The very recognition of the difficulties and complexities that have been outlined here is itself evidence of significant progress in classification research. More and more emphasis is being placed on fundamentals, and greater caution is practiced in making identifications. Today the taxonomists tend increasingly not to venture specific identifications in groups that have not been thoroughly studied and revised, although a generation or two ago, when the problems involved were not so well understood, determinations, often inaccurate, were freely made in the same groups. With the increase in knowledge about any group of insects, identification has grown more difficult but it has, at the same time, become more accurate and precise.

The recent history of taxonomic research in the mosquitoes is a good example of the progress that is being made as the result of intensive, specialized study of one family of insects. At the same time it indicates rather clearly how vast the task of the insect taxonomist is. From the time of the discovery of the transmission of malaria, Yellow fever, and dengue by mosquitoes, this family, which now contains approximately 2,000 species, was studied actively. It soon became one of the best known insect groups of comparable size. The basic classification, after going through a period of great instability in the first two decades of the twentieth century, had reached a high degree of stability, thanks largely to the efforts of H. G. Dyar in the United States and F. W. Edwards in England. This taxonomy was based on both adult and larval characters, which greatly enhanced its strength. Large revisionary works had been published for the mosquito faunas of most of the regions of the world, and it seemed possible to make definite determinations rather readily for mosquitoes from anywhere. With the outbreak of the Second World War, when rapid recognition of the mosquitoes encountered in remote parts of the world was important, however, it became evident that much of what had been done on the classification of this family was out of date and that many species could not be determined satisfactorily.

Under that demand, intensive study of the family was undertaken by many taxonomists, and comprehensive keys were prepared to the anophelines (malaria vectors) of the world, as well as keys to other mosquitoes of medical importance occurring in certain crucial areas. The presence of military entomologists in the war theaters made possible the extensive and careful collection of specimens, many of the adults being individually reared and associated with larval and pupal forms. Such material was studied in the field laboratories and in the museums where the specimens were finally deposited; a flood of papers describing new species and revising genera and species groups resulted. Much was learned about the taxonomic relationships of species, and many new characters usable for distinguishing the different species were discovered. The study of mosquitoes in the pupal stage was given a great impetus, and the results are proving fruitful in the continuing attempts to improve the classification of the family. Although the gaps in the knowledge of mosquitoes are being filled in rapidly, however, much remains to be done. That is even truer of other insect groups, including the scale insects, ants, fleas, lice, aphids, grasshoppers, and certain small families of moths and beetles which have been rather intensively studied because of their conspicuous economic importance.

If this, then, is the situation in the relatively small groups that have received special attention because of their unusual importance to man's welfare, it must be evident that an immense amount of work will need to be done before the many larger groups that have had comparatively little study are thoroughly investigated and classified.

Thus, in 200 years since Linnaeus, during which time the number of known species of insects has increased from fewer than 2,000 to approximately 700,000, insect classification has become an elaborate and complex activity. At first it consisted essentially of the mere sorting of specimens into a series of figurative pigeonholes on the basis of differences that were often purely superficial. Gradually it has had to take into consideration many factors that have increased immeasurably the difficulty of the work. These include variation in all its aspects, dimorphism, the correlation of biological characteristics with structural peculiarities insofar as knowledge of biology will allow, and the identification of immature insects. The last virtually is a separate field in itself, because it is concerned with forms utterly unlike the adults with which they belong.

Insect classification is now recognized as a task that is never finished. Adjustments or complete revisions of the classifications of all groups become necessary as more new species are discovered and new information is accumulated about those already known.

C. F. W. MUESEBECK, who received his academic training in entomology at Cornell University before joining the Bureau of Entomology in 1916, has been in charge of the division of insect identification since 1935. In recognition of his service, the Department of Agriculture awarded him a distinguished service medal in 1951. Before he joined the Department, Mr. Muesebeck spent several years in Europe in search of specific parasites of mine injurious insect pests that had been introduced into the United States.