering in irregular masses. Besides these etements there are many droplets of oil (e), recognizable by their smooth spherical outlines and golden-brown color. The fat cells are mostly irregu- larly ovoid or elliptical in shape; their protoplasm is filled with large and small oil globules, and contains also masses of fine granules like those floating free in the blood. These granules are the protoplasmic substances formed within the fat cells. Many of the cells have irregular or broken out- lines (b, c), as if their outer walls had been partly dis- solved, and the contents of such cells appear to be escap- ing from them. In fact, many are clearly in a state of dis- solution, discharging both thëir oil globules and their pro- teid inclusions into the blood; and it is clear that the similar matter scattered so profusely through the blood liquid has come from fat cells that have already disin- tegrated. All these materials will gradually be consumed in the building of the tissues of the adult, the organs of which are now in process of formation. In Chapter IV we learned that every animal consists of a body, or soma, formed of cells that are differentiated from the germ cells usually at an early stage of develop- ment. The function of the soma is to give the germ cells the best chance of accomplishing their purpose. An insect that goes through two active forms during its life, a larval and an adult form, differs from other animais in having a do«ble soma. The entire organism, of course, is hot double, for, as we have just seen in the study of the caterpillar, many of the more vital organs are continuous from the larva to the adult; but there is a group of organs which, after reaching a definite form of development in the larval stage, at the end of this stage virtually die and go into dissolution, while a new set of tissues develops into new organs or into new tissues replacing those that have been lost. The groups of somatic cells that-form the tissues and organs that undergo a metamorphosis, there-

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fore, are differentiated in the embryo into two sets of cells, one set of which will form the special organs of the larva, while those of the other will remain dormant during the larval life to form the adult organs when the larval cells have completed their functions. The cells of the second set carry the hereditary influences that will cause them to develop into the original, or ancestral, form of the species; the cells of the first set produce the temporary larval form, which may retain certain primitive characters from the embryonic stage, but which does not represent an ancestral form in the evolution of the species. An extreme case of anything is always more easily understood when we can trace it back to something simple, or link it up with something familiar. The metamorphosis of insects appears to be one of the great mvsteries of nature, but reduced to its simplest terres it becomes only an exaggerated case of a temporary growth in certain groups of cells to form something of use to the young, which disappears by resorption when the occasion for its use is past. Innumerable simple cases of this kind might be cited from insects; but there is a familiar case of well- developed metamorphosis even in our own growth, namely, the temporary development of the milk teeth and their later substitution by the adult teeth. If a similar process of double growth from the somatic cells had been carried to other organs, we ourselves should have a meta- morphosis entirely comparable with that of insects. TIaE Mo'rla For three weeks or a little longer the processes of re- construction go on within the pupa of the tent caterpillar, and then the creature that was a caterpillar breaks through its coverings and appears in the form and costume of a moth (Fig. I59 J). The pupal shell splits open on the forward part of the back (E) to allow the moth to emerge, but the latter then only finds itself face to face with the wall of the cocoon. It has left behind its cutting instru-

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ments, the mandibles, with its discarded overalls; but it bas turned chemist and needs no tools. The glands that furnished the silk for the larva have shrunken in size and have taken on a new function; they now secrete a clear liquid that oozes out of the mouth of the moth and acts as a solvent on the adhesive surfaces of the cocoon threads. The strands thus moistened are soon loosened from one another suflqciently to allow the moth to poke its head through the cocoon wall and force a hole large enough to permit of its escape. The liquid from the mouth of the moth turns the silk of the cocoon brown, and the lips of the emergence hole are alwavs stained the same color - evidence that it is this liquit that softens the silk--and the fraved edges of the hole left in the cocoon of the tent caterpillar show many loose ends of threads broken by the moth in its exit. The most conspictu)us features of the moth (Fig. 6) are its furry covering of hairlike scales and its wings. The wings are short when the insect first emerges from its cocoon (Fig. 159 J), but they quickly expand to normal length and are then folded over the back IFig. 16 A). The colors of the moths of the tent caterpillar are various shades of reddish-brown with two pale bands obliquely crossing the wings (Plate 14 G, H). The female moth (Plate 4 H, Fig. 16 B) is somewhat larger than the male, her body being a little over three-fourths of an inch in length, and the expanded wings one and three-fourths inches across. The tent caterpillars perform so thoroughly their duty of eating that the moths have little need of more food. Consequently the moths are hot encumbered with imple- ments of feeding. The mandibles, which were such large and important organs in the caterpillar (Fig. 52, Md) but which shrank to a rudimentary condition in the pro- pupa (Fig. I59 H, J/d), are gone entirely in the moth (Fig. I6). The maxillae, which were fairly long lobes in the propupa (Fig. 59 H, Mx), have likewise been

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THE CATERPILI.AR AND THE MOTH

reduced to mere rudiments in the adult, where they appear as two insignificant though movable knobs (Fig. 162, Mx). The median part of the labium has been reduced to almost nothing in the moth; but the labial palpi (LbPlp) are long and three-segmented, and when normally covered with hairlike scales they form two conspicuous feathery brushes that project in front of the face. The mouth parts of the tent caterpillar moth are hOt typical of these organs of moths and butterflies in gen-

Fro. 6. Moths ofthe tent caterpillar, Malacosoma americana. (A little greater than natural size)

eral, for most of these insects are provided with a long proboscis by means of which they are able to feed on liquids. Evervone is familiar with the large humming- bird moths, or'hawk moths, that are to be seen on sumnaer evenings as they dart from flower to flower, thrusting into each corolla a long tube uncoiled from beneath the head; and we have all seen the sunlight-loving butterflies carelessly flitting over the flower beds, alighting here and there on attractive blooms to sip the sweet liquid from the nectar cups. Moths and butterflies carry the proboscis tightly coiled, like a tiny watch spring (Fig. I63 A, Prb), be- neath the head and just behind the mouth. It can be unwound and extended (B, Prb) whenever the insect wants to extract a drop of nectar from the depths of a

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 -- flower corolla, or when it  would merely take a drink of water or other liquid. The proboscis consists of the greatly lengthened maxillae firmly attached to each other by dovetailed grooves and ? ", ridges. The inner face of tlf / each maxilla is hollowed in the form of a groove run-  kQ -Z//IP ning its entire length' and the two apposed grooves

FIG. 162. Facial view of the head of between the united maxillae the tent caterpillar moth, with cover- are converted into a central ing scales removed, and antennae cut channel of the proboscis. off near their bases tl,,t, base of antenna; E, compound The two blades of the pro- eye; Lb, labium; LbPIp, labial palpus; boscis spring from the sides Lm, labrum;Mth, mouth;Mx, maxilla of the mouth. The first part of the alimentary canal just back of the mouth is transformed into a bulblike sucking apparatus. The

Fro. 163. Head and mouth parts of the peach borer moth A, side view. B, three-quarter facial view. /lnt, basal part of antenna; E, compound eye; LbPlp, labial palpus; O, ocellus; Prb, proboscis

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THE CATERPILLAR AND THE MOTH

upper wall of the structure is ordinarily collapsed into the cavity of the bulb, but it is capable of being lifted by strong muscles inserted upon it from the walls of the head. The alternate opening and closing of the bulb sucks the liquid food up through the tube of the pro- boscis and forces it back into the gullet. The moths and butterflies are thus sucking insects, as are the aphids and cicadas, but they are hot provided with piercing organs, though some species have a rasp at the end of the pro- boscis which is said to enable them to obtain juices from soft-skinned fruits. ,Vith the tent caterpillar, it is interesting to note, the maxillae are much longer in the pupa (Fig. I59 I, Mx) than they are in either the caterpillar or the adult moth (Fig. I62, Mx), as if nature had intended the tent cater- pillar moth to have a proboscis like that of other moths, but had then changed her mind. The real meaning of this is that the moths of the present-day tent cater- pillars are descended from ancestors that had a functional proboscis in the adult stage like that of other moths, and that the reduction of the proboscis of the modern moths has taken place in rimes so recent that the organ has hot vet been suppressed to the same degree in the pupa. The alimentary tract of the tent caterpillar moth is very different from that of the caterpillar. In the cater- piller, the organ consists of three principal parts (Fig. J64 A), the first comprising the oesophagus (Oe) and the crop (.Cr), the second being the stomach, or ventriculus (lZent), and the third the intestine (Int). In an adult moth that is almost mature, but which is still inside the pupal shell (B), the oesophagus bas become a long narrow tube IOe) at the rear end of which the crop forms a small sac (Cr) projecting upward, which may contain a bubble of gas. The stomach has contracted to a pear-shaped bag with very thin transparent walls, and is usually filled with a t{ark-brown liquid. The intestine has changed radically in form, for it now consists of a long, slender,

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tubular part, the small intestine (SDt), and of a great terminal receptacle, the rectum (Rert), filled with a mass of soft orange-colored matter. In the fully-matured insect (C), after it has escaped from the cocoon, still fu,'ther

C  8Int

FiG. I64. Transformation in the form of the alimentary canal of the tent caterpillar from the larva to the adult moth A, alimentary canal of the caterpillar. B, the same of the pupa. C, the same of the moth Cr, crop; Int, intestine; Mal, Malpighian tubules (hOt shown full length); Oe, oesophagus; Rect, rectum; Slnt, small intestine; lent, centriculus

alterations have taken place. The crop sac (Cr) is now greatly distended into a spherical vesicle tensely filled wlth gas--air, probably, that the moth has swallowed, perhaps to aid it in breaking the pupal shell, for there are sometlmes small bubbles also in the tubular oesophagus.

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THE CATERPII.I.AR AND THE MOTH

The stomach is contracted to a mere remnant of its former size (A, l'ent), and its walls are thrown into thick corrugations. The intestine (SI,t) is about the same as in the earlier stage of the moth (B). Since the moth of the tent caterpillar probably eats nothing, it has little use for a stomach. The intestine, however, must serve as an outlet for the lalpighian tubules (Mal), since the latter remain functional through the pupal stage. The secretion of the tubules contains great numbers of minute spherical crystals, which accunqu- late in the rectal sac (Rect) where they form the orange- colored mass contained in this organ and discharged as soon as the moth leaves the cocoon. Most of the male moths of the tent caterpillar emerge from the cocoons several davs in advance of the females. At this time their bodies contain an abundance of fat which fills the cells of the fat tissue as droplets of oil. This fat is probably an energy-forming reserve which the male moth inherits from the caterpillar, for the internal reproductive organs are hOt vet fully developed and do hOt become functional until "about the time the females are out of their cocoons. The bodies of the female tent caterpillar moths, on the other hand, contain little or no fat tissue; but each female is fully matured when she emerges from the cocoon, and her ovaries are full of ripe eggs ready to be laid as mon as the fertilizing element is received from the male (Fig. I65, Or). The spermatozoa will be stored in a special recep- tacle, the spermatheca (Spm), which is connected with the exit duct of the ovaries (I'Ç¢) by a short tube. Each egg is then fertilized as it issues from the oviduct. The ma- terial that will form the covering of the eggs when laid is a clear, brown liquid contained in two great sacs (Fig. 65, Res) that open into the end of the median oviduct (lg). Each sac is the reservoir of a long tubular gland (C/G/). The liquid must be somehow mixed with air when it is discharged over the eggs to give the egg covering

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IN  ECTS

its ffothy texture. It soon sets into a jellylike substance, then becomes firm and elastic like soif rubber, and finally turns dry and brittle. The d'are of the egg laying depends on the latitude of the region the moths inhabit, varying flore the middle

Spm Bcpx Dov VF b An Fro. 16 5. The female reproductive organs of the moth of the tent caterpillar, as seen from the left side a, external opening of the bursa copulatrix; .4n, anus; b, opening of the vagina; Bcpx, bursa copulatrix; CIGI, colleterial glands, which form the substance of the egg covering; Dot', duct of the left ovary; upper ends of the ovarian tu bules; Rect, rectum; Res, reservoirs of the colleterial glands (CIGI); 8pro, spermatheca, a sac for the storage of the spermatozoa; ri. terminal strand of the ovary; I/g, vagina

of May in the southern States to the end of June or later in the north. While the eggs will hot hatch until the fol- lowing spring, they nevertheless begin to develop at once, and within six weeks young caterpillars may be round fldly formed within them (Fig. 166 B). Ech little caterpillar has its head against the top of the shell and its body bent U-shaped, with the tail end turned a little to one side. The long hairs of the body are all turned for- ward and form a rhin cushion about the poor creature, which for crimes yet uncommitted is sentenced to eight

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THE CATERPILLAR AND THE MOTH

months' solitary confinement in this most inhuman posi- tion. Yet, if artificially liberated, the prisoner takes no advantage of the freedom offered. TlSough it can move a little, it remains coiled (A) and will fold up again if forcibly straightened, thus asserting that it is more com- fortable than it looks. It is surprising that these infant caterpillars can remain inactive in their eggshells all through the summer, when the warmth spurs the vitality of other species and speeds them up to their most rapid growth and development.

External conditions in general appear to have much to do with regulating the lives of insects, and if the tent caterpillars in their eggs seem to give proof that the crea- tures are hOt entirely the slaves of environ- ment, the truth is probably that all in- sects are hOt gov- erned by the same

Fro. 166. The young tent caterpillar fully formed within the egg by the middle of summer A, the young caterpillar removed from the egg. B, the caterpillar in natural position within the egg

conditions. We have seen that some of the grasshoppers and some of the aphids will hot complete their develop- ment except after being subjected to freezing tempera- tures, and so it probably is with the tent caterpillars-- it is hot warmth, but a period of cold that furnishes the condition necessary to the final completion of their de-- velopment. Whatever mav.be the secret source of their patience, however, the young tent caterpillars will bide their time through all the heat of summer, the cold of winter, and hot till the buds of the cherry or apple leaves are ready to open the following spring will they awake and gnaw through the inclosing shells against which their faces have been pressing all this while.

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