Popular Science Monthly/Volume 79/November 1911/Insect Parasitism and its Peculiarities

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INSECT PARASITISM AND ITS PECULIARITIES[1]
By Professor WILLIAM MORTON WHEELER

BUSSEY INSTITUTION, FOREST HILLS, MASS.

IT is universally admitted that economic entomology, like such other branches of applied biology as medicine and sanitary science, is to a very considerable extent the strategics of our warfare with a host of parasites, which are forever endeavoring to destroy our bodies, our domestic animals, our food supply, our clothing and the very materials with which we construct our dwellings and on which we write or print our interpretations of the wonderful world in which we live. In other words, economic entomology is, to nearly all intents and purposes, merely that portion of applied parasitology which deals with insects. Naturally, therefore, the destruction of the insect parasites of man and of the plants and animals on which his very existence depends, must always constitute the basic interest of this science.

A vague notion of putting certain of the parasites themselves to some use in the struggle to which I have referred, seems to have been apprehended even in pre-scientific times and among primitive peoples. We have read of savage tribes, which, like monkeys, eat their hexapod ectoparasites. The Aztecs invented another use for these creatures, as we learn from a quaint work published many years ago by Cowan.[2] He cites the following story from Torquemada "respecting the revenue of Montecusuma which consisted of the natural products of the country, and what was produced by the industry of his subjects. During the abode of Montecusuma among the Spaniards, in the palace of his father, Alonzo de Ojeda one day espied in a certain apartment of the building a number of small bags tied up. He imagined at first that they were filled with gold dust, but on opening one of them, what was his astonishment to find it quite full of lice? Ojeda, greatly surprised at the discovery he had made, immediately communicated what he had seen to Cortes, who then asked Marina and Anguilar for some explanation. They informed him that the Mexicans had such a sense of their duty to pay tribute to their monarch that the poorest and meanest of the inhabitants if they possessed nothing better to present to their king, daily cleaned their persons, and saved all the lice they caught, and that when they had a good store of these, they laid them in hags at the feet of their monarch." A more scholarly, not to say more spiritual, use of parasites, seems to have been invented by no less a personage than the founder of the Dominican order of monks, since it is related in the same work "that the Devil, teasing St. Domingo in the shape of a flea, skipped upon his book, when the saint fixed him as a mark where he left off, and continued to use him so through the volume."

Although we may infer from such personal and therefore very trivial uses of fleas and lice as food, book-marks and taxes, that both saints and savages have occasionally endeavored to make their parasites subserve a useful purpose, it is only within very recent times that what may be properly called an economic use has been suggested for certain parasitic and predatory insects; namely, that of controlling the insects injurious to our crops, forests, domestic animals, stored foods and fabrics. The notion of using predatory beetles in destroying garden pests seems first to have occurred to Boigiraud de Poitiers in France in 184.[3] and in the following year to Antonio Villa in Italy. 3 The latter country also produced the two entomologists Eondani and Ghiliani, who, during the fifties and sixties of the past century, first suggested the use of parasitic insects for similar purposes. Since 1870 this suggestion has taken firmer hold of entomologists, especially in France, Italy and the United States, largely owing to the remarkable results achieved by Riley, Howard and their collaborators in our federal Bureau of Entomology. To mention only a single example, it has been found that the fluted scale (Icerya purchasi), so destructive to the orange, can be controlled by an Australian ladybird (Novius cardinalis), and this control has been successful in California, New Zealand, Cape Colony, Hawaii, Florida, Portugal, Italy, Syria and Egypt. The scale was accidentally introduced into all of these countries and in all of them the beetle, when in turn introduced, showed itself capable of preventing the pest from spreading and destroying the orange trees. This and many similar, though perhaps less striking, cases, have led entomologists to ransack remote regions of the globe for parasites to rear and turn loose on the noxious insects, which, after accidental introduction into our country, increase so alarmingly and do so much damage, owing, in great measure at least, to the absence of the parasites and other enemies that keep them in check in their native environment. The most elaborate experiment of this nature and one which is being followed with keen interest by all economic entomologists, is now being carried on at the Parasitological Laboratory at North Melrose, near Boston. Here for several years past great numbers of parasites have been received from Europe and northern Asia, carefully reared and studied, and, when found to be sufficiently promising, liberated in the hope that they will multiply and eventually control the gypsy and brown-tail moths.[4]

The fact that such economic uses have been suggested for insect and not for any other parasites seems to imply that the former must be peculiar in certain important particulars. This I believe to be true and has led to the following considerations. That I have chosen to read them to you, who are primarily interested in the problems of zoology in its broadest sense, is due to a conviction on my part that many of the accounts of parasitism, even in the best of our zoological hand-books, are more or less one-sided and anthropomorphic, probably as a result of the stepmotherly treatment necessarily bestowed upon the insects in such treatises. Before I say more about the insects, however, I wish to make a few remarks on animal parasitism in general.

Parasitism is, of course, a form of "behavior," and may be described as one of several complex types of the reactions of organisms to the most important source of their energy, their food supply. Other reactions to this element of the environment are predatism, commensalism, scavengerism and mutualism. There is in the main sufficient consensus of opinion concerning the distinctions between these different phenomena. Predatory animals kill other animals and devour them wholly or in part. Parasites put other organisms in the position of "hosts" by living directly on their tissues in such a manner as not to cause their immediate death. The parasite thus draws indirectly on the food supply of another organism by permitting or compelling it to do the hard work of procuring the food and of converting it into much more accessible and much more easily assimilable compounds. The parasite may be said, therefore, to use its host as an instrument not only for procuring, but for predigesting, its food. The commensal also uses another animal as an instrument, but merely in gaining access to a food-supply which the latter has procured but has not yet assimilated. The scavenger, like the saprophyte among plants, may be described as a parasite of the dead, deriving its sustenance from decomposing animals or plants or from the excretions of the former. The mutualist, finally, as the name implies, lives in a condition of balanced energetic or nutritional cooperation with another organism.

Of all these types of reactions to the food supply, parasitism is far and away the most prevalent; so prevalent, in fact, that it may be doubted whether there is any animal that does not resort to it, at least during a brief portion of its life, even if this be only during the period when, as an egg, it is drawing its supply of food-yolk from its parent. That parasitism has been most frequently developed from predatism is certain, that it may occasionally have its origin in commensalism, mutualism or scavengerism is highly probable, that it can, especially when it affects a considerable portion of the life-cycle of an organism, develop into anything but a more extreme form of parasitism, is very doubtful.

It would be easy to show by the citation of many examples that parasitism is an extremely protean phenomenon, one which escapes through the meshes of any net of scholastic definitions in which we may endeavor to confine it. Nor is this surprising when we stop to consider its great prevalence and the fact that during the course of time the organic world, pari passu with its increasing differentiation, has become ever more and more heavily weighted with parasitism and mutualism. That this nutritive dependence of organisms on one another has been steadily growing during paleontological time is clearly seen in the comparatively recent development of viviparity in mammals and many other animals, in the development of the alternating generations of plants into a condition in which the gametophyte is parasitic on the sporophyte (gymnosperms and angiosperms) or the sporophyte on the gametophyte (ferns and mosses), in the increasing mutualistic relations between insects and angiosperms, in the enormous development of parasitism among the highest orders of insects, the Diptera, Hymenoptera, Coleoptera, Lepidoptera and Homoptera, which are not known to have existed before Jurassic and Triassic times, and even in many apparently more primitive parasites like the true lice, bird lice, bat lice, fleas and many tape worms, flukes and round worms, which could not have developed till after their mammalian and avian hosts had made their appearance. Social life, too, which is hardly more than a mixture of parasitism and mutualism, shows a similarly recent development. Man himself, with whom we do not commonly associate the idea of parasitism, although the term is derived from a certain type of man well known to the ancient Greeks, not infrequently displays an extraordinary variety of parasitic activities. As an embryo he is always entoparasitic, using his allantois in a manner that vividly suggests the rootsystem of a Sacculina attached to a crab. At birth he becomes a kind of ectoparasite on his mother or nurse, and throughout his childhood and youth he is commonly what might be called a family parasite, depending for his sustenance on his parents, brothers and sisters or remoter relations. At maturity, in addition to the possibility of becoming parasitic on his wife, he has a choice of many kinds of social parasitism. As a member of a trust, political party or legislative body, not to mention many other organizations and institutions, he may graft successfully on the community at large or on some particularly lucrative portion of it, and should he fail through these activities to store up a sufficient corpus adiposum in the form of a bank-account, he may parasitize, with advancing years and till the end of his days, on his own offspring.[5]

But the roots of parasitism may be traced even deeper within the very fabric of the organism itself. The theories of Roux and Weismann have made us familiar with the struggle among the parts of the individual organisms, i. e., among its organs, tissues, cells and the components of its cells, a struggle in which these elements often grow and develop at the expense of other elements in a manner that can only be regarded as parasitic. The more modern theories of mutation and Mendelism, with their insistence on unit-characters and "factors," obviously admit of an interpretation in similar terms. We can even shift this interpretation to the psychic plane, where we find the fixed ideas, obsessions and monomanias behaving as so many processes which draw their sustenance from other psychic processes to such an extent that they may in the end not only dominate but destroy the whole personality.

Some of you will be shocked at this account of what we are in the habit of describing in very different language, for the same emotional reason that we all admire the tiger and the tiger-beetle and loathe the tape-worm and the louse, namely, because our instinctive horror of the parasites to which our species is so constantly exposed, prevents us even as twentieth-century zoologists from appreciating the extent to which all life, ourselves included, is saturated with parasitic proclivities. I fear, however, that this attempt to justify my shocking language will fail to convince some others among you, who will accuse me of being myself a host of one of the obsessions to which I have just alluded—of the parasitic obsession, namely, of the idea of parasitism. You will say that in thus subtilizing or volatilizing what has always seemed to be a concrete biological phenomenon, and in thus diffusing the concept of parasitism over the whole organic world, I have not only distorted it beyond recognition, but have deprived it of any usefulness which it may have had. To such accusations I can only reply that I gladly concede that it is admissible for practical purposes to circumscribe parasitism by arbitrary names and definitions in special fields of biological, sociological and psychological study, but I must insist, nevertheless, that it is a very fundamental and far-reaching phenomenon, which, for theoretic and heuristic purposes, may properly be said to include any complex of vital processes, which maintain themselves at the expense of other vital processes, in the same or in other organisms, without reacting on these processes in a similarly sustentative manner. But let us return to the more conventional conception of the subject.

When an organism becomes parasitic it, of course, undergoes structural and physiological changes. These express themselves in the loss or modification of previously existing characters and in the acquisition of new characters. The amount of this loss, modification and acquisition depends, first, on the intimacy of relationship of the parasite to the host; second, on the nature of this relationship; third, on the time in the parasite's ontogeny when this relationship is established, and fourth, on the portion of the ontogeny which it covers. Ectoparasites, as we all know, are, as a rule, less modified than entoparasites, but each of these categories includes very different degrees of modification, according as the parasite is confined to a particular organ of the host or is capable of moving more freely over its surface or through its tissues. The habitus of a parasite is most profoundly influenced and characterized by the moment in its ontogeny when it joins its host, and especially by the length of the period during which this association is maintained. According as this association is coextensive with the parasite's life or merely for a briefer period, we may distinguish permanent and temporary parasites. The latter, again, may be divided into those that are free from their hosts only during larval or early life and those that are free as adults. To these three types practically all animal parasites can be referred. They are best represented by such forms as certain tape-worms and flukes, by such crustaceans as Sacculina, and by such insects as the Ichneumonidæ and Chalcididæ among Hymenoptera and the Tachinidæ among Diptera. Permit me to describe very briefly the salient peculiarities of a typical example of each of these groups.

The tape-worm is an excellent example of a permanent parasite. It produces an enormous number of very minute eggs, and either these or the singular embryos, known as onchospheres, which they contain, are passively swallowed by the host. The onchosphere passes from the alimentary tract into the tissues of the host and there becomes a bladderworm. This, in order to become a sexually mature tape-worm, must enter the alimentary tract of a second host. The transfer is effected passively by the second, or definitive, devouring the first, or intermediate host. Both stages of the parasite exhibit extreme modification of structure, the second being characterized by an enormous development of the hermaphroditic gonads and of the alimentary surface, which is merely the integument, and is therefore in immediate contact with the food supply. To this type of parasitism we may also refer the Dicyemids, and many of the flukes and round-worms. In many of these cases the association with the host may be effected without any effort on the part of the parasite, and the small size, the enormous number and the method of distribution of its eggs are properly interpreted as so many direct and necessary adaptations to chance.

The Rhizocephalous crustacean Sacculina, which may serve as a paradigm of the second type, or that of temporary parasitism with free early ontogenetic stages, also produces an enormous number of minute eggs. These, however, develop into free-swimming Nauplii, which in turn become Cypris larvæ and as such seek out their Decapod or Isopod hosts. Owing to the activity and comparatively high organization of these larvæ, the element of chance in bringing about the host association, though still considerable, is not as great as it is in the tape-worm. When it has joined its host, the Cypris larva, through one of the most remarkable methods of development known to exist among animals, proceeds to undergo structural modifications so extreme that, without a knowledge of the earlier stages, the crustacean affinities of the organism would never be suspected. "In the adult state the body consists of two portions: a soft bag-like structure, external to the host, carrying the reproductive, nervous and muscular organs and attached to some part of the host's abdomen by means of a chitinous ring; and a system of branching roots inside the host's body, which spring from the ring of attachment and supply the external body with nutriment."[6] In Sacculina, as in the tape-worm, the gonads are hermaphroditic and reproduction takes place by a continual round of self-fertilization. To this type of temporary parasitism with free larval stage we may also refer the myzostomes and other parasitic annelids and the parasitic mollusks. In all these cases metamorphosis supervenes while the animal is still very small and hence precedes growth and the incidence of the modifications produced by the parasitic habit.

As an example of the third type or that of temporary parasitism with free adult stage, we may select the Ichneumonid Hymenopteron. The eggs are few in number and rather large and are deposited by the mother directly in or on the host, which is the larva of some other insect. The sluggish, bag-shaped parasitic larva, on hatching from the egg, feeds for some time on the blood-tissues and fat-body of the host, but is careful not to prevent the latter from moving about, procuring its food and growing to maturity. When it has reached this stage, however, the parasite quickly destroys it by consuming its vital tissues. It then completes its own growth, pupates and eventually emerges as a very active, highly organized and beautifully colored fly, provided with a splendid nervous system, exquisite sense-organs and powerful locomotor organs in the shape of legs and wings. It is either a male or a female and, if of the latter sex, soon proceeds to place its offspring in immediate contact with the host. Although the larval Ichneumon exhibits modifications of structure almost as extreme as those of the adult Sacculina, these produce no effect on the organization of the adult insect. The association of the larva with its host is the work of the mother insect, a creature gifted with complex instincts that enable her to ferret out the host even in the most intricate concealment. The large size and small number of her eggs and her highly specialized method of oviposition indicate very clearly that chance, which plays such a role in the life-cycle of the tape-worm and Sacculina, has given way to an almost inevitable association of the parasite with its host.

Of course, the Ichneumon represents only one of many forms of parasitism among insects. I have chosen it because it is the most characteristic and most highly specialized. There are insects like the Strepsiptera and the Rhipiphorid and Meloid beetles which seem to combine the Sacculina with the Ichneumon type in that they produce many small eggs that hatch as very active triungulin larvæ and only later develop into legless, bag-like larvæ of the Ichneumon type. It is interesting to note that in the Strepsiptera the adult female prolongs the parasitic habit of the larva, while the adult Meloidæ or oil-beetles are rather sluggish and seem to show other after-effects of their larval life. There are also many insects, like the true lice and bird lice which are, to all intents and purposes, permanent parasites comparable with the ectoparasitic flukes, though they never exhibit such extreme modifications. And, finally, there are other animals besides insects that have parasitic larval and free adult stages, e. g., the fresh-water mussels.[7]

Zoologists have naturally been deeply impressed by such wonderful parasites as the tape-worms, flukes and Sacculina and have regarded these as fine examples of degeneration or degradation. Many, indeed, have dwelt on these words in a manner which leaves no doubt that they are used in a purely anthropomorphic sense as implying deterioration or "an impairment of natural or proper qualities" in the parasites, notwithstanding Ray Lankester's assertion that "degeneration may be defined as a gradual change of the structure in which the organism becomes adapted to less varied and less complex conditions of life."[8] It is easy to trace the source of this anthropomorphism to the atrophy of the parasite's neuro-muscular system, a system by which we as intellectual beings necessarily set great store, and the hypertrophy of the alimentary and reproductive organs, which, notwithstanding their immense biological significance, have nevertheless been assigned a very inferior place in our scheme of ethical values. But parasites may properly be regarded as more advanced organisms than the predators, for they have not only had a more eventful phylogenetic career, but, during their long history, have learned to use other organisms in a very economical manner as instruments of nutrition. From a consistent biological point of view, therefore, and from one embracing insect as well as vermian and crustacean parasites, it is evident that the peculiar convergent complexions of these organisms should be attributed to specialization. "Degeneration" is properly a pathological term, and parasites, however pathogenic they may be, are, of course, no more pathological or diseased than predatory animals. There is some evidence to show that the myzostomes have persisted in their modern form since Silurian times, with a conservatism equalled only by that of their Crinoid hosts. If all the generations of these peculiar annelids have been pathological for millions of years, they should long since have disappeared from the waters of the globe, but we find that though many or all of the original species have doubtless become extinct, this was probably due simply to the extinction of their hosts, for nearly every extant species of Crinoid supports at least one species of Myzostoma. Moreover, if we regard parasitic modification as an expression of degeneration, we must suppose that such forms as the adult Ichneumon are produced by a post-larval regeneration. Apart from adding an unusual meaning to the word "regeneration," this fails to express the actual conditions correctly. The whole ontogeny of such insects is in reality very highly specialized, the adult representing in many particulars as great a departure from the primitive insect type as the larva, albeit in a very different direction. In discussions of this subject I would therefore substitute the words "parasitic specialization" for such terms as "degeneration" and "degradation." Together with these, another term, "retrogression," should be avoided, for the reason that the parasitic modifications of structure to which it is often applied can be more properly attributed to "arrest of development."

It will be seen from the foregoing discussion that the leading peculiarity of insect parasitism, at least in such groups as the Hymenoptera and Diptera, which are almost the only ones of value in controlling noxious insects, is the restriction of the parasitic habit to the sluggish larva and the specialization of the free adult for the purpose of disseminating the species and of placing the coming generation in intimate contact with the host. No one who observes one of our large, graceful Ichneumonids, such as Thalessa lunator, alighting on a tree-trunk and then conveying its greatly attenuated eggs by means of its long hair-like ovipositor through some three inches of hard wood into the burrow of a Tremex larva, the presence of which it has been able to detect by means of its marvelously acute sense-organs, can fail to appreciate the advantages of such a method of bringing a parasite to its host, rather than by the tape-worm's shot-gun method of scattering minute eggs about promiscuously, or by the Sacculina's almost equally haphazard method of employing minute, feeble, aquatic larvæ.

Another peculiarity of economic importance in the parasitism of Hymenopterous and Dipterous insects is its highly predatory character, for the voracious larvæ of these orders almost invariably kill their hosts.[9] Other forms, like the Strepsiptera, which permit their hosts to reach the adult stage, nevertheless destroy their gonads and thus decrease the reproductivity of the host species. In some cases, indeed, it is impossible to decide whether we are dealing with parasitism or predatism. The Sphex, that lays her eggs on caterpillars which she has carefully paralyzed, is commonly regarded as a predatory insect, but she is from another point of view, an even more specialized parasite than the Ichneumon. Her sting immobilizes but does not kill the active fullgrown or nearly full-grown caterpillars, and her larvæ are careful to feed in such a manner as to spare as long as possible the life of their victims. We have here merely a further extension of the maternal instincts primarily devoted exclusively to bringing about the union of the parasite with the host, to a unique and effective preparation of the host's body for easier exploitation by the parasite.

A third peculiarity of economic importance in the Hymenopterous and Dipterous insects is their pronounced tendency to confine their attacks to species of large, recently developed and eminently noxious groups, such as the Lepidoptera, Coleoptera, Homoptera and other plant-destroying insects.

There are also a number of peculiarities some of which are of less practical but of no less theoretical interest. These, which I must consider very briefly in the limited space at my disposal, are the following:

1. The occurrence of hypermetamorphosis which is frequently exhibited by parasitic insects often of the most remote taxonomic affinities, such as the Proctotrypids, and certain Chalcidids (Orasema and Perilampus) among the Hymenoptera, Mantispa among the Neuroptera, the whole order of Strepsiptera, and the Meloidæ and Ehipiphoridæ among Coleoptera. The complication of development arises in all of these cases from an inability of the mother insect to find the host or at any rate to reach it during the proper ontogenetic stage, and hence from the need of an active and inquisitive first larval stage to supply this defect.

2. The prevalence of hyperparasitism. We may distinguish primary, secondary, tertiary, quaternary and even quinary parasites among insects, according to the principle of the "little fleas ad infinitum." The numerical appellations in this series have been restricted to insects parasitic on other insects, although the primary parasites are really secondaries when they attack insects like caterpillars, since these are, of course, plant-parasites.[10]

3. The absence in parasitic insects of hermaphroditism, a phenomenon so prevalent among vermian, crustacean and annelidan parasites. Only one small group of insects is known to be hermaphroditic, namely, the Termitoxeniidae, comprising a few genera of extraordinary flies that live in termite nests.[11]

4. The rare occurrence of heterœcism, or change of host, a phenomenon very prevalent among tapeworms and flukes. It has been developed, however, within apparently very recent times in such groups as the plant-lice and in certain myrmecophilous beetles of the genera Atemeles in Eurasia and Xenodusa in North America.

5. The increasing development of viviparity as seen in such a series of parasites as Hemimerus, which, according to Hansen and Heymons, develops within the ovary of its mother,[12] the larviparous Tachinidæ and Sarcophagidæ, the nymphiparous Hippoboseidæ, Nyeteribidæ and plant-lice, and the Termitoxeniidæ which, according to Wasmann, are born practically as adult insects.[13]

6. The development of polyembryony among the Chalcidid and Proctotrypid Hymenoptera. Owing to the greatly increased reproductivity of these parasites through the formation of dozens or even hundreds of adults from a single original egg by a process not unlike that employed in the egg-shaking experiments of our laboratories, this phenomenon, though restricted to comparatively few species, is nevertheless of considerable economic importance.[14]

7. The development of social life among insects. This, as I have shown on former occasions, has its origin both ontogenetically and phylogenetically in the parasitism of the offspring on the parent.[15]

Paleontology seems to show very clearly the conditions that have favored the enormous development of parasitism among insects especially within comparatively recent times. Some of these conditions are:

1. The diminution in insect stature which occurred in the late Carboniferous and during the Permian and seems to have been originally in great part an adaptation to increased reproduction and dispersal. Other things equal, a small animal will, for very obvious reasons, become a parasite more easily than a large one.

2. The development of metamorphosis. This was already clearly established in the earliest known insects, the Paleodictyoptera, which were predatory and amphibiotic like the may-flies of the present-day, living in the water during their apterous larval stages and spending their winged imaginal stage in the air. They show plainly the great peculiarity of insect development, i. e., metamorphosis succeeding growth and not preceding it as in the crustaceans, mollusks and annelids. Dimly foreshadowed in this method of development are the more complete modern types of insect metamorphosis, which have their morphological origin, as we now know, in a doubling of most of the rudiments of the organs in the embryo. On hatching, one set of these rudiments develops immediately into the larval body, while the other set remains in abeyance in the form of minute germinal centers, or histoblasts, from which the body of the adult will be fashioned during the quiescent pupal stage. The higher insects are therefore beautiful examples of double personality, much more perfect examples of this phenomenon, in fact, than any that has been discovered in man. The larval insect is, if I may be permitted to lapse for a moment into anthropomorphism, a sluggish, greedy, self-centered creature, while the adult is industrious, abstemious and highly altruistic, concentrating its activities on reproduction and the dissemination of the species. Unlike ourselves, who are Mr. Hydes and Dr. Jekylls in varying degrees, for brief alternating periods in our lives, or even simultaneously, the youthful insect sows its wild oats with a vengeance as a glutton or even as an assassin and then experiences a change of heart and reforms for good and all.

Parasitism must have been very easily grafted on to such a sharply dichotomic method of development as that of the holometabolous insects, for the larvæ of the predators are already much inclined to sloth and gluttony when the food supply is abundant, and comparatively little modification would be required to convert them into parasites. But the same peculiarities of metamorphosis have also made the holometabolic insects ideal hosts. We have already seen that insects, as a rule, are themselves not only parasitic during larval life, but also prefer larvæ as hosts. It is not improbable that this is the primitive, and that parasitism on the egg, pupa or adult is a secondary, or derivative condition. The real secret of both host and parasite being larvae lies in the peculiar significance of anabolism in this stage. The host accumulates great quantities of fats and proteids as a so-called "fat-body," which is of little or no immediate use to the organism itself, but is stored up to be utilized during metamorphosis. This fat body may, therefore, be devoured by the parasite and converted into its own fat-body without seriously injuring the host. Furthermore, the fact that the parasite, too, stores up its food in the form of a fat-body instead of at once turning it over to its gonads and becoming reproductive, accounts for the striking differences between the insect parasite, on the one hand, and the tape-worm and Sacculina, on the other. The few exceptions among insects, such as the female Strepsiptera, in which the food taken by the larval parasite from its host is soon turned over to the gonads and used for reproduction, leads to a permanent parasitism resembling that of the tapeworms or the adult Sacculina. The larva is at once arrested in its development and begins to reproduce by pasdogenesis. We may conclude, therefore, that the existence of parasites of the Ichneumon type, with free, active and highly developed adults is rendered possible by an inhibition of gonadic growth during larval life; whereas parasites which begin to reproduce while still living with their hosts are thereby prevented from either leaving them or undergoing further morphological differentiation.[16]

3. A third primitive peculiarity of holometabolic insects, which seems greatly to have favored parasitism, is the astonishing rapidity of their larval metabolism and growth and the equally remarkable quiescence of their pupal stages. These have, of course, converted insects into the most wonderful opportunists, through enabling them to take advantage not only of the changing seasons and the very diverse physical conditions of our planet, but also of the most evanescent supplies of food, both living and in process of decomposition.

4. Parthenogenesis may also be cited as a widely prevalent phenomenon, which has been put to good use by parasitic insects. Like polyembryony, it has an economic significance, because it enables such noxious parasites as the plant-lice to multiply enormously under conditions that would preclude reproduction in non-parthenogenetic species, and for the same reason greatly assists many Hymenopterous parasites in checking the undue multiplication of these and other plant-destroying insects.

Although I may have had little difficulty in convincing you that parasitism is a very specialized kind of behavior, you will probably still be of the opinion that there is something inherently and radically wrong with animals that resort to it rather than to predatism, mutualism or some other means of maintaining their vital activities. It must, of course, be admitted that in becoming satellites of their hosts, parasites have renounced the primitive, wasteful and erratic freedom of the predator and are compelled to mould their activities on those of the host. This necessarily puts them in a condition of such abject dependence that their very existence as individuals and species is imperilled whenever they overstep that margin of vitality which the host, like every other healthy organism, can afford to sacrifice to the accidents of its environment.[17]

The parasite not only tends to restrict itself to the use of a particular host as a food-procuring instrument, but is also compelled to exercise the most exquisite care in the use of this instrument. From the very nature of the situation, therefore, parasitism is an extremely precarious type of behavior. But this is true also of all highly specialized behavior, that of biologists included, and points the way to, but does not constitute, the real difficulty with parasitism. This, I take it, is the suppression of the voluntary movements, which necessarily results from the intimate host relations, especially when these are confined, as is so often the case, to some one particular organ or tissue. It is not, therefore, the parasite's habit of taking something for nothing from another organism, that is so fatal, for all creatures, in matters relating to nutrition, find it more blessed to receive than to give, but the acceptance of the most important supply of its energy under conditions that preclude an exercise of the muscular and hence also of the sensory and nervous activities and restrict its vital activities to a round of assimilation, metabolism and reproduction. This unbalancing of functions is probably hastened by a kind of intraorganismal parasitism or "Kampf der Theile" in Roux's sense, the alimentary and reproductive tissues drawing their nutriment not only from the host but also from the more inactive tissues of the parasite's own body. That this torpor, or inactivity of the neuromuscular system is at the bottom of the peculiar disability of parasites is shown by many non-parasitic organisms, which have easy access to an abundant food supply consisting of dead or inorganic substances. Most plants and many invertebrates, such as the barnacles, and especially the scavengers among insects, exhibit essentially the same modifications as parasites. In fact, the larval stages of many insects that feed on carrion or decomposing animal and vegetable matter, are quite indistinguishable from parasitic larvæ. This and the further fact that plant-eating species are not generally regarded as parasites by entomologists have led to considerable confusion in certain accounts of insect parasitism.

While most parasites among the lower invertebrates have never succeeded in freeing themselves from the tyranny of the host relation and the fatal torpor to which it inevitably leads, this is, as we have seen, by no means true of the typical insect parasites. To the ontogeny of these organisms the dictum "once a parasite, always a parasite" most certainly does not apply. That it is inapplicable to their phylogeny I am not prepared to say, although I am unable to think of any nonparasitic insects that show evidence of descent from parasitic species. There can be no doubt, however, that parasites are still able to give rise to new specific forms. This capacity is without doubt very feeble or languishing in the permanent parasites of the tape-worm and Sacculina types as compared with that of the insects. Indeed, there is much evidence to show that in insects, parasitism, far from interfering with the process of species formation, may actually have a tendency to favor or accelerate it. Sharp estimates the number of species of parasitic Hymenoptera on our globe at 200,000, and of this vast number probably 80,000 belong to a single family, the Chalcididæ, of which only some 6,000 species have been described. Another parasitic family, the Tachinidæ, belonging to the great order Diptera, seems to be in such an active stage of species formation that the most diligent and thoughtful students of the group flounder about in it with a dazed and almost ludicrous helplessness. And not only is practically the whole enormous group of moths and butterflies to be regarded as parasitic, but the same is true also of untold legions of plant-lice, scale-insects and beetles. Hyperparasitism, which may be regarded as a kind of permutation of parasitism, must also be mentioned in this connection, because it gives us a glimpse of the virgin fields which the holometabolic insects, owing to their peculiar method of development, are beginning to invade.

I believe that the foregoing discussion of the peculiarities of insect parasites adequately supports the view that these organisms are eminently fitted to function in controlling the depradations of injurious insects. That they can not be regarded as instruments of extermination is obvious from the fact that under natural conditions the complete extinction of the host species involves the destruction of the parasitic species, unless the later is able to live on more than one host. Although it is not improbable that during geological time such joint extermination of host and parasite has repeatedly occurred, we are unable to cite any case that has fallen under the observation of the entomologist. Purely local extermination of injurious hosts by their parasites has, however, been observed.

Before bringing my lecture to a conclusion two matters must be briefly discussed. One of these, which is mainly of theoretical interest, relates to the development of the parasite's association with its host, the other, of more practical significance, to the methods of greatest promise in the study of insect parasitism. We need not stop to consider cases of the tape-worm type which reach their hosts by chance. In the two other types which I have distinguished, we have the association with the host established through the initiative of the larval parasite itself (Sacculina type) or through the parasite's mother (insect type). While the former type does not seem to call for any special explanation, there seems, at first sight, to be something mysterious in the insect parasite, for when we see an adult organism, such as an Ichneumon coming from a distance—out of the blue, so to speak—and seeking out a concealed larva in which to deposit its egg, we are tempted to turn to some teleological explanation, such as is implied in the common conception of "instinct," or perhaps to something in the nature of a "divinatory sympathy" between parasite and host. Although such conceptions are necessarily anthropomorphic, I would not deny them a certain, albeit provisional, value. As biologists, however, we are fortunately in a position to suggest a simpler explanation. The intimate practical knowledge (sit venia verbo) which the mother Ichneumon possesses of the host, loses much of its mystery when we stop to consider that she has, during her own larval life, devoured just such an insect, for the same reason that we may be said to have an excellent practical knowledge of an orange after we have eaten it. The Ichneumon is therefore familiar with the location, feeling, odor and taste of the creature in which she will lay her eggs, if we make the not improbable assumption that the results of her own larval experience persist as mnemic factors, notwithstanding the profound morphological and physiological changes which she has undergone during metamorphosis. There would then be nothing surprising in her tropism-like reactions to the mechanical and chemical stimuli represented by the host larva and its immediate environment.

As the time at my disposal is nearly exhausted, I must bring my discussion to a close. Having made the pilgrimage to the American Mecca of experimental zoologists, I could hardly hope for salvation if I departed without at least saluting the Kaaba. This I can do most effectively, perhaps, by calling attention to the great need of experimental work in animal and especially in insect parasitology. Biologists, during the romantic period of Darwinism, made much of the parasites. These organisms, in fact, supplied them with no end of ammunition in defence of natural selection, the influence of the environment and the biogenetic law. Then came the period of morphological minutiæ with its tacit assumption that particles of a dead organism are vastly more interesting and illuminating than the whole of a living one. During this period the parasites were, of course, sectioned and studied in the same manner as other organisms, but, since it is impossible to explain a living whole by pulling it to pieces and sticking the inert fragments together again, parasitism, which is a process and not a thing, retained its ethological interest mainly for biologists who were engaged in the practical applications of their science.[18]

Now that we have reached the third period, or that of emphasis on experiment with the living organism as the best means of elucidating the life-processes, those of us who had the misfortune to live and exhaust our greatest enthusiasm during the romantic and morphological periods, can, I suppose, do nothing better with the meager remnant of our vitality than pray for breadth of sympathetic vision on the part of our younger, more numerous and more vigorous contemporaries. The splendid achievements of the investigators who assemble here every summer certainly whet one's desire to see experimental work of the same character accomplished in parasitology. A certain amount of simple experimental work on social parasitism in ants has been inaugurated by Wasmann and myself and continued with interesting results by Santschi, Emery, Viehmeyer, Donisthorpe and others, but more important work, having for its object the artificial production of individual parasites and such studies on the behavior of their descendants as those recently made by Kammerer on the offspring of Amphibia, whose breeding habits had been artificially modified, have not yet been undertaken. Here again, as in so many other cases, the botanists are blazing the trail for the zoologists. The familiarity of the former with grafting, which is merely an artificially induced parasitism, has led them to undertake interesting experiments, like those recently published by Pierce[19] and MacDougal and Cannon.[20] And although these experiments yielded less striking results than might, perhaps, have been expected, they nevertheless emphasize an important fact, which all biologists, except systematists and paleontologists, are too apt to overlook, namely, the extraordinary stability of specialized characters.

Experimental zoologists, including the students of animal behavior, are most keenly interested in the modifiability of the organism, and their experiments are usually devised for the special purpose of determining the amplitude and peculiarities of this modifiability. The entomologist, however, who is attempting to use parasitic insects as tools or implements in controlling the depredations of other insects, is primarily interested in the stability of structure and constancy of behavior. This follows from the very nature of his work. As the essential excellence of a tool consists in its remaining the same as it was when it left the hands of the manufacturer, so a parasitic species can be used as an efficient tool only if it behaves generation after generation with uniform constancy. Hence in combating pests, only those

parasitic insects can be utilized to advantage that are not only prolific and will endure the climatic conditions into which they have been artificially introduced, but will maintain very definite relations only with individuals of a single or of a very few host species and destroy them in their earliest possible ontogenetic stage before they can do extensive damage.[21] Such constancy is especially necessary in primary and tertiary parasites, since whenever these show a tendency to become secondaries and quaternaries, as is sometimes the case, they become harmful instead of beneficial.[22]

It is clear that the determination of the constancy or invariability of parasitic reactions as a basis for practical applications requires, if anything, an even greater insistence on the experimental method than does the determination of the range and character of modifiability for purely theoretical purposes. Ever since the days of Redi both theoretical and practical entomologists have resorted to the experimental method and therefore have no reason to regard themselves as behind the times in appreciation of what some zoologists have been heralding as a recent dispensation. In other respects, however, the students of insect life are "old fashioned" and resemble the botanists more closely than the zoologists, in that they are constrained by the extraordinary intricacy of their science to maintain the closest and most sympathetic cooperation with the taxonomists, morphologists, and students of geographical distribution. Without this cooperation their studies of insect parasitism would resolve themselves into a weltering chaos.

  1. A lecture delivered at the Marine Biological Laboratory, Woods Hole, Mass., August 8, 1911. In preparing the lecture for publication several footnotes have been added and the concluding paragraphs have been rewritten.
  2. "Curious Facts in the History of Insects," J. B. Lippincott & Co., Philadelphia, 1865.
  3. For a fuller account of the work of these and other early promulgators of the use of predators and parasites in combating noxious insects, see Trotter, "Due precursori nell' applicazione degli insetti carnivori a difesa delle piante coltivate," Bedia, V., 1907, pp. 126-132.
  4. Excellent general accounts of the subject here touched upon are contained in the following papers: Marchal, "Utilization des Insects Auxiliaires Entomophages dans la Lutte contre les Insects Nuisibles a 1 'Agriculture, "Ann. de l'Inst. Nat. Agronom. (2), VI., 2, 1907, 74 pp., 26 figs.; translation in part in Pop. Sci. Monthly, LXXII, 1908, pp. 352-370, 406-419; Silvestri, "Sguardo alio Stato Attuale dell' Entomologia Agraria negli Stati-Uniti del Nord America, etc.," Boll. Soc. Agric. Ital., XIV., No. 8, 1909, 65 pp.; Howard and Fiske, "The Importation into the United States of the Parasites of the Gypsy Moth and the Brown-tail Moth," Bull. No. 91, Bur. of Ent., Dep. Agric, 1911, 312 pp., 73 figs., 25 pls.
  5. Certain general aspects of social parasitism in man are admirably presented by Massart and Vandervelde in their work entitled "Parasitisme Organique et Parasitisme Social," Bull. Sci. de France et de la Belg., XXV., 1893, 68 pp., and by Ross in Chapter XXVIII. of his "Social Control," Macmillan Co., New York, 1910. The conception of viviparity as a form of parasitism has been developed by Giard ("Sur la signification générale du parasitisme placentaire," C. R. Soc. Biolog., 1897), Houssay ("La Forme et la Vie. Essai de la Methode Mcanique en Zoologie," Paris, 1900) and Faussek ("Viviparity and Parasitism," in Russian, Busskoje Bogatswo, 1893).
  6. Geoffrey Smith in the Cambridge Natural History, Vol. IV., p. 95.
  7. Cf. Lefevre and Curtis, "Reproduction and Parasitism in the Unionidæ," Journ. Exper. Zool., IX., No. 1, 1910, pp. 79-115, 5 pls.
  8. "Degeneration," p. 32.
  9. It would seem that the death of the insect host is necessitated either by the relatively very large size of its insect parasite at maturity, when acting alone, or (in cases of polyembryony and simultaneous infestation by several individuals of the same species) to the equally considerable bulk of a number of small parasites acting together. The comparatively slight difference in stature between host and parasite is certainly one of the most remarkable peculiarities of insect parasitism.
  10. Fiske ("Superparasitism: An Important Factor in the Natural Control of Insects," Journ. Econ. Ent., III., 1910, pp. 88-97) and Pierce ("On Some Phases of Parasitism Displayed by Insect Enemies of Weevils," ibid., III., 1910, pp. 451-458) have distinguished between "hyperparasitism" and "superparasitism." The former term is denned by Pierce as "the normal attack of a parasite species upon another parasite species," whereas superparasitism "occurs when a normally primary parasite attacks a host already parasitized, and the result is that the latest comer generally attacks its predecessor." The distinction is important, but for the sake of brevity I have not introduced into the text.
  11. Wasmann, "Termitoxenia, ein neues flugelloses, physogastres Dipterengenus aus Termitennestern," 2 Pts., Zeitschr. f. wiss. Zool., LXVIL, 4, 1900, pp. 559-617, LXX., 2, 1901, pp. 289-298, and Assmuth, "Termitoxenia Assmuthi Wasm.; Anatomisch-histologische Untersuchung, "Inaug. Dissert., 1910,53 pp.
  12. Cf. Hansen, "On the Structure and Habits of Hemimerus talpoides Walk.," Entom. Tijdskr. Arg., XV., 1894, and Heymons, "Eine Plazenta bei einem Insekt {Hemimerus)," Verh. deutsch. zool. Gesellsch., 1909, pp. 97-107, 3 figs.
  13. Loc. citato.
  14. The occurrence of polyembryony was first clearly recognized and thoroughly investigated by Marchal in Eucyrtus fuscicollis ("Recherches sur la Biologie et le développement des Hyménoptères Parasites. I. La Polyembryonie Specifique on Germinogonie," Arch. Zool. Expér. Gén. (4), II., 1904, pp. 257-335, 5 pls., and an earlier paper: "La dissociation de l'oeuf en un grand nombre d'individus distincts chez l'Encyrtus fuscicollis," C. R. Acad. Sci. Paris, CXXVI., 1898, pp.-662-664), although Bugnion ("Recherches sur le développement postembryonnaire, l'anatomie et les mœurs de l'Encyrtus fuscicollis," Rec. Zool. Suisse, V., 1891, pp. 435-534, 6 pis.) had previously studied the same insect. Silvestri has published several valuable papers on polyembryony, the most important being "Contribuzioni alia Conscenza Biologica degli Imenotteri Parassiti. I. Biologia del Litomastix truncatellus (Dalm.)," Ann. B. Scuola Sup. d'Agric. Portici, VI., 1906, pp. 1-51, 5 pls.
  15. Wheeler, "Ants, their Structure, Development and Behavior," Columbia Univ. Press, 1910. Recently Holmgren ("Termitenstudien, I. Anatomische Untersuchungen," R. Svensk. Vetensk. Handl., XLIV., No. 3, 1909, 216 pp., 3 pls., 76 text-figs.) and Eseherich ("Termitenleben auf Ceylon," Gustav Fischer, Jena, 1911, 262 pp., 3 pis., 68 text-figs.) have accumulated much evidence to support the conclusion that the mutual attraction among the individuals and the development of the castes of the termite colony are due to the habit of these insects of feeding on the fatty exudates of one another's bodies and on that of their queens. This may also be true of ants and other colonial insects. A very similar method of feeding on the surface secretions of their host-ants is adopted by certain myrmecophiles (Oxysoma, Attaphila and Myrmecophila) and certain parasitic ants (Leptothorax emersoni).
  16. This singular ability of the insect to inhibit the development and growth of its gonads till adult life is not only significant in connection with the development of parasitism within the group, but is also of fundamental importance in the development of colonial life among all the social insects. In the worker castes of these organisms the inhibition of the gonads, except under unusual conditions, is simply prolonged into and throughout adult life. Perhaps in last analysis this inhibition is merely a special manifestation of the extraordinary independence of the insect soma and germ-plasm, as has been so beautifully shown in the castration and transplantation experiments of Oudemans, Meisenheimer, Regen and Kopeć. For a discussion of this subject see my paper, "The Effects of Parasitic and Other Kinds of Castration in Insects," Journ. Exp. Zool, VIII., No. 4, 1910, pp. 377-438, 7 figs.
  17. Within this "margin of vitality" must also be included the reproductivity of the host species. Thus certain ants, like Formica fusca, throughout the north temperate zone, are able to survive the inroads of a number of parasitic ants (Polyergus rufescens, Formica sanguinea, F. rufa, F. exsecta and many allied species), largely on account of its great reproductive powers, coupled with an ability to live in the most diverse physiographic conditions.
  18. In support of this statement the reader may be referred to the following general articles on insect parasitism, written by well-known economic entolologists in our own country during the period characterized by a very exclusive occupation with morphology in our universities: Riley, "Parasitism in Insects," Proceed. Ent. Soc. Wash., II., No. 4, 1893, 35 pp.; Webster, "Insect Parasites," 15 pp. (reprint without date); Osborn, "Insects Affecting Domestic Animals," Bull. No. 5, U. S. Dept. Agric, 1896, 302 pp., 170 figs.; Howard, "A Study in Insect Parasitism," Tech. Ser. No. 5, U. S. Dept. Agric, 1897.
  19. "Das Eindringen von Wurgeln in lebendige Gewebe," Botan. Zeitg., III., 1894, pp. 169-176; "Artificial Parasitism," Botan. Gazette, XXXVIII, 1904, pp. 214-217.
  20. "The Conditions of Parasitism in Plants," Carnegie Inst. Publ., Washington, 1910, 60 pp., 10 pis., 2 text-figs.
  21. Howard and Fiske (loc. cit., p. 204) express a similar opinion when they say that "it is probably true also that among those parasites which are the most closely restricted in their host relationships are to be found those which are the most effective in bringing about the control of their respective hosts. This is primarily due to the fact that a correlation usually exists between the life and seasonable history of such a parasite and some one or more hosts which it is particularly fitted to attack. The existence of a correlation between parasite and host of such intimate character makes possible the continued existence of the parasite independently of alternate hosts, and it is thus enabled to keep pace with the ODe species upon which it is peculiarly fitted to prey when other circumstances are favorable to its increase. Some of the most interesting examples of correlation of this sort which have yet come to attention are to be found among the tachinid parasites of the gypsy moth or the brown-tail moth, and on this account as well as on a purely empirical basis they are now considered much more likely to become important enemies of these hosts than before their characteristics were so well understood."
  22. A very instructive case of such instability in hyperparasitism, or rather superparasitism, is seen in Pteromalus egregius, which was introduced into Massachusetts as a primary parasite of the brown-tail caterpillar. This European parasite, as Fiske has recently shown (Howard and Fiske, loc. cit., p. 267 et seq.) has not only spread over a great area in eastern New England, since it was first liberated in 1906 and 1907, but besides acting as a primary parasite, it may also behave as a secondary, tertiary or quaternary superparasite.