Popular Science Monthly/Volume 76/March 1910/Insects and Entomologists: Their Relation to the Community at Large I
|INSECTS AND ENTOMOLOGISTS: THEIR RELATIONS TO THE COMMUNITY AT LARGE|
By Professor JOHN B. SMITH, Sc.D.
WHEN your president first wrote me, suggesting that I should deliver the popular lecture required by the constitution of this society, he also suggested a subject: "What entomology has done for the world, and its future." The subject is an attractive one; but it required little consideration to decide that within the time at my disposal for preparation and presentation it was impossible for me to do justice to it. The mere compilation of what has been accomplished would require all the time, and I am distinctly doubtful concerning my ability as a prophet. There are no records of any successful ones in my family history and I have never observed any suggestive symptoms in my own case. I therefore secured a compromise on a much less ambitious topic, and find that quite large enough, for, until systematically set down, the importance of insects in their relation to man, direct and indirect, is scarcely appreciated. It is only within the last decade that our conceptions in this matter have become at all clear, and among the public at large extreme haziness is still the dominant condition.
And it was not even easy to determine just what constitutes an entomologist under our present-day methods of specialization, for 'while not so long ago any person interested in the study of insects at all might be called an entomologist, there are now many students of insects who know nothing at all about them as a whole, but a very great deal about some small, almost or quite invisible part of a single species or group.
So we must at the outset classify our students and determine what is really meant by an entomologist:
First of all, we have those individuals who devote their energies to the study of adult specimens only; describing species and genera, revising and monographing groups and, in short, devoting themselves altogether to systematic work. This is essential work, for until species are made known and tagged there is nothing to speak or write about and, no matter how interesting their structure or habits, the information is absolutely useless or unavailable to others, until it can be applied by some definite term to some definite concept.
The systematist then, no matter how little he may know of the insects outside of the dry specimens with which he works, is entitled to be called an entomologist and to have his good deeds recorded here.
Then we have, secondly, those students to whom the systematic position of an insect is a matter of little account; but who are interested in its life history, in its development, in its relation to its surroundings and more or less, perhaps, in its economic importance to man or to some set of men. Without question, these students also are eminently entitled to be considered as entomologists and there is no body of men whose work is of greater importance to the community than those falling under this heading.
In a third category come those who see in the specimen before them a combination of structures of greater or less interest or importance; who care little or nothing for its life history or economic importance, and nothing at all for its systematic position. They need the name of the species only to designate the particular organism that was studied. The work of these students is of the highest possible importance; but they are not entomologists, though their studies may be confined to insect structures. They are anatomists or histologists, depending upon whether they study it grossly, with dissecting needles and low power lenses, or whether they first slice it into sections and then use the high power microscope to look through them. It goes without saying that any member of the first and second division may be a member of the third as well, and I would not be understood as in any way belittling the importance of the work done by these men.
A fourth class is interested in certain species of insects only because of their relation to some other animal or to man, and only in so far as that relation exists. Such are they who study mosquitoes only as intermediate hosts of diseases of man, or bot flies only as parasites of animals. The work done by these students is of intense scientific and practical interest and of the utmost importance to the community, but they are not entomologists, although some of those carrying on this kind of work are entitled to rank as such because of other work done.
And now, what about him who falls under none of my classifications; the man who uses all his leisure in scouring woods and fields, swamps and running waters, for its wealth of insect life; who works in good weather and bad, for whom the woods at night hold no terrors when in pursuit of specimens? What about the collector? Of a surety he is entitled to rank among the elect, for without him the systematist would have little to work with and the student of insect ecology but a poor basis for his branch of the science. It is the collector who in the past has formed the body of all our entomological societies, and now forms the working majority of most of them. It is the collector upon whom the science rests as a foundation and he is entitled to rank by himself, although aside from this he may and often does belong to one of the other divisions as well.
Now, dropping the entomologist for the moment, let us consider the insects themselves, and here we find their influence extending in every direction; sometimes to our benefit, more often to our injury. Those that affect us injuriously we are able to subdivide into those that attack us directly either as parasites or merely as a source of food supply, and those that prey upon our crops, supplies or farm stock. And even the list of directly injurious forms is not a small one for, to begin with, there are no less than three species of sucking lice that attack the human animal and are confined to him, favoring his head, and other hairy regions, and his clothing when he wears any. It is an interesting matter for reflection that the egg laying habit of the body-louse is an adaptation that must have required ages to develop and that could not even begin to develop until man wore clothing of some kind.
And wherever man goes wholly or partly unclothed, he shares with other animals the danger of becoming infested with creatures like bots, screw worms and other dipterous maggots, or penetrating insects like jigger fleas and their allies. Man, then, stands in the relation of host to a not inconsiderable number of insects species, only a few of which, however, are really dependent upon him.
But as prey, his usefulness to insects is infinitely greater. In his home a variety of bloodsuckers have established themselves; even in his bed they may be found, and they range from the reasonably sized creatures found in the temperate regions to the infinitely more formidable creatures found in the tropical countries, where the bites often produce unpleasant and even dangerous results.
Where man has hairy pets, like cats and dogs, the fleas that infest them primarily often attack him as a compliment, and in some sections of the world and of own country fleas are not insignificant either in numbers or effects. Outdoors, Tabanids or horse and other flies in great variety make life miserable in the woods or on the sea-shore, while even in New Jersey an occasional mosquito may yet be met with, ready to demonstrate that he likes you none the less because of any campaigns waged against him—or her.
Gnats and midges of various kinds, whether we call them black-flies, punkies or other names, all manifest an affectionate interest in the human visitor to their homes, and I do not mind saying that there have been occasions when I abandoned the field to them and admitted defeat. It is wonderful how well fitted these insects are for their work and how well they understand the use of the tools with which kind nature has provided them!
Some kinds of insects have no grudge against man and as such never bother unless interfered with, but they are quite ready to manifest their displeasure if they are wasps or hornets, or to make it unpleasant in other ways to the ignorant meddler, as in the case of many of our Limacodid larvæ or nettling caterpillars. Sometimes an insect becomes a nuisance quite without intent, as in the case of the caterpillar of the brown-tail, which distributes its hair so liberally that it produces severe irritations and inflammations, as those residents of Massachusetts that have suffered from "brown-tail rash," know to their sorrow.
And this brings me, naturally, to the consideration of those forms that are troublesome or even dangerous to man because they are agencies in the transmission of diseases, either as carriers or as intermediate hosts. Note that I use two terms; carriers and intermediate hosts, because there is a vast difference between them. Carriers are such insects as merely pick up by accident disease "germs"—to use a current expression—and transport them to another place where they may or may not find a suitable medium to propagate, and where they may or may not be in position to get into the proper portion of the human animal. Almost any sort of insect may be a carrier, although there are some few peculiarly adapted for the purpose, and such a carrier may be the transmitting agent for a variety of diseases: it is not itself affected by any, and is in no sense a fellow sufferer.
It is different in the case of intermediate hosts; here the insect itself harbors one stage of the morbific organism and is itself a sufferer from one form of the disease. Its power of transmission is strictly limited and is restricted to one disease alone.
The best known and most abundant of the germ carriers is the common house-fly more recently called "typhoid fly." Now it undoubtedly
is a typhoid fly, but it is only one of several species that may be equally effective, and it is by no means a carrier of typhoid germs only. Its habits are such that it may be a transmitting agent for any intestinal disease and for many of the pulmonary and bronchial troubles as well. In a typhus or cholera epidemic it is a "cholera" or "typhus fly" and to call it the "typhoid fly" gives an unfounded suggestion of definite relationship between disease and insect.
But it certainly is marvelously well adapted as a carrying agent. Its omnivorous feeding habits, its persistence in seeking entrance at places where savory or other pungent odors attract it, and its foot and mouth structure make a combination difficult to equal. The pulvilli of the feet with their numerous minute hooked hairs are ideal collectors of microorganisms, and the lobed, lip-like mouth structure, with its array of pseudo-trachea for surface scraping, can hardly be surpassed in effectiveness. No doubt the house-fly is a danger of the first order, and there is no economic problem now before the sanitarian, of more importance than the elimination of this pest. That it can be done there is no doubt, and that in time it will be clone is equally certain.
There are numerous other flies and insects that serve as carriers, and there are numerous diseases due to more or less uncleanly habits, that are carried by insects; but the process is so simple and direct that it offers little of scientific interest.
Much more complicated is that transmission of disease in which the insects act as intermediate hosts, and here again the members of the order Diptera lead the way. We assume that man is highest in the scale of vertebrate development, and it is no part of my thesis to dispute this. It is equally assumed that the Diptera are most highly specialized in their development among the insects, and it is significant that the association of humanity with these flies has been so close and
so long continued that it has been possible for a common parasite to develop whose continuance depends absolutely upon the intimate association of the two.
If all insects were eliminated absolutely, typhoid, cholera and associated diseases might still continue to plague mankind; but eliminate Stegomyia calopus and yellow fever would be equally eliminated, because without that particular species the parasite causing the disease would find it impossible to continue its existence. Not that we know very much about the "germ" of yellow fever, and its life-cycle is largely guess-work; but we do know enough, from direct observation and experiment, to warrant the statements that I have just made.
It is different in the case of malerial fevers and their association with certain species of Anopheles. Here we do know from direct observation the entire life-cycle of the parastic organism from the time it enters the circulation through the beak of the mosquito until it reaches its limit of growth in the human body. We have observed the blasts and sporozoits entering a red blood corpuscle; we have observed the gradual growth in this corpuscle; we have followed the gradual breaking down of the cells and have observed their rupture and the discharging spores. The development of the gametes has been observed; the flagellation and conjugation of the micro-and macro-gamete and the development in the Anopheline stomach from vermicule to zygote. All this has been demonstrated, and so little guess-work is there about it that the elimination of Anopheles breeding places is now the first step in dealing with an outbreak of real malarial trouble.
The relationship between rats, fleas and plague has also been practically established, and elimination of the disease in man is sought by the destruction of rats. I have already mentioned, in another connection, that the cat and dog flea would, when opportunity served,
bite human beings; the statement may be made more generally that almost any flea will, when there is occasion, bite almost any warm blooded animal upon which it finds its way. So the fleas upon plague stricken rats, leaving their natural hosts, may and do infest men instead of other rats when men herd where rats abound.
In more torrid or tropical countries insect-borne diseases are more numerous than in our more temperate clime, and there the mosquitoes, bearing a much greater variety of fevers, also transmit organisms of much higher character than "germs." Filariasis is a disease caused by minute thread-worms or filaria, and these also require the mosquito as an intermediate host to complete their development.
In recent years the records of studies made of certain tropical diseases have been largely records of studies in insect transmission of disease; and the studies made on insects, by physicians, have been as important as those made on the afflicted patient.
The identification of a species of Glossina or tsetse fly as an agent in the spread and transmission of the "sleeping sickness" is one of the more recent accomplishments in this direction, and opens a way for dealing with this plague that has practically depopulated great areas in Africa.
This seems like a series of heavy indictments against a lot of insignificant creatures whom, heretofore, we have deemed ourselves justified in ignoring. But the case has been understated rather than otherwise, and it is time that we recognized insects as among the most dangerous enemies to man from the sanitary standpoint.
Let me put it in a somewhat different way: could we at a blow eliminate all the members of the single order Diptera—including the fleas—we should at the same time absolutely eliminate malaria, yellow fever, dengue, jungle and several other kinds of tropical fevers, the bubonic plague, sleeping sickness, filariasis, several forms of eye diseases certain ulcerating sores in tropical countries, and we should reduce to a minimum enteric fevers of all kinds, lessen the death rate from tuberculosis and pulmonary troubles, and probably modify or lessen leprosy and kindred diseases.
But this is not all our plaint; for besides attacking man directly in his bodily health and comfort, they attack his domestic and other animals and lessen their value if they do not absolutely destroy them.
Every one of our domestic animals and all our feathered friends of the barnyard are infested by lice—biting and sucking and some of them harbor several species. All of them are well adapted in form to the conditions under which they live; and even the hog, which is not usually thought of as a hairy animal, has a species that manages to move about as freely as need be among the bristles. Naturally, animals so infested can not do their best for their masters; they become mangy in appearance, do not grow well, the fowls lessen in egg production and the cows in milk.
To the dairyman, flies—comprehensively speaking—are nuisances from all points of view. In the pasture Tabanids in great variety get after them; in the stable Stomoxys is always on hand. Occasionally the fauna of one country contributes a pest to that of another, as was the case when the so-called horn-fly was introduced about twenty years ago from Mediterranean Europe and rapidly spread throughout the United States and Canada.
Buffalo gnats and similar species make certain sections of the Mississippi Valley region almost uninhabitable for cattle at times, and dairying in some places is barred because of the abundance of mosquitoes. These are all external attacks which reduce the condition of the creature attacked because of the irritation, pain or actual loss of blood.
There are others that are more truly parasitic, like the bots. Where ever sheep are raised in numbers the herders have their troubles with the species that gets into the nasal passages, causing blind staggers and often death. Cattle are infested by species that lodge under the skin,
forming suppurating sores along the back. These reduce the condition of the animal and materially lessen the value of the skin. Horsemen know to their sorrow the species that fixes on the walls of the stomach, often in such numbers as to kill the animal. To realize the amount of suffering caused to their hosts by these insects and others with similar habits is almost impossible. To estimate the money loss caused to man has been attempted and the figures run to astounding sums.
But the end is not even yet, for these same animals often suffer from diseases similar to those of man and like them transmissible through or carried by insects. Ticks are not insects, but the economic entomologist is often expected to deal with them, hence we may just mention the cattle tick in connection with Texas fever, which has destroyed and still destroys its thousands and causes enormous money losses.
Many of our birds suffer from maladies similar to malaria, caused by a Proteosoma, and even the common and ever vociferous sparrow is not exempt. These species of Proteosoma find their intermediate hosts in certain species of Culex or whatever the present equivalent of that generic term may be. Some of these diseases have been carefully worked out, but how many more remain of which we know nothing as yet, I would not even dare to guess.
The effect of the tsetse fly or its bite on horses in South Africa has been long known, but just how that effect is produced is a matter of more recent knowledge. We know now that the disease is produced by Trypanosomes carried by the flies, and this information has opened the way to intelligent treatment.
Here again, be it noted, the order Diptera contributes the bulk of the dangerous species to the mammalian types, and to our horses,
cattle and sheep, the elimination of all flies would be as great a boon as to man himself.
Heretofore I have spoken only of insects that either attack or influence the health of man or other animals. But there are numerous others that live with him and are messmates, sharing in the produce that he has stored for his own use, and in this work members of almost every insect order are concerned.
The Thysanura are represented by various species of Lepisma or silver-fishes, and sometimes by spring-tails and bristle-tails where produce is stored in damp places.
The Neuroptera have quite a variety of forms, ranging from the Psocidæ which the housewife usually regards with a disgust intended for parasites of similar form, to the Termites or white ants whose work is noted more frequently than the insects themselves.
Among the Orthoptera, the voice of the cricket on the hearth calls up only pleasant association; but the appearance of roaches in kitchen and pantries arouses feelings of quite a different character. And roaches of many varieties are found in human habitations throughout the world.
As to the Hemiptera or true bugs, they have been already referred to as direct feeders or parasites upon man himself or his domestic animals. They do not add to their sins attacks on his supplies.
The Coleoptera or beetles contain numerous messmates and some that do not even confine themselves to his stores. The carpet beetles feed upon his supplies of woolens, whether on the floors or on the shelves, and occasionally they get into the feathers of his pillow. But these are minor troubles compared with those that arise when the feeding is on the feathers of the madam's hat or the fur of her winter coat. Similar species get into our closets and pantries to feed upon the meat supplies, and occasionally we find them already established in provisions received from the packing houses, so that similar species occur the world around. Our grain, flour and meal supplies furnish homes to more kinds of species than the ordinary householder cares to consider, and nowhere does neglect or lack of cleanliness produce quicker infestation than in the pantry where our grain products are stored. Our lentils, peas, beans and other legumes are attractive to a variety of "weevils," especially in barns and granaries.
Even the den of the master of the house is invaded—if he is master and has a den—and in his supply of cigars and cigarettes the cigarette beetle and its larvæ hold forth—scarcely improving the flavor of the tobacco for smoking.
Among the Lepidoptera we find valiant aids to the beetles in destructive work on stored products. There are various kinds of meal-moths,
feeding on whole or prepared grains, and some of these, like the Mediterranean flour moth, cause serious troubles in mills. The well-known "Angoumois grain moth" not infrequently ruins entire crops of wheat for milling purposes, and in barns and granaries breeds continuously. Dried fruits are attacked by similar species and indeed scarcely anything in the pantry is exempt from the small caterpillars which usually live in silken tubes of their own construction.
And then there are the clothes and carpet moths which cause troubles of their own and are sources of much worry and expense to a part of the population not ordinarily interested in entomological matters. Incidentally they are sources of income to others who thrive on
Fig. 12. A clothes-moth, with its larva free and in a case; from the Division of Entomology, U. S. Department of Agriculture.
selling compounds more or less effective in protecting fabrics against the ravages of the "moths."
In the order Hymenoptera there are comparatively few species, confined practically to one series of families—the ants—that cause trouble in our households. But these, where they do occur, may exceed a combination of all the others in the annoyance and positive injury that they cause. There are several native forms, large and small, red and black, that are frequently pestiferous, and within the few years last past we have acquired another, the "Argentine Ant," which, as is customary with introduced pests, does more harm within its present range than all the native kinds combined.
Again we have reached the Diptera or. flies; but this time there is very little fault to find, for those species, other than the pests already mentioned, that do come into our houses, do so mostly as scavengers.
And there is yet another heading under which we have to consider insects in their effect upon man, and that is in the light of pests on the crops which are grown by him. A very large percentage of all insects are vegetable feeders and, naturally enough, when large areas are planted to one crop, so as to eliminate for the insect the problem of food supply, and cultural methods lessen their natural enemies, these vegetable feeders flourish out of all proportion to their normal natural limitations. The result is that a considerable percentage of the crop is destroyed, and most of the destroyed percentage represents the farmers' profit. That is to say, suppose it costs $5.00 to cultivate, plant, harvest and thresh an acre of wheat, and the harvest sets for a 15 bushel crop. With wheat at $1.00 per bushel at the station, the farmer might count on a profit of $10.00 per acre. But if in spring chinch-bug attack reduces the yield by 5 bushels, and the wheat-head army worm destroys 3 bushels more, the net return is reduced to $2.00 per acre, which is not living wage to the owner.
That this sort of reduction occurs with discouraging frequency on many sorts of crops throughout our country, is within the experience of every economic entomologist, and the total calculated loss per annum from insect attack in the United States alone amounts to $1,500,000,000. Surely a terrific tax to pay and one which is not paid without protest.
Not only is practically every crop attacked, but every portion of the plants may be infested. There are maggots that burrow into the roots of vegetables; borers that live in the roots of trees, shrubs and even meadow plants, and wire-worms and grubs that eat off the rootlets of grasses, strawberries and the like. Potatoes are gnawed ground by a variety of insects, and not a tuber nor a bulb escapes infestation.
The stems of herbaceous plants and shrubs and the trunks and branches of trees harbor borers without number and belonging to several orders. Vigorous growing vines like those of cucurbits may harbor several borers in a single stem, while even a wheat straw affords ample accommodation for several species, from the minute joint-worms to the caterpillars of owlet moths.
Fruit and other trees are attacked even in the nursery, and many a seedling never gets beyond the earliest stage of development. The temptation is great to enlarge on these points; but the difficulty would then be to find a stopping place. It must suffice to say that there is no part of a plant from the tip above to the rootlet below ground that
Fig. 14. The San José Scale as it appears on an infested shoot; from the Virginia Experiment Station.
may not be inhabited by a borer. Nor, on the other hand, is there a part of the outside of the plant above ground that may not be infested by scales or plant-lice.
Not so many years ago the Pacific Coast feared for its fruit industry because of the Cottony Cushion Scale, an imported pest. Still more recently the eastern United States was invaded by the San Jose Scale, another imported species which is responsible for more legislation, more organization, more expenditure of money and a greater revolution in methods of fruit growing, than any insect in history. No one who has not been in this fight from the beginning and who has not seen the changes in development, can really appreciate what has happened in the last decade. Incidentally, this insect has made more positions for entomologists and has stimulated more interest in entomological work than all other species combined; in which respects it may not be considered an unmitigated pest.
As for plant-lice, their name is literally legion and their study is only begun. We find their eggs in winter and the insects themselves throughout the year. With the beginning of plant-growth the Aphids also begin development and the character of the infestation is as various as the plants or parts of plants attacked. They are not even confined to the overground parts of the plant, but may be on the roots as well; either permanently or in an alternate stage. It is just allowable to mention the grape Phylloxera as a species that does all its real injury in the subterranean stage, and to record that this is one of the few contributions that America has made to European agriculture, in return for the many that we have received.
The host of other plant bugs that suck the juices of vegetation can only be hinted at. Mention must be made, however, of the chinch bug, which in the middle west has been the subject of more careful study and experiment, and has done, perhaps, more wide-spread damage than any other of its ordinal allies.
As to feeders on foliage, there seems no end to them and they are of all orders. Nor are their injuries of recent notice. The plague of locusts which devoured all crops was one of those visited upon Egypt in the days of Moses, and similar plagues of locusts exist to this day in African countries. They have not been unknown in the United States in years past and it is not yet safe to say that there will be no more.
Gypsy and brown-tail moths afford excellent illustrations of the expense that caterpillars may impose on a community, for they have cost Massachusetts alone not less than $2,000,000 directly and indirectly, while the general government has already spent more than half a million.
No part of a plant being free from insect attack, the fruit and seeds should also be infested, and so we find it. Codling Moth and Plum Curculio are terms known to horticulturists throughout the country, while cotton-boll weevils have more recently taken a prominent position in our southern states.
I trust that I have succeeded in convincing my audience of the important part that insects play in the community, and how vitally they affect man in his life, his health and his pocket. I might add to emphasize this still more, that were all natural checks removed from plant-feeding insects for two successive years, not a green thing would be left on the face of the earth; and that with the same checks removed from the forms parasitic and predatory on the higher animals, the third year would probably see the end of all vertebrate terrestrial life.
The questions will naturally arise—is there no brighter side to this subject? Are not insects of some use, and do we not derive some benefit or advantage from them?
Both of these questions are answerable in the affirmative, for insects are distinctly and importantly useful to man both directly and indirectly, altogether aside from the fact that parasitic and predatory species materially reduce the amount of injury that would otherwise be caused by those already mentioned.
Leaders among the directly beneficial species are the honey-bees and silk worms. Bee products, wax and honey, amount to millions of dollars
|Fig. 16. Honey bee.||Fig. 17. A burying beetle, Necrophorus sp.|
annually, and there is no more attractive food than good honey—a matter that is much more appreciated in continental Europe, where honey is a usual part of breakfast, than in the United States, where it is rarely seen on our tables at any meal. Bees-wax is, of course, an important commercial product, although its place for many purposes has been usurped by the cheaper paraffin.
As for silks, I would not dare to estimate the amount of money invested in them annually. The silk worm makes up in value for much of the injury caused by other caterpillars, and it would be well sometimes for our grand ladies in rustling garments to realize that they owe a large percentage of their exterior magnificence to a nasty, inconspicuous caterpillar.
Before the development of the aniline dyes there was no more beautiful scarlet than that obtained from the cochineal insect, a miserable coccid, allied to some of our destructive mealy bugs. As a pigment it produced a beautiful carmine, and this may make us look more leniently upon creatures like the cottony cushion and San José scales. Several of our common soft scales, notably that huge creature that infests the tulip tree, give a purplish extract in alcohol, and so do some of the Lachnid plant lice; but they will hardly be considered now-a-days as a source of coloring materials.
Insects figure to some extent in the pharmacopoeia, although not so much now as they did in days gone by. That powdered roaches had medicinal qualities was known to the ancients, and I believe that tinctures and extracts of these insects are still obtainable. Spanish flies or cantharides have a well deserved reputation for blistering properties and have been used to promote the growth of hair. They have also been made into extracts and tinctures, and used internally for a variety of troubles.
Bees produce an acid that is useful in some rheumatic affections, and it is asserted that the simplest effective method of application is to allow the bee itself to administer the remedy directly to the affected part. Incidentally, by this method the patient gets back at the operating physician, for whether the patient is benefited or not, the administrator dies as the result of the application.
But decoctions and other preparations of insect species are no longer so much thought of as in days gone by, and we now run to sanitation and other preventive measures, to serums and antitoxins, to antiseptics and to coal-tar products, so I need hardly claim very much more for insects under this head.
I might mention that insects as food material are not unknown, and while I have already mentioned locusts as a plague, I might add in mitigation that they also serve as an important article of food for the natives in the countries where they occur in such numbers. Wood-boring larvæ and even insect eggs have been and are even yet eaten; but their value in these respects can hardly be deemed sufficiently great to give them a favorable standing in our present communities.
Indirectly valuable to man are a long series of species that act as scavengers, reducing dead animal and vegetable matter to its inorganic compounds, and another series that serves as an aid to plant propagation. The former need little attention because man can do scavenger work much better and quicker himself; but in the work of plant pollination the importance of insect work is scarcely appreciated by the public at large.
Numerous plants depend absolutely upon insects for continued existence, and there are all stages of dependence from those where the plant depends on one species only to those for whom any insect that can carry pollen at all will serve. An illustration of a dependent plant is the Yucca which is visited by a little white moth, a Pronuba, which proceeds in its work as deliberately and purposefully as if it realized the importance of every step in the process. Another of still greater interest is that of the edible Smyrna fig, which depends upon Blastophaga for its fruit development. It is a case where the establishment of a horticultural interest depended upon the possibility of introducing with the fruit its specific pollinator as well.
The relation of bumble-bees to red clover is so well understood now that it is not really necessary to do more than mention it, and in general, the most effective pollinators are found among the bees, because they, more than any other insects, are structurally adapted for the gathering and transportation of pollen. Not only are the mouth structures adapted for getting deep into the flower cups but the hairy covering itself is modified so that it holds the grains dislodged as the insect moves among them.
To be sure butterflies and moths are also of use, and the butterfly tongue is well adapted for its purpose of reaching down and lapping nectar from concealed nectaries. Some of these tongues indeed are so developed as to permit the insects to feed while on the wing and to get into depths beyond the reach of less favored species.
(To be concluded)
- ↑ Popular lecture delivered at Boston, December 30, 1909, before the Entomological Society of America, its friends and guests.