fine the facics, or general aspects, of the different kinds. They were trial classifications and were naturally ex- Progress in Insect tremely artificial. They brought to- gether things that were in no sense re- Classification lated and separated widely forms that belonged close together. With the C. F. W, Muesebeck growth of knowledge about insects and the rapid increase in the number of known kinds, however, the search for The accurate identification of an new characters usable in the develop- insect is the key to all past recorded ment of more satisfactory classifica- experience with that species. Without tions was intensified. Methods and it, costly mistakes may be made in the concepts are improving steadily; the application of control measures, in- result is that insect taxonomists grad- effective or unjust quarantine prac- ually are producing in their classifica- tices may be instituted, or much work tions an interpretation of insect life may be unnecessarily duplicated. If that is much closer to existing facts there were only a few hundred, or even than any of the arrangements previ- a few thousand, different kinds of in- ously developed. sects it would not be very difficult for In the efforts to attain the goal of an entomologist to learn to recognize having classifications in accord with them all and to call their names—but the natural relationships of insects, one nearly 700,000 different kinds have has to take into account not only ana- been described and named, and it is tomical characteristics but also facts estimated that at least twice that num- pertaining to the physiology, biology, ber remain to be identified. distribution, ecology, and sometimes Obviously it is quite hopeless there- cytology,-of the species. Sometimes, in- fore to determine what a given insect deed, anatomical distinctions arc lack- really is without the help of some or- ing or at least are not evident, although derly arrangement or classification of conspicuous differences have been ob- all the known kinds. To be sure, a com- served in the life habits of the insect paratively small number of common populations in question—differences in and distinctive insects will always be time of appearance, food preferences, readily recognizable without special method of hibernation, or even in reac- aids, but the vast majority can only be tion to certain insecticides. identified by the skillful use of keys, Such facts, when known, are usually descriptions, and other guides that re- indicators of fundamental distinctions sult from the painstaking research of between forms that at first appeared to many specialists. It is this research in be identical. They suggest that a re- classification that makes definite iden- study of series of specimens may reveal tification possible. How accurate and structural differences formerly over- complete the identification will be looked, and often it does. That was so depends on how thorough and critical with the screw-worm flies, the Euro- the research has been. pean spruce sawflies, and the Califor- Classification of living things is an nia red scale and the yellow scale of effort to interpret nature. It attempts citrus, among others. The two screw- to bring together the kinds that arc worm flies were long regarded as a alike and closely related and to sepa- single species that fed sometimes as a rate those that are unlike and unre- true parasite on warm-blooded ani- lated. The earliest classifications of mals, including man, and sometimes as insects were based largely on habits a scavenger upon dead animals. These and habitats and on certain gross ana- differences in feeding habits led tax- tomical features that contribute to dc- onomists to a comparative study of flies 56 Progress in Insect Classification 57 reared from larvae of the two habit tain pests of stored products, ranges types, and to the discovery that the in color from completely yellow to flies from the two sources could, after wholly black. all, be distinguished on the basis of On the other hand, all the moths of anatomical differences and repre- all the species in the genus Rupela are sented two entirely different species. without exception entirely white. One That information resulted in an individual may be five times as large as abrupt change in control procedure another fully matured specimen of the against the parasitic form. With the same species. In another species, per- spruce sawflies it was a cytological haps a closely related one, the speci- study that established the distinctness mens may be of rather uniform size. of two species long regarded as one. Details of sculpture or of wing vena- In the case of the red and yellow scales tion may be strikingly constant or may of citrus, differences in susceptibility vary widely. It is important to deter- to attack by certain parasites and dif- mine the range of variability in each ferences in location on infested trees instance, but that requires large num- eventually led to the discovery of struc- bers of specimens and these are not tural differences by which the two always to be had. Seldom, therefore, species could be identified. can a character that is employed in a By the utilization, then, of all avail- key be considered absolute, and no key able information, taxonomists are at- can be regarded as more than a tem- tempting to correlate behavior and porary guide to identification. It will other life characteristics of insects with inevitably require modification, or anatomical features, since it has come even complete recasting, as knowledge to be realized that only in this way can of the particular group involved in- sound, natural classifications be de- creases; its usefulness will depend to veloped. In the formation of keys, no small extent on the aptitude, experi- which are the guides to identification ence, and perhaps even intuition of the and which reflect the judgment of user. taxonomists with respect to relation- Dimorphism among adult insects of ships, however, it is necessary to de- the same species is a common phe- pend on the use of physical character- nomenon. It is often a cause of serious istics of the insects. Only those are difficulty in the development of classi- always definitely determinable from fications. The winged sex forms in the specimens themselves. Therefore termites and ants, for example, bear the principal efforts of the research little resemblance to the wingless work- taxonomist are necessarily directed to- ers of the same species ; in the mutillid ward the search for physical peculi- wasps, which are popularly and inac- arities, however small, that seem likely curately known as velvet ants, the to be relatively constant and more or winged males are so unlike the wing- less distinctive. That that is no simple less females that their identity can only task must be obvious from the enor- be established by biological associa- mous number of known kinds and the tion. That is true also of the canker- continued addition to that number of worm moths, in which the female is a 10,000 or more new species annually. grublike ^gg sac but the male is a nor- A key that will infallibly lead the user mal winged moth. Such striking caste to the correct name for a given insect or sexual dimorphism occurs in vari- can rarely be constructed even for a ous sections of all the major insect comparatively small group of insects. groups. Furthermore, since the biologi- Few characteristics are absolutely fixed, cal association of conspicuously differ- and the extent and direction of the ent castes or sexes of the same species variation are themselves extremely var- may be difficult and slow, it sometimes iable. Bracon hebetor, an abundant happens that the male and female of a and widely distributed parasite of cer- single species are long treated as two 58 Yearbook of Agriculture 1952 distinct things and are known under group of plant pests comprising the different names. Only field observa- family Aleyrodidae, the members of tions or biological studies can establish which are known as whiteflies. They the facts in such cases. are tiny insects that are not collected More often than not it is in the larval abundantly in the adult stage but are stage that an insect is destructive. Be- commonly seen fixed on leaves of in- cause it is harmless and is scddom seen, fested plants when they are in the the adult may be unknown to the pupal stage. The family, which con- grower whose crop is being damaged. tains such devastating pests as the In order that the right control meas- citrus blackfly, was long ignored, and ures may be applied, the insect has to most of the present knowledge con- be identified in its larval stage. For cerning its classification has been ac- practical reasons, then, it has become cumulated during the past 50 years, the essential, in the case of various insect first significant and basic w^ork being groups^ to supplement classifications done by American taxonomists. Good founded on adults with keys to the characters upon which to base a classi- larvae. The development of such keys fication were discovered in the pupae, is particularly difficult and slow be- and nearly the whole classification of cause the identity of the larvae must the whiteflies is founded on this stage. first be definitely established. Other- In fact, it is rarely possible to identify wise, however good the key might be, adults in this group because adults have it would not lead the user to the avail- been definitely associated with the im- able information on habits and control mature forms in only a few instances. of the pest. The name originally pro- In keeping with its growing com- posed for the adult insect is the clue, plexity, taxonomy has gradually be- and the larva must be identifiable by come increasingly specialized, until the same name. For definite association now a worker usually confines himself of the larva with the adult, however, to a single limited field, as, for ex- field or laboratory studies must usually ample, aphids, or fleas, ants, biting be conducted, and these often demand lice, cutworm moths, leafhoppers, scale facilities not readily available. Accord- insects, termites, thrips, w^eevils, cer- ingly, the number of different kinds of tain sections of the wasps or bees, grass- insects for which this type of associa- hoppers, gall flies, or mosquitoes. tion has been w^orked out is very small Such specialization is essential for and grows slowly. Even in the Lepi- thoroughly competent and authorita- doptera (moths and butterflies) and tive work in the identification and the Coleóptera (beetles), where more classification of insects. Even to keep work on immature forms has been done abreast of the taxonomic literature in than in the other major groups of in- any one of the restricted fields cited is sects, fewer than 3 percent of the de- a time-consuming task, for taxon- scribed species are known in the larval omy—the description, nomenclature, stage. Since about 1910, however, a and orderly classification of organ- great deal of progress has been made isms—-is international, and a taxono- in this field of taxonomic work, and mist must take into account everything because emphasis has naturally been that is published in his specialty placed on the injurious species, most throughout the world. Furthermore, of the major pests are now identifiable new approaches and more refimed in the immature stages. techniques must be sought continu- Although, as 1 have indicated, the ally in dealinsi; with problems involv- normal course is to base names on the ing the classification of so complex a adults and to develop the original and group of variable and evolving organ- principal classifications from adult isms as the insects have been found to characteristics, there is one conspicuous be. Final decision as to identity often exception. That involves the important rests on features of certain internal Progress in Insect Classification 59 structures that must be dissected out greatly enhanced its strength. Large and mounted on slides after more or revisionary works had been published less elaborate preparatorial treatment. for the mosquito faunas of most of the Indeed, such meticulous preparation regions of the world, and it seemed of slides, which usually involves stain- possible to make definite determina- ing the tissues, is now a routine pre- tions rather readily for mosquitoes requisite for the study and identifica- from anywhere. With the outbreak of tion of the whole insects of many the Second World War, when rapid groups, including aphids, whiteflies, recognition of the mosquitoes encoun- scale insects, thrips, fleas, and lice. tered in remote parts of the world was The very recognition of the diffi- important, however, it became evident culties and complexities that have that much of what had been done on been outlined here is itself evidence of the classification of this family was out significant progress in classification re- of date and that many species could search. More and more emphasis is not be determined satisfactorily. being placed on fundamentals, and Under that demand, intensive study greater caution is practiced in making of the family was undertaken by many identifications. Today the taxonomists taxonomists, and comprehensive keys tend increasingly not to venture spe- were prepared to the anophelines cific identifications in groups that have (malaria vectors) of the world, as well not been thoroughly studied and re- as keys to other mosquitoes of medical vised, although a generation or two importance occurring in certain cru- ago, when the problems involved were cial areas. The presence of military not so well understood, determina- entomologists in the war theaters made tions, often inaccurate, were freely possible the extensive and careful col- made in the same groups. With the lection of specimens, many of the increase in knowledge about any group adults being individually reared and of insects, identification has grown associated with larval and pupal forms. more difficult but it has, at the same Such material was studied in the field time, become more accurate and pre- laboratories and in the museums where cise. the specimens were finally deposited; The recent history of taxonomic re- a flood of papers describing new species search in the mosquitoes is a good ex- and revising genera and species groups ample of the progress that is being resulted. Much w^as learned about the made as the result of intensive, special- taxonomic relationships of species, and ized study of one family of insects. At many new characters usable for dis- the same time it indicates rather clearly tinguishing the dififerent species were how vast the task of the insect tax- discovered. The study of mosquitoes in onomist is. From the time of the dis- the pupal stage was given a great im- covery of the transmission of malaria, petus, and the results are proving fruit- yellow fever, and dengue by mosqui- ful in the continuing attempts to toes, this family, which now contains improve the classification of the family. approximately 2,000 species, was stud- Although the gaps in the knowledge of ied actively. It soon became one of the mosquitoes are being filled in rapidly, best known insect groups of com- however, much remains to be done. parable size. The basic classification, That is even truer of other insect after going through a period of great groups, including the scale insects, ants, instability in the first two decades of fleas, lice, aphids, grasshoppers, and the twentieth century, had reached a certain small families of moths and high degree of stability, thanks largely beetles which have been rather inten- to the efforts of H. G. Dyar in the sively studied because of their conspic- United States and F. W. Edwards in uous economic importance. England. This taxonomy was based on If this, then, is the situation in the both adult and larval characters, which relatively small groups that have received special attention because of their unusual importance to man's wel- fare^ it must be evident that an im- mense amount of work will need to be Values of done before the many larger groups that have had comparatively little Insect Collections study are thoroughly investigated and classified. Clarence E. Michel Thus, in 200 years since Linnaeus, during which time the number of known species of insects has increased The pleasure and challenge of taking from fewer than 2,000 to approxi- part in one vital scientific activity can mately 700,000, insect classification be his who makes a collection of insects. has become an elaborate and complex He starts for the fun of it, the joy activity. At first it consisted essentially in the endless variety of form, color, of the mere sorting of specimens into behavior, and universality of insects. a series of figurative pigeonholes on the Before long he wants to know the cor- basis of differences that were often rect scientific names of the specimens purely superficial. Gradually it has he has and to expand his collection to had to take into consideration many include examples of other species. His factors that have increased immeasur- interest grows wâth his collection and ably the difficulty of the work. These both may attain considerable size. include variation in all its aspects, di- Whatever his age and schooling he is morphism, the correlation of biologi- a scientist then, one of a group whose cal characteristics with structural pe- work has great economic value to culiarities insofar as knowledge of farmers and everybody else. biology will allow, and the identifica- He will discover the basic value of a tion of immature insects. The last vir- collection of correctly identified speci- tually is a separate field in itself, be- mens—that the correct scientific name cause it is concerned with forms utterly of a species is the key to all published unlike the adults with which they information about that species, its belong. habits, and the damage or good it does. Insect classification is now recog- He will also discover that there is still nized as a task that is never finished. a great deal to be learned about all Adjustments or complete revisions of insects and that his own careful obser- the classifications of all groups become vations arc of value in adding to the necessary as more new species are dis- store of knowledge about them. covered and new information is ac- His collection may be small, from cumulated about those already known. twenty to several hundred specimens, with examples of the orders and prin- C. F. W. MuESEBECKj who received cipal families of insects. Or it may in his academic training in entomology tim.e embrace thousands of specimens at Cornell University before joining and be restricted to certain groups of the Bureau of Entomology in igiS^ insects, such as a family or a genus. has been in charge of the division of He may prefer to make what we call insect identification since igs§. hi a general collection. His aim then is to recognition of his service, the Depart- accumulate representative specimens ment of Agriculture awarded him a of the common insects in his own distinguished service medal in ig^i. neighborhood so that he can enjoy their Before he joined the Department, Mr. beauty of color and form or use them Muesebeck spent several years in Eu- to learn to recognize the insects or rope i?i search of specific parasites of simply to satisfy his instinct for collect- some injurious insect pests that had ing. The scope and size of his collection been introduced into the Uiiited States. will depend on him and the breadth of

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