Progress in Insect Classification

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 together 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 wingless 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.

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