C. T. Brues.
The Food of Insects Viewed from the Biological and Human Standpoint.
Psyche 37:1-14, 1930.

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PSYCHE
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VOL. XXXVII MARCH, 1930
No. 1
THE FOOD OF INSECTS VIEWED FROM THE
BIOLOGICAL AND HUMAN STANDPOINT 1
Our present-day views concerning human food and nu- trition are in such a state of active revolutiony that it may seem futile to discuss the food of insects on the basis of the fragmentary knowledge we ' possess concerning these small animals. I have avoided the term nutrition, however, since food relates to actual materials and does not necessarily introduce chemical and physiological connotations. It isy therefore? clear that "entomological chop suey" might more adequately? if less elegantly? express the content of my sub- ject matter? provided? of course? that we first geparate and accurately identify all the disguised components of this delicacy. This separation, and identification of insect food- stuffs has, as a matter of fact? been rather thoroughly done by entomologists and affords the basis for an understanding of at least some of the principles that underlie the trophic behavior of insects.
That these peculiarities have determined to a great extent the evolution and differentiation of insects is very clear, and as I hope to indicate later we must also attribute to them a very important role among the many factors that have directed the course of organic evolution since the time when insects became a dominant figure in the living world. The most striking feature of the diet of insects is the high degree of specificity which exists in the selection of 'Presented by the retiring president at the annual meeting of the . Entomological Society of America at Des Moines, Iowa, Dee. 28, 1929. Contribution from the Entomological Laboratory of the Bussey Insti- tution, Harvard Uuiversity, No. 331.




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2 Psyche [March
food by a very large percentage of the species. This is
paralleled almost nowhere else in the animal or plant king- doms, with the notable exception of certain parasitic or- ganisms. In the case of these parasitic types, such as path- ogenic bacteria, fungi and worms, it has of course never been questioned that they are important factors in influ- encing the abundance, distribution and, finally, the evolu- tion of their hosts and of competing organisms. The same is self-evident with respect to parasitic insects, including those that carry diseases, and the recognition of the role played by entomophagous parasites has led to the develop- ment of the method of biological control that we have ap- plied with success to the reduction of noxious insects. Such facts are so generally appreciated that we must not allow them to draw our attention at the present time from the less patent relationships that I wish to discuss. The conventional classification of food habits as first applied to vertebrate animals, and later extended to other less familiar groups may be readily applied to insects and m7e may thus more or less accurately group them in the following categories which are by no means either clear- cut or mutually exclusive, since they may grade into one another or appear in combination in the diet of a single species of insect.
OMNIVOROUS = PANTOPHAGOUS
HEREXVOROUS = PHYTOPHAGOUS
PUTRIVOROUS = SAPROPHAGOUS
minimivorous = microphagous
fungivorous = mycetophagous
CARNIVOROUS ZOOPHAGOUS
predatory = harpactophagous
parasitic = biophagous
So far as insects are concerned it is difficult to arrange these in any linear order and certainly no single arrange- ment could be made which would indicate the phylogenetic sequence of the different types in the several major groups where they occur. Every type except the parasitic one is to be found among the most generalized groups of insects. Thus, the omnivorous cockroach, the vegetarian walking stick, or the predatory dragonfly are to-day emulating their forebears who feasted likewise in the forests of carbon- if erous times.




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19301 The Food of Insects 3
It may seem idle to speculate concerning the most primi- tive type of food habits among the insects, although there seems good reason to believe that the earliest insects9 like their somewhat problematical ancestors, may have subsisted upon dead or moribund animals in combination with mis- cellaneous plant material, or to speak more concisely, in terms of human dietetics, the balanced chop suey ration mentioned a few moments ago.
Such was undoubtedly the diet of the early multitudinous cockroaches that swarmed throughout those carboniferous forests. These very insects are today a prominent feature of the entomological fauna of tropical jungles, and, moreover9 they have even invaded the overheated tenements of our great cities. The cockroaches, therefore, show three char- acteristic features: a mixed, more or less indiscriminate diet ; great morphological &ability over extended periods of time? and an adaptability to changing conditions and to intensive competition with other, more modern types of insects.
On the other hand, purely carnivorous habits are char- acteristic of several very primitive groups, notably of the earliest dragonflies. The predatory habits of these insects are very pronounced and predatism has attained a wonder- ful degree of perfection among the modern dragonflies. The imagines are admirably fitted for the capture of prey while on the wing, and the nymphs are even more marvel- ously adapted for the seizure of prey beneath the water in which they live. The mechanism peculiar to the nymph and unparalleled elsewhere is a unique, pincers-like? bristly organ, known as the mask formed by the highly modified labium. The form of this structure is so similar throughout the Odonata that there can be no doubt that it was char- acteristic of the earliest representatives of the order and perhaps of the ancestral Protodonata as well. So far then, as structural adaptation is concerned, the dragonflies must be rated very high. They have persisted over an extremely long period with little more tendency toward morphologiml change or specitation than the cockroaches. Even though the nymphal mouthparts are most exquisitely suited for the unfailing capture of prey, the diet of individual genera and species has not become highly specialized. We might



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almost my that the great eEmcy of the -k has made umexssary any great. specialization in instinct; and, con- sequently, the diet of my species or individual varies great1 y, depending upon circumstances. This sbtement is abundant1y mpported by observations made on the diet of dragonfly nymphs by several entorno1ogists. These show that there is practically no selection since the imagines of a single species of dragonfly wi11 devour a greatly diverse mixture of insects, while the nymphs consume also many smll crustaceans, other invertebrates, and even some Protozoa,
Among the dragonflies, then, a long history with com- paratively dow evolutionary change is associated with an indiscrimhate diet during both the nymphal and reproduc- tive stages.
If we now turn to certain predatory typea among some of the more recent holome~bol~u~ insects we find a condition stxikingly in contrast to that just described. In the Diptera, for example, there is a series of rather clmely re!ated fam- ilies, that inchdes the robber-flies or Asilid~, whose mem- bers are highly predaceous. Although only fragmentary dab are available for these flies, some species are seen to have very strong predilections in the choice of prey. Thus, among the large flies of the gems Proctacmthus, one species captures almost entirely acdeate Hymenoptera, more than half of ita prey consisting of honeybees- and hornets, while the mcond greatly prefers small scaxab~id beetles in com- bination with other m~scellaneou~ insects. Other robber- flies are butterfly huntem, but many are more or less gem eral feeders, and zi tabulation shows that? dthough there is a consistent choice of food among species, there is not the dose re&rktion that prevails among parasitic insects, nor among vegetarian types which we wdl discuss in a moment, A very high specificity in the sdection of prey obtains among the soliky wasps, With them insects the prey is stung and stored in the nest, where it forms the food of the developing larva. Thus? the choice of- fwd rests with the mother wasp, while the larva ohedientJy eats what is put before it, thriving to maturity thereon. Although a very wide range of prey, including spiders and the most diverse imects is uti1ized by these wasps, individual species corn-



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19301 The Food of Insects 5
monly restrict their hunting to the capture of a series of related forms or even to a single species. One American Aphilanthops stores only queen ants and a related European wasp captures ants also. Our common American mud- dauber wasps collect small spiders of various kinds and certain crabronids capture flies of a single or of several species. The fixity and persistence of their instincts is shown by the tendency of genera or larger groups to re- strict themselves to related types of prey? and this may extend to the members of a large family like the Psam- niocharid~~ where the spider-storing habit is so general that the vernacular name of "spider wasps?? has been bestowed upon them by common consent. In another group? stages through which the change from a somewhat indiscriminate diet to a specific one has taken place are still preserved. Thus, in the genus Sphex (Chlorion) some species store a variety of Orthuptera in their nests, others only a few, and finally some only one. As we shall see in a moment this condition prevailing among the solitary wasps is wholly analagous to that which obtains among phytophagous insects.
The tendency among these diverse types of predatory in- sects is clearly toward a restriction of the dietary although m7e cannot consistently detect any orderly arrangement whereby a relationship of predators implies to any great extent a similarity of prey. Sufficient kvidence has been presented? however? to show that we cannot make any broad generalizations. Thus predaceous insects do not exhibit the close correlations characteristic of parasites nor of vege- tarian insects. At this point? we must emphasize the fact that many predatory insects have narrowed down their food relations to a point where their direct contacts with the environment are restricted to certain definite components of the fauna of which they themselves form an integral part. The significance of these facts may be best understood after we have examined the food relations of vegetarian insects.
Insects that feed on plants are far and away the most impodant series to illustrate the adherence of species or larger groups to restrict diet. On account of their complex relationships toward these plants directly? and indirectly



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6 Psyche [March
through them to other animals, coupled with their great abundance and diversity, they are of extreme interest. Their ecological relations are far reaching for they extend, ten- tacle-like, into the innermost corners of every type of terres- trial, aerial, or fresh-water environment. From the general biological or evolutionary standpoint they are of peculiar interest for it is this vast horde of vegetarian species (for they include about half of the living kinds of insects) that have made their influence felt over the long lapse of geo- logical time since these types became highly differentiated during the periods preceding the tertiary. The chronology of this process, at least with regard to specific food rela- tions, is difficult to determine, but taxonomic groups similar to those of today were so well established in the eocene and oligocene that we can rest assured that their food relations were already equally complex at that time. Thus, the time
during which the factors introduced by these insects have been active in affecting the evolution of other animals and plants is much more extended than that included in the period just mentioned. As I have shown previously, there is good reason to believe that the differentiation of feeding habits among phytophagous Coleoptera was well under way while that of the Lepidoptera was just beginning at the time when the modern types of trees appeared on the earth. There is no need to attempt at the present time any de- tailed account of the specificity of food selection among the Lepidoptera as this is well known and I have already dealt extensively with it elsewhere. Briefly, we may say that the members of this order may be considered as forming two or three groups with respect to specificity of food. These are: first, those which utilize a very considerable and not necessarily related series of food plants, occasionally a hun- dred or more in number, like the cecropia moth or the leop- ard moth; second, those having a much more restricted dietary that includes a few, usually related, species; and finally, some that are confined to a single plant host or to several very closely related and genetically similar members of a single genus. Again, these categorical divisions are only relative; but experience shows them to be very con- venient, and we may unquestionably regard them as suc- cessive phylogenetic stages. We may conveniently refer to



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19301 The Food of Insects 7
the insects concerned as polyphagous, oligophagous and monophagous, respectively.
The labors of economic entomologists have gone far to- ward an elucidation of the interactions of these several types of food-habits as they determine the competition for food among insects and the devious ways in which they in- fluence the bioccenotic relations of insect food plants. And, since no plants appear to be immune to insect attack this question is seen at once to involve the whole terrestrial flora. Several factors concerned in these relationships between insects and plants may be considered separately. The effect of insect feeding on the flora is by no means the same in the case of polyphagous, oligophagous and monophagous spe- cies. With the former a long series of plants suffer to a more or less equal degree. Thus, with grasshopper out- breaks there is general injury to all kinds of vegetation; with the gypsy moth a considerable series of trees and also other plants suffer, but not to an equal extent. This means that there is a simultaneous reduction in the abundance of a number of different plants, and an opportunity is offered for many others to increase, at least temporarily, while many competing insects decline due to a lessened food sup- ply. Thus, in brief, outbreaks or fluctuations among poly- phagous species involve many other insects and plants to a major extent. We can also see how such feeding might actually cause the extinction of certain rare or poorly adapted plants.
The feeding of oligophagous insects results in the injury to a greatly restricted series of plants and has, of course, no direct effect upon any others. If dominant species of plants be affected there as a very considerable opportunity for many rarer forms to increase, while if a reverse con- dition prevails and the scarcer forms are affected, the in- fluence upon the remainder of the competing flora is negli- gible. Incidentally we must notice that the extinction of certain plants might result from the feeding of oligopha- gous species, although the chances for such an occurrence are less than those noted above in the case of polyphagous insects since a great reduction in one of a few food plants will at once considerably reduce the food supply. This is then immediately reflected in a lesser abundance of the in-



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8 Psyche [March
sect, and injury decreases. In general, therefore, the feeding of oligophagous insects does not involve simultaneous fluc- tuations in a considerable number of plants, especially if those concerned are not dominant forms, and likewise, a smaller number of species of insects is affected directly. The fluctuations that may occur among associated insects are to a greater extent in an inverse ratio rather than in a direct one.
Monophagous species present a very remarkable series of phenomena. Aside from any parasites they may support or predators that they may feed, their relations to the living environment are entirely restricted to contacts through the medium of the host plant. They can never become so abun- dant as to rise up and destroy it, since for obvious reasons their fluctuations in abundance trail very closely those of their host plants.
Therefore, we may never attribute the
extinction of any plant, even in a restricted region, exclu- sively to the activities of a monophagous insect. Other plants are affected to a varying extent, dependent mainly upon the rarity or abundance of the host plant. If it be a
dominant species, its fluctuations increase or decrease the struggle for existence among competing plants; if it be a rare speciea, this influence upon a series of other plants is negligible. It also affects a number of associated insects which feed upon the same food plant. The number of these is, of course, extremely variable, but careful studies of plant fauns indicate that dominant types of plants support sur- prisingly large numbers of vegetarian insects mounting into hundreds of species in the case of common types such as willows, figs, oaks and maize, although the average for plants in general falis far below this mark. Certain plants which produce poiaona or violently irritating substances, (like our common American poison-ivy) , enjoy comparative, but by no means complete immunity from injury by phy- tophagous insects.
In regard to their relations with the living environment, we may say that monophagous insects live a life apart. Their association with plants is such that the vicissitudes of life for both members of the pair are greatly reduced on ac- count of the lesser number of variable factors that affect each. So far as abundance of fluctuation in numbers they



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19301 The Food of Insects 9
are mutually adaptive. The instincts determining food se- lection are firmly fixed in the germ-plasm and the insect is doomed to feed to the end of its days on beans, cabbage, yeast or what-not, unless some fortunate shift or mutation of instinct may add pork to the beans, or perchance combine hops and malt with the yeast ration. Such persistence over long periods during which whole groups of insects and plants have evolved in mutual adaptation seem only to be explained on the basis of instinctive behavior. This view which I have upheld in the past has been recently questioned by some, who would place the matter upon a purely physi- ological basis, but I cannot see that there is convincing evi- dence to support this conclusion or to controvert my own contentions that we are dealing with persistent instincts rather than with digestive necessities.
With the foregoing considerations on phytophagous in- sects as a basis it is possible to draw certain conclusions of a general nature which indicate some of the ways in which the development of oligophagy and monophagy has in- fluenced the evolution of the higher plants as well as that of the insects themselves. Due to its tendency to reduce the chances of extinction of plant species whose existence might be jeopardized by an abundance of polyphagous insects we can see that it has tended to increase the diversity of the flora. The development of many mutual adaptations of plants with both monophagous and oligophagous insects are dependent upon the specific food habits of the insects con- , cerned and it is thus clear that we must attribute to the development of these instincts many of the remarkable morphological modifications seen in both plants and in- sects. In the present state of our knowledge, at least, there is no other causal explanation for their existence. Since speciation and the rapidity of evolutionary change in gen- eral are highly correlated with adaptive modifications, we must attribute to the phenomena of restricted food a highly important role in determining the trend of these evolution- ary processes.
A relationship similar to the one just given was early noticed by Darwin in connection with anthophilous insects and entomophilous flowers and his interpretation of the mutual modifications of insects and flowers has long since



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10 Psyche [March
become solidly entrenched among the unassailable biological doctrines, after having further engaged the attention of a score of later naturalists. I mention it here not for discus- sion, but only to show the similarity to to views just ex- pressed concerning phytophagous insects and their food- plants.
Really no time remains for the discussion of the other types of food habits among insects which I had hoped to mention in connection with their biological interest. They would serve to clarify the statements already made, but since they might also cloud the issue somewhat and since they show how thoroughly the insects have exploited the world's food supply they are more appropriately dealt with in connection with my concluding remarks on insect food as viewed from the human standpoint.
From a purely human standpoint, we must regard every- thing as either beneficial or harmful, unless it appear to be utterly unimportant or indifferent and the scientific mind will not admit the third possiblity. As entomologists are prone to look upon insects in this light we may view them thus at the moment.
harmful to man directly
harmful to useful plants
harmful to useful animals
a. harmful to useful insects
destroyers of injurious insects
destroyers of undesirable plants
destroyers of obnoxious substances
producers of useful substances.
I think this classification reflects the usual attitude to- ward the economic relations of insects, and it serves well to emphasize the fact that the importance of every species is gauged by what it harms, injures or destroys, with the sole exception of the small handful of "producers" like the



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19301 The Food of Insects 11
honey bee, silkworm, etc. This can only mean that, practic- ally, the activities of insects that appear to be of greatest human interest, are those which serve to destroy other things. As the ultimate purpose of the human species is to destroy and make over the face of nature, quite naturally insects and man are at once both in accord and conflict, and our most competent practical entomologists predict a battle to the death between insects and man. Unfortunately for ourselves, man has proceeded to change the face of nature as rapidly and completely as possible. In so doing, he has found his progress seriously impeded by an unfor- seen increase in the numbers of insects that feed upon agri- cultural, horticultural and forest crops, combined with an equally unexpected migration or spread of many phytopha- gous species into regions where they did not previously occur. There is no need to press this point for we all realize that this is a serious situation that confronts agriculture, horticulture and forestry today. It is equally clear that this distressing condition has been brought about mainly by two factors inherent in our prevailing mode and philosophy of life.
One is the growing tendency throughout the world,