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PSYCHEVOL. XXXVII MARCH, 1930 No.

THE FOOD OF INSECTS VIEWED FROM THEBIOLOGICAL AND HUMAN STANDPOINT

BY CHARLES T. BRUES

Our present-day views concerning human food and nu-trition are in such a state of active revolution, that it mayseem futile to discuss the food of insects on the basis of thefragmentary knowledge we possess, concerning these smallanimals. I have avoided the term nutrition, however, sincefood relates to actual materials and does not necessarilyintroduce chemical and physiological connotations. It is,therefore, clear that "entomological chop suey" might moreadequately, if less elegantly, express the content of my sub-ject matter, provided, of course, that we first Separate andaccurately identify all the disguised components of thisdelicacy. This separation, and identification of insect food-stuffs has, as a matter of fact, been rather thoroughly doneby entomologists and affords the basis for an understandingof at leat some of the principles that underlie the trophicbehavior of insects.

That these peculiarities have determined to a great extentthe evolution and differentiation of insects is very clear,and as I hope to indicate later we must also attribute tothem a very important role among the many factors thathave directed the course of organic evolution since /he timewhen 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

1presented by the retiring president at the annual meeting of theEntomological Society of America at Des Moines, Iowa, Dec.. 28, 1929.Contribution from the Entomological Laboratory of the Bussey Insti-tution, Harvard Uuiversity, No. 331.

2 Psyche [March

food by a very large percentage of the species. This isparalleled 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 a.s path-ogenic bacteria, fungi and worms, it has of course neverbeen questioned that they are important factors in influ-encing the abundance, distribution and, finally, the evolu-tion o their hosts and of competing o.ranisms. The sameis self-evident with respect to parasitic inse.c/.s, includingthose that carry diseases, and the recognition of the roleplayed 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. Suchfacts are so generally appreciated that we must not allowthem to draw our attention at the present time rom theless patent relationships that I wish to discuss.The conventional classification of food habits as first

applied to vertebrate a.nimals, and later extended to oherless familiar groups may be readily applied to insects andwe may thus more or less accurately group them in thefollowing categories which are by no means either clear-cut or mutually exclusive, since they may grade into oneanother or appear in combination in the die of a singlespecies of insect.

O:M:NIVOROUS PANTOPttAGOUSHERBIVOROUS PttYTOPI-IAGOUSPUTRIVOROUS SAPROPItAGOUS

minimivorous microp,hagousfungivorous mycetophagous

CARNIVOROUS ZOOPI-IAGOUSpredatory harpactophagousparasitic biophagous

So far as insects are concerned it is difficult to arrangethese in any linear order nd certainly no single arrange-ment could be made which would indicate the phylogenetiesequence of the different types in the several major groupswhere they occur. Every type exeep the parasitic one isto be found among the mos,t generalized groups of insects.Thus, the omnivorous cockroach, the vegetarian walkingstick, or th predatory dragonfly are to-day emulating theirforebears who feasted likewise in the forests of carbon-iferous times.

1930] The Food of Insects 3

It may seem idle to speculate concerning the most primi-tive type of food habits among the insects, although thereseems good reason to believe that the earliest insects, liketheir somewhat problematical a.ncestors, may have subsistedupon dead or moribund animals in combination with mis-cellaneous plant material, or to speak more concisely, interms of human dietetics, the balanced chop suey rationmentioned a few moments ago.

Such was undoubtedly the diet of the early multitudinouscockroaches that swarmed throughout those ca.rboniferousforests. These very insects are today a prominent feature ofthe entomological fauna of tropical jungles, and, moreo,ver,they have even invaded the overheated tenements of ourgreat cities. The cockro,a.ches, therefore, show three char-aceristic features" a mixed, more or less indiscrimina.tediet; great morphological stability over extended periods oftime, and an adaptability to changing conditions and tointensive competition with other, more. modern types ofinsects.On the ocher hand, purely carnivorous habits are char-

acteristic of several very primitive groups, notably of theearliest dr,agonflies. The predatory habits of these insectsare very pronounced and predatism has attained a wonder-ful degree of perfection among the modern dragonflies.The imagines are admirably fitted for the cap,ure of preywhile on the wing, and the nymphs are even more marvel-ously adapted for the seizure of prey beneath the water inwhich they live. The mechanism peculiar to the nymph andunparalleled elsewhere is a unique, pincers-like, bris,tlyorgan, known a,s the mask formed by the highly modifiedlabium. The form of this structure is so similar throughoutthe Odonata that there can be no doubt tha,t it was char-acteristic of the earliest representatives of the order andperhaps of the ancestral Protodonaa as well. So far then,as structural adaptation is concerned, the dragonflies mustbe rated very high. They have persisted over an extremelylong period with little more tendency toward morphologicalchange or speciation than the cockroaches. Even thoughthe nymphal mouthparts are most exquisitely suited forthe unfailing capture of prey, the diet of individual generaand species has not become highly specialized. We might

4 Psyche [March

almost say that the great efficacy of the mask has madeunnecessary any great specialization in instinct; and, con-sequently, the diet of any species or individual variesgreatly, depending upon circumstances. This statement isabundantly supported by observations made on the diet ofdragonfly nymphs by several entomologists. These showthat there is practically no selection since the imagines ofa single species of dragonfly will devour a grea.tly diversemixture of insects, while the nymphs consume also manysmall crustaceans, other invertebrates, and even someProtozoa.Among the dragonflies, then, a long history with com-

paratively slow evolutionary change is associated with anindiscriminate diet during both the nymphal and reproduc-tive stages.

If we now turn to certain predatory types among some ofthe more recent holometabolous insects we find a conditionstrikingly in contrast to that just described. In the Diptera,or example, there is a series of rather closely related am-ilies, that includes the robber-flies or Asilide, whose me.m-bets are highly predaceous. Although only ragmentarydata are available ior these flies, some species are seen tohave very strong predilections in the choice of prey. Thus,among the large flies oi the genus Proctacan/hus, one speciescaptures almost entirely aculeate Hymenoptera, more thanhalf of its prey consisting of honeybees and hornets, whilethe second greatly prefers small scara.bmid beetles in com-bination with other miscellaneous insects. Other robber-flies are butterfly hunters, but many are more or less gen-eral feeders, and a tabulation shows th.t, although thereis a consistent choice of o.od among species, there is not theclose restriction that prevails among parasitic insects, noramong vegetarian types which we will discuss in a moment.A very high specificity in the selection of prey obtains

among the solitary wasps. With these insects the prey isstung and stored in the nest, where it forms the iood of thedeveloping larva. Thus, the choice of food rests with themother wasp, while the larva obediently eats what is putbefore it, thriving to maturity thereon. Although a verywide range of prey, including spiders and the most diversei:sects is utilized by these wasps, individual species com-


monly restrict their hunting to the capture of a series ofrelated forms or even to a single species. One AmericanAphilanthops stores only queen ants and a related Europeanwasp captures ants also. Our common American mud-dauber wasps collect small spiders of various kinds andcertain crabronids capture flies of a single or of severalspecies. The fixity and persistence of their instincts isshown by the tendency of genera or larger groups to re-strict themselves to related types of prey, and this mayextend to the members of a large family like the Psam-mocharidm, where the spider-storing habit is so general thatthe vernacular name of "spider wasps" has been bestowedupon them by common consent. In another group, stagesthrough which the change from a somewhat indiscriminatediet to a specific one has taken place are still preserved.Thus, in the genus Sphex (Chlorion) some species store avariety of Orthoptera in their nests, o/hers only a few, andfinally some only one. As we shall see in a moment thiscondition prevailing among the solitary wasps is whollyanalagous to that which obtains among phytophagousinsects.The tendency among these diverse types of predatory in-

sects is clearly toward a restriction of the dietary although,e cannot consistently detect any orderly arrangementwhereby a relationship of predators implies to. any greatextent a similarity of prey. Sufficient evidence has beenpresented, however, to show that we cannot make any bro’adgeneralizations. Thus predaceous insects do not exhibit theclose correlations characteristic of parasites nor of vege-tarian insects. At this point, we must emphasize the factthat many predatory insects have narrowed down their foodrelations to a point where their direct contacts with. theenvironment are restricted to certain definite componentsof the fauna of which they themselves form an integralpart. The significance of these facts may be best understoo.dafter we have examined the food relations of vegetarianinsects.

Insects that feed on plants are far and away the mostimportant series to illustrate the adherence of species orlarger groups to restrict diet. On account of their complexrel,ationship.s toward these plants directly, and indirectly

6 Psyche [March

through them to other animals, coupled with their greatabundance and diversity, they are of extreme interest. Theirecological 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 generalbiological or evolutionary standpoint they are of peculiarinterest for it is this vast horde of vegetarian species (forthey include about half of the living kinds of insects) thathave made their influence felt over the long lapse of geo-logical time since these types became highly differentiatedduring the periods preceding the tertiary. The chronologyof this process, a,t least with regard to specific ood rela-tions, is difficult to determine, but taxonomic groups similarto those of today were so well established in the eocene andoligocene that we can rest assured that their food rela,tionswere already equally complex at that time. Thus, the timeduring which the factors introduced by these insects havebeen active in affecting the evolution of other animals andplants is much more extended than that included in theperiod just mentioned. As I have shown previously, thereis good reason to believe that the differentiation of feedinghabits among phytophagous Coleopter,a was well under waywhile that o the Lepidoptera was just beginning at thetime 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 theLepidoptera as this is well known and I have already dealtextensively with it elsewhere. Briefly, we may say that themembers of this order may be considered as forming twoor three groups with respect to specificity of food. Theseare: first, those which utilize a very considerable and notnecessarily relai/ed 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 restricteddietary that includes a few, usually related, species; andfinally, some that are confined to a single, plant host or toseveral very closely related and genetically similar membersof a single genus. Again, these categorical divisions areonly 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


the insects concerned as polyphagous, oligophagous andmonophagous, respectively.The labors of economic entomologists have gone far to-

ward an elucidation of the interactions of these severaltypes of food-habits as they determine the competition forfood 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 thisquestion is seen at once to involve the whole terrestrial flora.

Several factors concerned in these relationships betweeninsects and plants may be considered separa.tely. The effectof insect feeding on the flora is by no means the same in thecase of polyphagous, oligophagous and monophagous spe-cies. With the former a long series of plants suffer to amore 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 alsoother plants suffer, but not to an equal extent. This meansthat there is a simultaneous reduction in the abundance ofa number of different plants, and an opportunity is offeredfor many others to increase, at least temporarily, whilemany 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 toa major extent. We can also see how such feeding mightactually cause the extinction of certain rare or poorlyadapted 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 ofplants be affected there as a very considerable opportunityfor 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 ofcertain plants might result from the feeding of oligopha-gous species, although the chances for such an occurrenceare less than those noted above in the case of polyphagousinsects since a great reduction in one. of a few food plantswill at once considerably reduce the food supply. This isthen immediately reflected in a lesser abundance of the in-

8 Psyche [March

sect, and injury decreases. In general, therefore, the ieedingo2 oligophagous insects does not involve simultaneous fluc-tuations in a considerable number o:2 plants, especially ifthose concerned are not domina.nt 2o.rms, and likewise, asmaller number of species of insects is affected directly.The fluctuations that may occur among associated insectsare to a greater extent in an inverse ratio rather than ina direct one.Monophagous species present a very remarkable series

of phenomena. Aside 2rom any parasites they may supportor predators that they may 2eed, their relations to the livingenvironment are entirely restricted to contacts through themedium of the host pla.nt. They can never become so abun-dant as to rise up and destroy it, since or obvious reasonstheir fluctuations in abundance /rail very closely those oftheir host plants. Therefore, we may never attribute theextinction of any plant, even in a restricted regipn, exclu-sively to the activities of a monophagous insect. Otherplants are affected to a varying extent, dependent mainlyupon the rarity or abundance of the host plant. If it be adominant species, its fluctuations increase or decrease thestruggle for exist.ence among competing plants; i it be arare species, this influence upon a series of other plantsis negligible. It also affects a number of associa.ted insectswhich feed upon the same food plant. The number of /heseis, of course, extremely variable, but careful studies of plantfaunm indicate that dominant types of plants support sur-prisingly large numbers of vegetarian insects mounting intohundreds o.f species in the case of common types such asvillows, figs, aks and maize, although the average orplants in general falls far below this mark. Certain plantswhich produce poisons or violently irritating substances,(like our common American poison-ivy), enjoy comparative,but by no means complete immunity rom injury by phy-tophagous insects.

In regard to their relations with the living environment,we may say that monophagous insects live a life apart. Theirassociation with plants is such that the vicissitudes of lifefor both members of the pair are greatly reduced on ac-count of the lesser number of variable factors that affecteach. So far as abundance of fluctuation in numbers they


are mutually adaptive. The instincts determining food se-lection are firmly fixed in the germ-plasm and the insect isdoomed to ieed to the end of its days on beans, cabbage,yeast or what-not, unless some fortunate shift or mutationof instinct may add pork to the beans, or perchance combinehops and malt wii/h the yeast ration. Such persistence overlong periods during which whole groups of insects andplants have evolved in mutual adaptation seem only to beexplained on the basis of instinctive behavior. This viewwhich I have upheld in the past has been recently questionedby some, who would place the matter upon a purely physi-ological ba,sis, but I cannot see that there is convincing evi-dence to support this conclusion or to controvert my owncontentions that we are dealing with persistent instinctsrather than wii/h digestive necessities.With the foregoing considerations on phytophagous in-

sects as a ha.sis it is possible to draw certain conclusions ofa general nature which indicate some of the ways in whichthe development of oligophagy and monophagy has in-fluenced the evolution of the higher plants as well a.s thatoi’ the i.nse.cts themselves. Due to its tendency to reduce thechances of extinction of plant species whose existence mightbe jeopardized by an abundance of polyphagous insects wecan see thai/ it has tended to increase the diversity of theflora. The development of many mutual adaptations ofplants with both monophagous and oligopha.gous insects aredependent upon the specific food habits of the insects con-cerned and it is thus clear that we must attribute to thedevelopment of these instincts many of the remarkablemorphological modifications seen in both plants and in-sects. In the present state of our knowledge, at least, thereis no other causal explanation for their existence. Sincespeciation and the rapidity of evolutionary change in gen-eral .are highly correlated with adaptive modifications, wemust attribute to the phenomena of restricted food a highlyimportant 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 insectsand entomophilous flowers and his ini/erpretation of themutual modifications of insects and flowers has long since

10 Psyche [March

become solidly entrenched among the unassailable biologicaldoctrines, after having 2urther engaged the attention o.f ascore of later naturalists. I mention it here not 2or discus-sion, but only to show the similarity to to views just ex-pressed concerning phytoph,agous insects and their food-plants.

Really no time remains for the discussion of the othertypes of ood habits among insects which I had hoped tomention in connection with their biological interest. Theywould serve to clarify the statements already made, butsince they might also cloud the issue somewhat and sincethey show how thoroughly the insects have exploited theworld’s 2ood supply they are more appropriately dealt within connection with my concluding remarks on insect oodas viewed 2rom the huma.n standpoint.From purely human standpoint, we must regard every-

thing as either beneficial or harmful, unless it appear to beutterly unimportant or indifferent and the scientific mindvill not admit the third possiblity. As entomologists areprone to look upon insects in this light we may view themthus at the moment.

INJURIOUS INSECTS

harmful to man directlyharmful to useful plantsharmful to useful animals

a. harmful to useful insects

BENEFICIAL INSECTS

destroyers of injurious insectsdestroyers of undesirable plantsdestroyers o.f obnoxious substancesproducers of useful substances.

I think this classification reflects the usual attitude to-ward the economic relations of insects, and it serves wellto emphasize the fact that the importance of every speciesis gauged by what it harms, injures or destroys, with thesole exception of the small handful of "producers" like the

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 greatesthuman interest, are those which serve to destroy otherthings. As the ultimate purpose of the human species isto destroy and make over the face of nature, quite naturallyinsects and man are at once both in accord and conflict,and our most competent practical entomologists predict abattle to the death between insects and man. Unfortunatelyfor ourselves, man has proceeded to change the face ofnature as rapidly and completely as possible. In so doing,he has ound his progress seriously impeded by an unfor-seen increase in the numbers of insects that eed upon agri-cultural, horticultural and forest crops, combined with anequally unexpected migration or spread of many phytopha-gous species into regions where they did not previouslyoccur. There is no need to press this point for we all realizethat this is a serious situation that confronts agriculture,horticulture and forestry today. It is equally clear that thisdistressing condition has been brought about mainly bytwo factors inherent in our prevailing mode and philosophyof life. One is the growing tendency throughout the world,and particularly in America, to devote more and more. ex-tensive areas to the propagation of easily managed cropsthat give promise of large pecuniary returns. This matterthen goes deep into the fabric of our commercial li2e. Theother factor concerned is the unprecedented increase in thebulk and variety of vegetables, fruits and food-stuffs of allkinds that are shipped here and there in all directions., aftento the uttermost corners of the earth wherever they may bedisposed of at a profit. If, as generally occurs, there is greatlocal over-production of certain foods, the urge for widerdistributional opportunities is so powerful that it mustneeds invoke the aid of advertising propaganda and everyother known method that may be put to the tune of modernjazz. No one has ever suggested the curtailment of suchshipping on economic grounds. This matter is, of course,likewise not a suitable subject for unbiased discussion ex-cept in purely academic circles, since it is too deeply con-cerned with the business of transportation, with advertis-ing, with the love of luxury and with other matters of vastand immediate pecuniary importance.

12 Psyche [March

Having thus bared a very pessimistic attitude and ex-posed to view what you have doubtless recognized as theclear, clinical picture of an inferiority complex, let us re-turn to some purely entomological phases of the matter.How do the several types of food-habits among vegetarianinsects affect their pracitical importance in relation to thehuman food supply? It is generally conceded that mono-phagous and oligophagous types are the ones most destruc-tive to cultivated crops. We can at once recall a long seriesof such species, the potato-beetle, the codling moth, theoriental peach moth, and many others whose importance isdue to the fact that they unerringly pick out valuable andwidely cultivated agricultural plants. On the other hand,many with a considerable range of food-plants, the boll-worm, the corn-.borer, the Mediterranean fruit fly, the melonaphid, the red scale and the like are very destructive, butquite generally less so on the whole, in spite of the muchgreater variety which may feed upon a particular speciesof plant. General feeders like the various types of locusts,army worms, etc., although very conspicuously destructivein some regions and on certain occasions, are in the longrun less generally dreaded by agriculturists. To return thento our earlier discussion of monophagy and oligophagy, wemust admit, without further ado, that our present civiliza-tion could well dispense with this interesting evolutionaryphenomenon, since it is a gift of Nal/ure that serves to in-crease injury to cultivated plants, at least under modernagricultural conditions. We might return the gift withthanks, but so far, science has been unable to write a suf-ficiently polite or forceful note to accompany the transfer.

There is just one more matter relating to these destruc-tive insects which I hesitate to mention as it is so familiarto all of us. It was one of the first choice bits ot’ informationwe acquired as students and we have religiously passed iton to our students ever since. The most destructive insectpests are those that have spread from their original habitatinto some other faunal region. The reasons for this areusually said to be well understood on the basis of preda.toryand parasitic control, but there is much to suggest thatother factors remain still to be elucidated.


Since human behavior is conventionally regulated by pro-hibitions, punishments and legislative action of endless va-riety, it is quite natural that the immigration of insectsshould have come to the attention of law-mkers as a fitsubject or similarly paternal consideration. The protectiono the human population 2rom communicable diseases byrestricting the movements oi diseased persons has been sosatisfactorily effected by qu,arantine measures that exclu-datory regulations have been instituted to protect countriesfrom invasion by insects rom other regions, without aclear statement that the two situations are neither identicalnor essentially similar. Thus, such regulations have beenbased upon the inspection o2 host plants or other ma.terials.The success of these has not been all that might have beenhoped and there is a wide divergence of opinion as towhether the results attained are in any sense commensuratewith the great expense and hardships entailed. It is quiteclear that the absolute exclusion o2 insects is a much moredifficult undertaking than the interception of cases of hu-man disease. Human beings are dealt with legally andotherwise as individuals and besides, the success o publichealth measures does not depend upon absolute exclusion’or all time. There is no question that such measures mustaid somewhat to delay the introduction of insect pests wheresome natural geographical barrier to animal or plant mi-gration exists. As time goes on, as transportation increasesand speeds up and as movement becomes our main interest;the poor old oceans, mountains and deserts are rapidlylosing their importance as separating aunas and floras.In our own country during the past ew years there has alsobeen an increasing movement to prevent the spread o2 par-ticular species of insects between states or other artificiallylimited areas where absolutely no natural barriers exist.This in itself is a most commendable endeavor when it canbe carried on unostentatiously and at reasonable expendi-ture, since it serves to delay the otherwise extremely rapidnaturalization of insects over contiguous areas. In someinstances it is obvious that such quarantines may becomeoppressive and entail expenditures and economic conse-quences to communities that are not at all warranted by anyadvantages that might reasonably be expected to accrue

14 Psyche [March

rom them. This is well illustrated by the supreme effortsmade to curb the spread of the Mediterranean fruit fly fol-lowing the recent discovery of its establishment in Florida.Many persons were even led to believe that the extermina-tion of this pest in the United States was a foregone con-clusion instead of hoped-for miracle. Certainly long ex-perience in connection with the establishment o exotic in-sect pests of cultivated plants holds out little prospect oftheir eradication. Another unfortunate circumstance in thisconnection is the great menace of repeated introductions ofthe same insect. I do not wish to pose as a critic of the leg-islative activities of entomologists but eel that it is ourduty to view such matters in an unbiased attitude and toshy clear of the idea that ll evils may be remedied by theenactment of laws and the expenditure of money. The useo the quarantine has increased at such a rapid rate thatwe should be sure o our ground before allowing it to usurpthe foreground of entomological practice. Another gravedanger lies in the fact that perfectly sane scientific pro-nouncements may be easily perverted by legislative enact-ment and subsequent enforcement by persons ar less expertthan the entomological authorities who formulated them.

Certainly the importance of these problems and the threatwhich they hold over our future comfort and welfare de-mand that we as entomologists approach them in an al-truistic spirit.

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