Insects, Their Ways and Means of Living/Chapter VI
==CHAPTER VI: PLANT LICE ==
"PLANT lice! Ugh," you say, "who wants to read about those nasty things! All I want to know is how to get rid of them." Yes, but the very fact that those sort green bugs that cover your roses, your nasturtiums, your cab- bages, and your fruit trees at certain seasons reappear so persistently, after you think you have exterminated them, shows that they possess some hidden source of power; and the secrets of a resourceful enemy are at least worth knowing--besides, they may be interesting. Really, however, insects are hOt our enemies; they are only living their appointed lives, and it just happens that we want to eat some of the same plants that they and their ancestors have always fed on. Our trouble with the in- sects is just that same old economic conflict that has bred the majority of wars; and, in the case between us and the insects, it is we who are the aggressors and the enemies of the insects. We are the newcomers on the earth, but we fume around because we find it already occupied by a host of other creatures, and we ask what right have they to be here to interfere with us! Insects existed millions of years before we attained the human form and aspirations, and they have a perfectly legitimate right to everything they feed on. Of course, it must be admitted, they do not respect the rights of private property; and therein lies their hard luck, and ours. The plant lice are well known to anyone who has a garden, a greenhouse, an orchard, or a field of grain. Some call them "green bugs"; entomologists usually call
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them aphids. A single plant louse is an aphis, or an aphid; more than one are usually called aphides, or aphids. The distinguishing feature of the plant lice, or aphids, as we shall by preference call them, is their manner of feed- ing. All the insects described in the preceding chapters eat in the usual fashion of biting off pieces of their food, chewing them, and swallowing the masticated bits. The
Fit. 88. Group of green apple aphids feeding along a rib on under surface of an apple leaf
aphids are sucking insects; they feed on the juices of the plants they inhabit. Instead ofjaws, they have a piercing and sucking beak (Fig. '?9), consisting of an outer sheath inclosing four slender, sharp-pointed bristles which can be thrust deep into the tissues of a leaf or stem (Fig. 89 B). Between the bristles of the innermost pair (Fig. 9 o, Mx) are two canals. Through one canal, the lower one (b), a liquid secretion from glands o( the head is injected into the plant, perhaps breaking down its tissues; through the other (a) the plant sap and probably some of the proto- plasmic contents of the plant cells are drawn up into the mouth. A sucking apparatus like that of the aphids is possessed by all insects related to the aphids, comprising the order Hemiptera, and will be more fully described
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in the next chapter, which treats of the cicada, a large cousin of the aphids. When we observe, now, that different insects feed in two quite different ways, some by means of the biting type of mouth parts, and others by means of the sucking type, it becomes evident that we must know which kind of insect we are dealing with in the case of pests we may be trying to control. A biting and chewing insect can be killed by the mere expedient of putting poison on the out- side of its food, if it does not become aware of the poison and desist from eating it; but this method would not work with the piercing and sucking insects, which extract their food from beneath the surface of the plants on which they feed. Sucking insects are, therefore, to be destroyed by means
of sprays or dusts that will kill them by contact with their bodies. Aphids are usually attacked with irritant sprays, and in general it is not a difficult matter to rid infested plants of them, though in most cases the spraying must be repeated through the season. When any species of aphis becomes well established on a plant,
F??. 8 9 . The way an aphis feeds on the juices of a plant A, an aphis with its beak thrust into a rib of a leaf. B, section through the midrib of a young apple leaf, showing the mouth bristles from the beak of an aphis penetrating between the cells of the leaf tissue to the vascular bundles, while the sheath of the beak is retracted by folding back beneath the head
the infested leaves (Fig. 88) may be al- most as crowded as an East Side street on a hot summer after- noon. But there is
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PLATE 2
E
D
The green apple aphis (.dphis pomi) A, adult sexual female; B, adult male; C, young femme; D, female lav- ing an egg; E, eggs, which turn from green to black after they are laid. (Enla?ed about ?-o times) LICE
no bustle, no commotion, for each insect has its sucking bristles buried in the leaf, and its pump is busy keeping the stomach supplied with liquid food. The aphis crowds are mere herds, not communities or social groups as in the case of the termites, ants, or bees. Wherever there are aphids there are ants, and in con- trast to the aphids, the ants are always rushing about all over the place as if they were looking for something and each wanted to be the first to find it. Suddenly one spies a droplet of some clear liquid lying on the leaf and gob- bles it up, swallowing it so quickly that the spherule seems to vanish by magic, and then the ant is off again in the same excited manner. The explanation of the presence and the actions of the ants among the aphids is this: the sap of the plants furnishes an unbalanced diet, the sugar content being far too great in proportion to the protein. Consequently the aphids eject from their bodies drops of sweet liquid, and it is this liquid, called "honey dew," that the ants search out so eagerly. Some of the ants induce the aphids to give up the honey dew by strok- ing the bodies of the latter. The glistening coat often seen on the leaves of city shade trees and the shiny liquid that bespatters the sidewalks beneath is honey dew dis- charged from innumerable aphids infesting the under sur- faces of the leaves. In studying the termites, we learned that it is possible for a single pair of insects to produce regularly several kinds of offspring differing in other ways than those of sex. In the aphids, a somewhat similar thing occurs in that each species may be represented by a number of forms; but with the aphids these different forms con- stitute successive generations. If events took place in a human family as they do in an aphid family, children born of normal parents would grow up to be quite different from either their father or their mother; the children of these children would again be different from their parents and also from their grandparents, and when mature they
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perhaps would migrate to some other part of the country; here they would have children of their own, and the new fourth géneration would be unlike any of the three pre- ceding; this generation would then produce another, again different; and the latter would return to the home town of their grandparents Cm Nx and great-grandparents, and \ / here bring forth children that \ .a would grow up in the like- / ness of their great-great- great-grandparents! This seems like a fantastic tale of fiction, too preposterous to- be taken seriously, but it is a commonplace fact among the ?? aphids, and the actual gen- / ealogy may be even more Lb complicated than that above outlined. Moreover, the Fro. 9 o. Cross-section through story is hOt yet complete, the base of the beak of an aphis, for i't musr be added that all (From Davidson) Theouter sheath of the b«ak is the the generations of the aphids, labium (Lb), covered basally by except one in each series, are the labrum (Lin). The four in- closed bristles are the mandibles composed entirely of females (Md) and the maxillae (Mx), the capable in themselves of re- latter containing between rhem a food canal (a) and a salivary canal production. In warm " cil- (b). Only th« inner walls of th« mates, it appears, the female labrum and labium are shown in thesection succession may be uninter- rupted. How insects do upset out generalizations and our peace of mind ! We have heard of feminist reformers who would abolish men. With patient scorn we have listened to their predictions of a millenium where males will be unknown and unneeded--and here the insects show us not only that the thing is possible but that it is practicable, at least for a certain length of time, and that the time can be in- definitely extended under favorable conditions.
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Since special cases are always more convincing than general statements, let us follo?y the seasonal history of some particular aphids, taking as examples the species that commonly infest the apple. Let the rime be a day in the early part of Match. Probably a raw, gusty wind is blowing from the north- west, and only the silver maples with their dark purplish clusters of frowzy flowers already open give any sug- gestion of the approach of spring. Find an old" apple tree somewhere that has hOt been sprayed, the kind of tree an entomologist always likes to have around, since it is sure to be fidl of insects. Look closely at the ends
of some of the twigs and you will probably find a number of little shiny black things stuck close to the bark, especially about the bases of the buds, or tucked under the projecting edges of scars and tiny crevlces (Fig. 9?). Each little speck is oral and about one thirty- sixth of an inch in length. To the touch the ob- jects are firm, but elastic, and if you puncture one a pulpy liquid issues from lt; or soit appears, at least, to the naked eve-- a microscope would show that in this liquid there is
Fro. 9 ?. Aphis eggs on apple twigs in March; an enlarged egg below
organization. In short, the tiny capsule contains a young aphid, because it is an aphid egg. The egg was de- posited on the twig last fall by a femme aphis, and its living contents have remained alive since then, though fully exposed to the inclemencies of winter.
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Immediately after being laid in the fall, the germ nucleus of the aphis egg begins development, and soon forms a band of tissue lying lengthwise on the under sur- face of the yolk. Then this scarcely-formed embryo undergoes a curious process of revolution in the egg, turning on a crosswise axis head foremost into the yolk and finally stretching out within the latter with the back down and the head toward the original rear end of the egg. Thus it rernains through the winter. In March it again becomes active, reverses itself toits first posi- tion, and now compIetes its development. The date of hatching of the apple aphis eggs depends much upon the weather and will vary, therefore, ac- cording to the season, the elevation, and the latitude; but in latitudes from that of Washington north, it is some time in April, usually from the first to the third week of the month. The eggs of most insects resemble seeds in
Fz?. 9?-- Eggs of the green apple aphis with outer cover- ings split before hatching; below, an egg removed from its covering
their capacity for lying inert un- til proper conditions of warmth and moisture bring forth the creature biding its time within. The eggs of one of the apple aphids, however, are killed by premature warm weather, or if artificially warmed too long be- fore the normal time of hatching. In general, the final development of the aphis embryos keeps pace with the development of the apple buds, since both are con- trolled by the same weather con- ditions, and this coordination usually insures the young aphids against starvation; but the eggs commonly hatch a little in ad- rance of the opening of the buds, and a subsequent spell of cold
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PLANT LICE weather may give the young lice a long wa]t for their first meal. The approaching rime of hatching is s]gnaled in most
Fro. 93- Hatching of the green apple aphis, ,'lphis pomi A, the agg. B, an egg with the outer coat split. C, the sarneegg with the inner shell split atone end. D-F, three successive stages in the emergence of the young insect. G-J, shedding the hatching membrane. K, the ernpty eggshell. L, the young aphid
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cases by the splitting of an outer sheath of the egg (Fig. 92), exposing the glistening, black, true shell of the egg within. Then, from one to several days later, the shell itself shows a cleft within the rupture of the outer coat, extending along hall the length of the exposed egg sur- face and down around the forward end (Fig. 93 C). From this split emerges the sort head of the young aphis (D), bearing a hard, toothed crest, evidently the instru- ment by which the leathery shell was broken open, and for this reason known as the "egg burster." Once ex- posed, the head continues to swell out farther and far- ther as if the creature had been compressed within the egg. Soon the shoulders appear, and now the young aphis begins squirming, bending, inflating its fore parts and contracting its rear parts, until it works its body mostly out of the egg (E, F) and stands finally upright on the tip of its abdomen which is still held in the cleft of the shell (G). The young aphis at this stage, however, like the young roach, is still inclosed in a thin, tight-fitting, membranous bag having no pouches for the legs or other members, which are all cramped within it. The closely swathed head swells and contracts, especially the facial part, and sud- denly the top of the bag splits close to the right side of the egg burster (Fig. 93 H). The cleft pulls down over the head, enlarges to a circle, slides along over the shoul- ders, and then slips down the body. As the tightly stretched membrane rapidly contracts, the appendages are freed and swing out from the body (I). The shrunken pellicle is reduced at last to a small goblet supporting the aphid upright on its stalk, still held by the tip of the abdomen and the hind feet (I). To lil?erate itself entirely the insect must make a few more exertions (J), when, finally, it pulls its legs and body from the grip of the drying skin, and is at last a free young aphid (L). The emergence from the egg and from the hatching membrane is a critical period in the lire of an aphid. The
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PLATE 3
_A
The rosy apple aphis (.qnuraphis roseus) A, apple leaves and young fruit distorted by the aphids; B, under surface of an infested leaf; C, immature wingless aphid (greatly enlarged); 1), immature winged aphis I.ICE
process may be completed in a few minutes, or it may take as long as hall an hour, but if the feeble creature should be unable to free itself at last from the drying and contracting tissue, it remains a captive .strugg!ing in the grip of its embryonic vestment until it expires. The young aphid successfully delivered takes a few uncertain, staggering steps on its weak and colorless legs, and then complacently rests awhile; but after about twenty rein-
F?c. 94- Young aphids on apple buds in spring
utes or half an hour it is able to walk in proper insect fashion, and it proceeds upward on its twig, a course sure eventually to lead it to a bud. While the aphid eggs are hatching, or shortly there- after, the apple buds are opening and unfolding their delicate, pale-green leaves, and from everywhere now the young aphids come swarming upon them, till the tips are often blackened by their numbers (Fig. 94)- The hungry horde plunges into the hearts of the buds, and soon the new leaves are punctured with tiny beaks that rob them of their food; and the young foliage, upon which
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the tree depends for a stunted and yellowed. to spray if he has not The entomologist, young aphids on the there are three kinds differing slightly, but to a separate species.
proper start of its spring growth, is Now is the time for the orchardist already done so. however, takes note that all the apple trees are not alike; perhaps of them in the orchard (Fig. enough to show that each belongs When the first buds infested are
A t3
F1ç. 95- Three species of young aphids round on apples in the spring A, the apple-grain aphis, Rhopalosiphum prunifoliae. B, the green apple aphis, ?lphis pomi. C, the rosy apple aphis, ?lnuraphis roseus
exhausted, the insects migrate to others, and later they spread to the larger leaves, the blossoms, and the young fruit. The aphids all grow rapidly, and in the course of two or three weeks they reach maturity. The full-grown insects of this first generation, those produced from the winter eggs, are entirely wingless, and they are all females. But this state of affairs in no wise hinders the multiplication of the species, for these re- markable females are able of themselves to produce off- spring (a faculty known as parthenogenesis), and further- more, they do not lay eggs, but give birth to active young. Since they are destined to give rise to a long line of sure- mer generations, they are known as the stem mothers. One of the three aphid species of the apple buds is known as the green apple aphis (Fig. 95 B). During the
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early part of the season the individuals of this species are round particularly on the under surfaces of the apple leaves. They cause the infested leaves to curl and to become distorted in a ?haracteristic manner (Fig. 96). The stem mothers (Fig. 97 .A., B) begin giving birth to young (C) about twenty-four hours after reaching ma- turity, and any one of the mothers, during the course of her lire of from ten to thirty days, may produce an aver- age family of fifty or more daughters, for all her offspring are females, too. When these daughters grow up, however, none of them is exactly like their mother. They all have one more segment in each antenna; most of them are wing- less (D), but many of them have wings--some, mere padlike stumps, but others well developed or- gans capable of flight (Fig. 97 E). Both the wingless and the winged individuals of this second generation are also parthenogenetic, and they give birth to a third generation like them- selves, including wing- less, half-winged, and fully-winged forms, but with a greater propor- Fie. 9 6. Leaves of apple infested and tion of the last. From distorted by the green apple aphis on under surfaces now on there follows a large number of such generations continuing through the season. The winged forms fly from one tree to another, or to a distant orchard, and round new colonies. In
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summer, the green apple aphis is round principally on young shoots of the apple twigs, and on water sprouts growmg in the orchard. During the early part of the summer, the rate of pro- duction rapidly increases in the ap.hid colonies, and in- dividuals of the summer generat?ons sometimes give birth to young a week after they themselves were born. In the fall, however, the period of growth again is length- ened, and the families drop offin size; until the last females of the season produce each a scant hall dozen young, though they may live to a much greater age than do the sunamer individuals. The young summer aphids born as active insects are inclosed at birth in a tight-fitting, seamless, sleeveless, and legless tunic, as are those hatched from the winter eggs. Thus swathed, each emerges, rear end first, from the body of the mother, but is finally held fast by the face when it is nearly free. In this position, the ena- bryonic bag splits over the head and contracts over the body of the young aphid to the tip of the abdomen, where it remains as a cap of shriveled membrane until it finally drops off or is pushed away by the feet. The infant, now vigorously kicking, is still held in the ma- ternal grasp, and eventually liberates itself only after some rather violent struggling; bt?t soon after if is free it walks away to find a feeding place anaong its com- panions on the leaf. The mother is but little concerned with the birth of her child, and she usually continues to feed dt?ring its delivery, though she may be somewhat annoved by its kicking. The average summer female gives birth to two or three young aphids every day. The succession of forms in the familles is one of the most interesting phases of aphid lire. Investigations have shown that the winged individuals are produced principally by wingless forms, and experiments have demonstrated that the occurrence of the winged forms is correlated with changes in the temperature, the food
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supply, and the duration of light. At a temperature around sixty-five degrees few winged individuals ever appear, but they are produced at temperatures either below or above this point. Likewise it has been found that when the food supply gives out through the drying
FIG. 97" The green apple aphis, .4phis l?omi. A, B, adult stem mothers. C, a newly-born young of the summer forms. D, a wingless summer form. E, a winged summer form
of the leaves or by the crowding of the aphids on them, winged forms appear, thus making possible a migration to ffesh feeding grounds. Then, too, certain chemical substances, particularly salts of magnesium, added to the water or wet sand in which are growing cuttings of plants infested with aphids, will cause an increase of winged forms in the insects subsequently born. This does not happen if the plants are rooted, but it shows that a change in the food ca11 have an effect on wing production.
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Finally, it has recently been shown experimentally by Dr. A. Franklin Shull that winged and wingless condi- tions in the potato aphis may be produced artificially by a variation in the relative amount of alternating light and darkness the aphids receive during each twenty-four hours. Shortening the illumination period to twelve hours or less results in a marked increase in the number of winged forms born of wingless parents. Continuous darkness, however, produces few winged offspring. Maxi- mure results perhaps are obtained with eight hours of light. The effect of decreased light appears from Doctor Shull's experiments to be directly operative on the young from thirty-four to sixteen hours before birth, and it lS not to be attributed to any physiological effect on the plant on which the insects are feeding. It is evident, therefore, that various unfavorable local conditions may give fise to winged individuals in a colony of wingless aphids, thus enabling representatives of the colony to migrate in the chance of finding a more suit- able place for the continuance of their line. The regular production of Spring and Fall migrants is brought about possibly by the shorter periods of daylight in the earlier and later parts of the season. The final chapter of the aphid story opens in the fall and, like all last chapters done according to the rules, it contains the sequel to the plot and brings everything out right in the end. AIl through the spring and summer the aphid colonies have consisted exclusively of virgin females, winged and wingless, that give birth to virgin females in ever-in- creasmg numbers. A prosperous, self-supporting femi- nist dominion appears to be established. When summer's warmth, however, gives way to the chills of autumn, when the food supply begins to fail, the birth rate slack- ens and falls off steadily, until extermination seems to threaten. By the end of September conditions have reached a desperate state. October arrives, and the
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surviving virgins give birth in forlorn hope to a brood that must be destined for the end. But now, it appears, another of those miraculous events that occur so fre- quently in the lives of insects bas happened here, for the members of this new brood are seen at once to be quite different creatures from their parents. When they grow up, it develops that they constitute a sexual generation, composed of females and males, t (Plate 2 A, B.) Feminism is dethroned. The race is saved. The mar- riage instinct now is dominant, and if m?rital relations in this new generation are pretty loose, the time is Octo- her, and there is much to be accomplished before winter comes. The sexual females dit}er from their virgin mothers and grandmothers in being of darker green color and in having a broadly pear-shaped body, widest near the end (Plate 2 A). T]?e males (B) are much smaller than the females, their color is vellowish brown or brownish green, and they have long sp"?derlike legs on which they actively run about. Neither the males nor the females of the green apple aphis have wings. Soon the females begin to pro- duce, not active young, but eggs (D). The eggs are de- posited most anvwhere along the apple twigs, in crevices where the bark is rough, and about the bases of the buds. The newly-laid eggs are vellowish or greenish (D), but they soon turn to green, then to dark green, and finally become deep black (E). There are not many of them, for each female lays only from one to a dozen; but it is these eggs that are to remain on the trees through the winter to produce the stem mothers of next spring, who will start another cycle of aphid lire repeating the his- tory of that just closed. The production of sexual forms in the fall in temperate climates seems to have some immediate connection with the lowered temperature, for in the tropics, it is said, the aphid succession continues indefinitely through par- thenogenetic females, and in most tropical" species sexual
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A, a cluster of infested and distorted leaves. B, an adult stem mother. C, young apples dwarfed and distorted by the feeding of the aphids
males and females are unknown. In the warmer regions of the West Coast of the United States, species that regularly produce males and females every fall in the East continue without a reversion to the sexual forms.
Of the other two species of apple aphids that infest the buds in the spring, one is known as the rosy apple aphis (Fig. 95 C). The name comes from the fact that the early summer individuals of this species have a waxy pink tint more or less spread over the ground color of green (Plate 3), though many of the adult stem mothers (Fig. 98 B) are of a deep purplish color. The early generations of the rosy aphis infest the leaves (Fig. 98 A, Plate 3 A) and the young fruit (Fig. 98 C, Plate 3 A), causing the former to curl up in tightly rolled spirals, and the latter to become dwarfed and distorted in form. The stem mothers of the rosy aphis give birtb parthenogenetically to a second generation of females which are mostly wingless like their mothers; but in the next generation many individuals have wings. Several more generations now rapidly follow, all females; in fact, as with the green aphis, no males are produced till late in the season. The winged forms, however, appear in increasing numbers, and by the first of July almost all the individuals born have wings.
PLANT LICE
Heretofore, the species has remained on the apple trees, but now the winged ones are possessed with a desire for a change, a cornplete change both of scenery and of diet. They leave the apples, and when next discovered they are found to have established thernselves in sumrner colonies on those cornmon weeds known as plantains, and mostly on the narrow-leaved variety, the rib-grass, or English plantain (Fig. 99)- As soon as the rni- grants land upon the plantains they give birth to offspring quite unlike themselves or any of the preceding generations. These individuals are of a yellowish-green color and nearly all of thern are wingless (Fig. 99)- So well do they disguise their species that ento- rnologists were a long tirne in discovering their identity. Generations of wingless vellow fernales now foll?»w upon the plantain. But a weed is no fit place for the stor- age of winter eggs, so, with the advent of fall, winged forms again ap- pear In abundance, and F,ç. 99- The rosy apple aphis on nar- row-leaved plantain in summer; above, a these migrate back to the wingless summer form (enlarged) apples. The rail mi- grants, however, are of two varieties: one is sirnply a winged fernale like the earlier rnigrants that carne to the plantain from the apple, but the other is a winged rnale (Fig. 1oo A). Both forrns go back to the apple trees, and
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there the females give birth to a generation of wingless sexual females (B), which, when mature, mate with the males and produce the winter eggs. The third of the aphid species that infest the spring buds of the apple is known as the apple-grain aphis, so called because, being a migratory species like the rosy
A
F*?. *oo. The winged male(A) and the wingless sexual female (B) of the rosy apple aphis
aphis, it spends the summer upon the leaves of grains and grasses. The eggs of the apple-grain aphis are usually the first to hatch in the spring, and the young aphids of this species (Fig. 95 A) are distinguished by their very dark green color, which gives them a blackish appear- ance when massed upon the buds. Later they spread to the older leaves and to the petals of the apple blossoms, but on the whole their damage to the apple trees is less than that of either of the other two species. The summer history of the apple-grain aphids is similar to that of the rosy aphis, excepting that they make their summer home on grains and grasses instead of on plantains. In the fall, the winged female migrants (Plate 4) come back to the apple and there give birth to wingless sexual females, which are later sought out by the winged males. It would be impossible here even to enumerate the
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Flç. Ioi. Some common aphids of the garden A, winged form of the potato aphis, Illinoia aolanifofii, one of the largest of the garden aphids. B, winged form of the peach aphis, Myzus persicae, which infests peach trees and various garden plants. C, wingless form of the peach aphis. D, wingless form of the melon aphis, ?Iphis gossypii. E, winged form of the melon aphis
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many species of aphids that infest our common field and garden plants (Fig. ?Ol) and cultivated shrubs and trees, to say nothing of those that inhabit the weeds, the wild shrubbery, and the forest trees. Almost every natural group of plants has its particular kind of aphid, and many of them are migratory species like the rosy and grain aphis of the apple. There are root-inhabiting species as well as those that live on the leaves and stems. The Ph.vlloxera, that pest of vineyards in California and France, ?s a root aphid. Those cottony masses that often appear on the apple twigs in late summer mark the presence of the woolly aphis, the individuals of which exude a fleecy covering of white waxy threads from their backs. The woolly aphis is more common on the roots of apple trees, being especially a pest of nursery stock, but it migrates to both the twigs and the roots of the apple from the elm, which is the home of its winter eggs. An underground aphid of particular interest is one that lives on the roots of corn. We have seen that all aphids are much sought after by ants because ol c the honey dew they excrete, a substance greatly relished and prized by the ants. It is said that some ants protect groups of aphids on twigs by building earthen sheds over them; but the corn-root aphis owes its very existence to the ants. A species of ant that makes its nests in cornfields runs tunnels from the underground chambers of the nests to the bases ofnearby cornstalks. In the fMI the ants gather the winter eggs of the aphids from the corn roots and take them into their nests where they are protected from freezing during the winter. Then in the spring the ants bring the eggs up from the storage cellars and place them on the roots of various early weeds. Here the stem mothers hatch and give rise to several spring gen- erations; but, as the new corn begins to sprout, the ants transfer many of the aphids to the corn roots, where they lire and multiply during the sumnler and, in the fall, give birth to the sexual males and females, which produce
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the winter eggs. The eggs are again collected by the ants and carried to safety for the winter into the depths of their underground abodes. All this the ants do for the aphids in exchange for the honey dew they receive from them. The ants have so domesticated these corn-root aphids that the aphids would perish without their care. The fariner, therefore, who would rid his cornfield of the aphid pest, proceeds with extermination measures against the ants. The crowded aphid colonies exposed on stems and leaves naturally form the happy hunting grounds for a
Fro. io?. A common ladybird beetle, Coccinella novemnotata, that feeds on aphids. (Enlarged 5 rimes) A, the larva. B, the adult beetle
host of predacious insects. Here are thousands of soft- bodied creatures, all herded together, and each tethered to one spot by the bristles ofits beak thrust deep into the tissues of t]?e plant--a pot-hunter's paradise, trulv. Consequently, the placid lires of the aphids have many interruptions, and vast numbers of the succulent creatures serve only as half-way stages in the food cycle of some other insect. The al?hids have small powers of active
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defense. A pair of slender tubes, the cornicles, projecting from the rear end of the body, eject a sticky liquid which the aphids are said to smear on the faces of attacking insects; but the ruse at best probably does not give much protection. Parthenogenesis and large families are the principal policies by which the aphids insure their race against extinction. The presence of "evil" in the world has always been a thorn for those who would preserve their faith in the idea of beneficence in nature. The irritation, however, is not
F?ç. ?o 3. The aphis-lion, feeding on an aphis held in its jaws in the flesh but in a distorted growth of the mind, and consequently may be alleviated by a change of mental attitude. The thorn itself, however, is real and can not be explained away. Beneficence is not a part of the scheme by which plants and animais have attained through evolution their present conditions and relations. On the other hand, there are not good species and bad species; for every creature, including ourselves, is a thorn to some other, since each attacks a weaker that may contribute to its existence. There are many insects that destroy the aphids, but these are "enemies" of the aphids only in the sense that we are enemies of chickens and of cabbages, or of any other thing we kill for food or other purposes.
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Recognizing, then, that evil, like everything else, is a matter of relativity and depends upon whose standpoint it is from which we take our view, it becomes only a par- donable bias in a writer if he views the subject from the standpoint of the heroes of his story. With this under- standing we may note a few of the "enemies" of the aphids. Everybody knows the "ladybirds," those little oral, hard-shelled beetles, usually of a dark red color with black spots on their rounded backs (Fig. Io? B). The female ladybirds, or better, lady-beetles, lay their orange- colored eggs in small groups stuck usually to the under surfaces of leaves (Fig. 132 B) and in the neighborhood of aphids. When the eggs hatch, they give forth, not ornate insects resembling lady-beetles, but blackish little beasts with thick bodies and six short legs. The young creatures at once seek out the aphids, for aphids are their natural food, and begin ruthlessly feeding upon them. As the young lady-beetles mature, they grow even uglier in form, some of them becoming conspicu- ously spiny, but their bodies are variegated with areas of brilliant color- red, blue, and yellow--the pattern differ- ing according to the species. A common one is shown at A of Figure lO2. When one of these miniature monsters becomes full-grown, it ceases its depredations on the aphid flocks, enters a period of quietude, and fixes the rear end of its body to a leaf by exuding a glue from the extremity of its abdomen. Then it sheds its skin, which shrinks down over the body and forms a spiny mat ad- hering to the leaf and supporting the former occupant by only the tip of the body (Fig. 132 E). With the shedding of the skin, the insect has changed from a larva to a pupa, and after a short rime it will transform into a perfect lady-beetle like its father or mother. Another little villian, a remarkably good imitation of a small dragon (Fig. lO3) , with long, curved, sicklelike jaws extending forward from the head, and a vicious rem-
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F..._?_?..¢.., « perament to match, ??' -"-?--? '>'??..//x is. also a common. ??/? ?? y trequenter ot the ??///ff/ ? / aphid colonies and V?ff?ff??l levies a toll on the /////ï{?? lives of the meek o///? k B ?.d h¢lplCs? i.s¢c?s. ?& ç This marauder is 0 ? ? well named the apkis-lion. He is the larva of a gentle, harmless creature with large pale-green lacy wings and brilliant golden eyes (F@ m4A). The parent femmes show a remarkable prescience of the nature of their ofl?pring, l?r they sup- port their eggs on the tips of long
threadlike FIa. Io4. Thegolden- eye, Chrysopa, the par- stalks, tlSU- ent of the aphis-lion, allyattached and its eggs tO the under A, the adult insect. B, a group of eggs sup- surfaces of ported on long thread- leaves (B). like stalks on the under surface of a leaf The device seems to be a scheme for preventing the first of the greedy brood that will hatch from devouring its own brothers and sisters still in their eggs. Wherever the aphids are crowded there is almost sure to be seen crawling among them? soft grayish or green wormlike creatures, mostly less than a quarter of an inch in length. The body is legless and tapers to the forward end,
FIG. ?o 5. A larva ofa syrphus fly feeding on aphids
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which has no distinct head but from which is protruded and retracted a pair of strong, dark hooks. Watch one of these things as it creeps upon an unsuspecting aphid; with a quick movement of the outstretched forward end of the body it makes a swing at the fated insect, grabs it with the extended hooks, swings it aloft kicking and struggling, and relentlessly sucks the juices from its body (Fig. ?o5). Then with a toss it flings the shrunken skin aside, and repeats the attack on another aphid. This heartless blood-sucker is a maggot, the larva of a fly (Fig. ?o6) belonging to a family called the Syrphidae. The adult files of this family are entirely harmless, though
FIG. 106. Two common species of syrphus flies whose larvae feed on aphids. (Enlarged about 3¼ rimes) A, .411ograpta obliqua. B, OEyrphus americana
some of them look like bees, but the females of those species whose maggots feed on aphids know the habits of their offspring and place their eggs on the leaves where aphids are feeding. One of them may be seen hovering near a well-infested leaf. Suddenly she darts toward the leaf and then as quickly is off again; but in the moment of passing, an egg has been stuck to the surface right in the midst of the feeding insects. Here it hatches where the young maggot will find its prey close at hand. In addition to these predacious creatures that openly and honestly attack their victims and eat them alive, the aphids have other enemies with more insidious methods of procedure. If you look over the aphid-infested leaves
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Fm. fo 7. A dead potato aphis that has contained a parasite, which when adult escaped through the door cut in the back of the aphis
home. The guest that so ravishes its protector is the grub of a small wasp- like insect (Fig. mS) with a long, sharp ovipositor by means of which it thrusts an egg into the body of a living aphid
Fro. mS. .4phMius, a coin- mon small wasplike parasite of aphids
on almost any plant, you will most likely note here and there a much swollen aphid of a brown- ish color. Closer examination reveals that such individuals are dead, and man)' of them have a large round hole in the back, perhaps with a lid standing up from one edge like a trap door (Fig. ?o7). These aphids have not died natural deaths; each has been made the involuntary host of another insect that con- verted its body into a temporary
Fro. ??9. A female .4phidius inserting an egg into the body of a living aphis, where the egg hatches; the larva grows to ma- turity by feeding in the tissues of the aphis. (From Webster) IFig. 1o9). Here the egg hatches and the young grub feeds on the juices of the aphid until it is itself full-grown, by which rime the aphid is exhat?sted and dead. Then the grub slits open the lower wall of the hol]ow corpse and spins a web between the lips of the opening and against the surface of the leaf below, which attaches the aphid shell to the support. Thus se- cured, the grub proceeds to give [178]
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its gruesome chamber a lining of silk web; which done, it lies down to test and soon changes to a pupa. After a short rime it again transforms, this rime into the adult of its species, and the latter cuts with its jaws the hole in the back of the aphid and emerges. In other cases, the dead aphid does not test fiat on the leaf but is elevated on a small mound (Fig. l IC)A). Such victims have been inhabited by the grub of a re- lated species, which, when full-grown, spins a fiat cocoon beneath the dead body of its host, and in this inclosure undergoes its transformation. The adult insect then cuts a door in the side of the cocoon (B), through which it makes its exit. Insects that usurp the bodies of other insects for their own purposes are called parasites. Parasites are the
FIc. I ?o. Aphids parasitized by a parasite that makes a cocoon beneath the body of the aphis, where it changes to a pupa and, when adult, emerges through a door cut in the side of the cocoon
worst enemies that insects have to contend against; but really they do not contend against them in most cases, except in the way characteristic of insects, which is to insure themselves against extermination by the number of their offspring. The aphid colonies are often, how- ever, greatly depleted during a season favorable to the predacious and parasitic insects that attack them; but no species is ever annihilated by its enemies, for this would mean starvation to the next year's brood of the latter. The laws of compensation usually maintain a balance
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in nature between the procreative and the destructive forces. The insect parasites and predators of other insects in general comprise a class of insects that are most beneficial to us by reason of their large-scale destruction of species injurious to our crops. But, unfortunately, parasites as a class do not respect our classification of other creatures into harmful and useful species. I?]ven as some predator is stalking its prey, another insect is likely to be shadowing it, awaiting the chance to inject into its body the egg which will mean finally death to the destroyer. lmmature insects are often fOtllid in a sluggish or half-alive condi- tion, and an internal examination of their bodies usually reveals that they are occupied by one or more parasitic larvae. A larva of any of the lady-beetles, for example, is frequently seen attacl?ed to a leaf for pupation (Fig. 11 ?), which, instead of transforming to a pupa, remains inert and soon becomes a lifeless form, though still adhering to the ]eaf and bent in the attitude that the pupa would assume. In a short time there issues through the dried skin a parasite, giving evidence of the fate that has be- (allen the unfortunate larva; even if the usurper is not seen, the exit hole in the larval skin bears witness to his former occupancy and escape. And the parasites themselves, do they lead unmolested lives? Are they the final arbiters of lire and death in the insect world? if you are fortunate sometime while study- ing aphids out-of'-doors, you may see a tiny black mite, no bigger than the small;est gnat, hovering about an in- fested plant or darting uncertainly from one leaf to another, with the air of searching for something but not knowing just where to look. You would probably suspect the intruder of being a parasite seeking a chance to place an egg in the body of an aphid; but here she hovers over a group of fat lice without selecting a victim, then per- haps alights and rtlns about on the leaf nervously and intensely eager, still finding nothing to her choice. Her
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senses must be dull, indeed, if it is aphids that she wants. Do not lose sight of her, however, for her attitude bas changed; now she certainly bas her eye upon something that holds her attention, but the object is nothing other than one of those swollen parasitized aphids. Yet she excitedly runs up toit, feels it, grasps it, mounts upon it, examines it all over. Evidently she is satisfied. She dismounts, turns about, backs her abdomen against the
inflated mummy; now out comes the swordlike .',?,t/. ovipositor, and with a thrust it is sunken into the already parasitized aphid. Two mmu tes
later her business is ended, the ovipositor is withdrawn, once morè sheathed, and the insect is off and awav. This tiny creature is a hyperparasite, which is to say, a parasite of a para- site. In the act just wit- Fro. III. A parasitized larva of a lady- bird beetle, and one of the parasites The larva of the beetle has attached itself to a leaf preparatory to pupation, but has not changed to a pupa because of the parasites within it. Above, one of the parasites, which escaped from the beetle larva through a hole if cut in the skin of the latter
nessed she, too, has thrust an egg into the aphid, but the grub that will hatch from it will devour the parasitic occupant that is already in pos- session of the aphid's
skin. There are also parasites of hyperparasites, but the series does hOt go on "ad infinitum" as in the old rhyme, for the limitation of size must intervene.