Mosquitoes

Harry H. Stage.

Mosquitoes have annoyed man and undermined his health for centuries. These voracious bloodsucking pests occasionally become sufficiently numerous to kill livestock. They have prevented industrial and agricultural development in many parts of the world. Merely as annoying pests they have kept large areas from becoming summer resorts. But all those losses are slight, compared to the damage done to human beings by mosquitoes as carriers of malaria, yellow fever, dengue, filariasis, and encephalitis. Practically every school child learns about the relationship of mosquitoes to malaria and yellow fever. There are few insects that have been studied more or about which more has been written, but only within the last 50 years have economical and effective control methods been developed. These methods are a result of extensive research by entomologists, engineers, malariologists, physicians, chemists, and others.

There are more than 2,000 different species of mosquitoes. All have different flight habits, food preferences, and climatological requirements. Mosquitoes breed only in water, but great swarms can be produced in extremely small quantities of water, whether foul or clean, salt or fresh. They breed not only in extensive marshes but also in empty cans, abandoned automobile casings, tree holes, rain gutters, and the axils of some plants.

MOST OF THE present-day research on mosquitoes is concerned directly with methods of killing them. Before that can be economically accomplished, however, we must first be able to tell the various species apart. The problems of classification have led to a great deal of biological work, with implications that go far beyond the immediate practical objectives. Perhaps the biological studies may seem to place an undue emphasis on morphology and taxonomy in entomological writing, but without it how could we refer with precision to a single mosquito species of a total of 2,000, around the world, having definite relationships? In all languages the name Culex pipiens, for example, can refer to only one species of living thing.

Taxonomic research on mosquitoes began with Linnaeus, the father of systematic zoology. In 1735 he named the first genus of mosquitoes Culex, and his tenth edition of Systema Naturae published in 1758 was the beginning of the systematic naming of animals. In that work we find the genus Culex with six included species, but only one of these Culex pipiens is recognized as a valid species of mosquito today. In 1818 J. W. Meigen described the genera Aedes and Anopheles, and in 1827 J. B. Robineau-Desvoidy added the genera Sabethes, Psorophora, and Megarhinus. Late in the nineteenth century there were more attempts at classification, led by F. Lynch-Arribalzaga, who recognized all the old genera and proposed a number of new ones.

When the importance of mosquitoes as disease transmitters became known, late in the 1890's, the systematic study on the classification of mosquitoes was accelerated considerably. The leading research worker on classification of mosquitoes at this period was F. V. Theobald, who, from 1901 to 1910, published a five-volume monograph on the Culicidae, a family which included all mosquitoes known to him. Many new genera and species were described and given scientific names, but their classification was based on superficial adult characters, which subsequent research has shown to be unreliable. Somewhat later, in 1912, H. G. Dyar, F. Knab, and D. W. Coquillett published larval characters and adopted much sounder adult characters. Dyar continued his research on mosquito classification, and in 1929 the Carnegie Institution published his Mosquitoes of the Americas, which revised the previous volumes by L. O. Howard, Dyar, and Knab, included many new species, and also increased the geographic scope of Dyar's knowledge to the entire Western Hemisphere.

The present classification of mosquitoes was rather firmly established by F. W. Edwards in Wytsman's Genera Insectorum in 1932. The family Culicidae, as established by Edwards, consisted of 39 genera and some 1,400 species. New genera and a number of new species have been described since then, and many changes in names have been made. During the Second World War military entomologists discovered more than 200 new species of mosquitoes, mostly in the Pacific area.

The classification of mosquitoes now is probably in the best condition of any comparable group of insects, since it is based on intensive studies of the morphology of adults, eggs, larvae, and pupae, as well as on biological information. There have been major publications on the mosquito faunas of Surinam, Australia, India, the Philippines, Ethiopia, Egypt, and the Americas, and more are in progress. Much remains to be done in describing species from out-of-the-way places, in describing presently unknown larvae and pupae, and particularly in clarifying the status of closely related species, such as the Culex pipiens and Anopheles maculipennis complexes.

RESEARCH ON THE BIOLOGY and life history of mosquitoes was started about 1670 by Jan Swammerdam, of Holland. In 1691 P. Bonanni of Italy studied and described the life history of the common European mosquito, Culex pipiens. About 25 years later Rene de Reaumur of France studied the same mosquito, and his account of the development of the species remained valid until 1886, when the Department of Agriculture published Howard's first full life history of an American Culex.

W. Raschke, a German, studied the larva of a European Culex in 1887, and in 1890 an Englishman, C. H. Hurst, wrote on the pupal stage of Culex. Both Raschke and Hurst included observations on the physiology of the respiratory tubes, the gill flaps, and the tracheae, by which mosquito larvae breathe.

By 1892 Howard had worked out the life history of the southern house mosquito. In 1896 he published illustrations of the egg, larva, pupa, and adult of the northern house mosquito, the common mosquito around Washington early in the summer and therefore a ready subject for Howard's interest. His research was followed shortly thereafter by the first complete history of the common malaria mosquito.

The Department of Agriculture published Howard's Notes on the Mosquitoes of the United States in 1900. In it he described the anatomy and biology and suggested practical controls, which served as background and guide for W. C. Gorgas and J. A. Le Prince in their clean-up of mosquitoes in Havana. The following year Howard published Mosquitoes How They Live; How They Carry Disease; How They Are Classified; How They May Be Destroyed, a book of 241 pages containing a chapter on the taxonomic characters of several of the common mosquito genera and an extensive account of the remedies suggested against mosquitoes. Howard stated that the results of this research appeared "at the psychological moment," and the book was widely distributed to members of the Army Medical Corps. Its recommendations were soon put to use by the authorities responsible for the construction of the Panama Canal.

G. M. Giles, an English naturalist for the Indian Marine Survey, in 1900 published a handbook on mosquitoes, which contained the results of a great amount of research on the life history and on the conditions affecting their abundance. In 1902 he published an enlarged edition of more than 500 pages.

A few years later Howard, stressing the lack of information which his papers and books disclosed, obtained a grant from the Carnegie Institution of Washington to finance the preparation of an extensive monograph, Mosquitoes of North and Central America and the West Indies, by himself, Dyar, and Knab. Much study and research went into the preparation of two volumes, which appeared in 1912, followed by another in 1915 and one in I9I7. The four volumes were an outstanding contribution to research on the biology of mosquitoes in the Western Hemisphere.

John B. Smith, of the New Jersey Agricultural Experiment Station, in 1904 published a monumental report on his investigations on the habits and life history of New Jersey mosquitoes. This report was revised by Thomas J. Headlee in 1915, 1921, and 1945. In his third revision Headlee stated that most of the chapter on biology first published in 1904 was used in the last edition because the research work by Smith and his assistants was so fundamentally sound that it still served as the principal basis for modern mosquito-control procedure.

Although biological research on mosquitoes has been somewhat hampered by the complex problems of classification, a voluminous literature has been accumulated on the life history and biology of these insects, albeit the original reports are scattered through a wide range of scientific periodicals in several languages.

It was not until 1949 that a detailed volume appeared on mosquito biology, or The Natural History of Mosquitoes, as Marston Bates, the author, preferred to call it. Reporting on his own research, and compiling thousands of notes and reports by other scientists in all parts of the world, Bates prepared one of the very few volumes on the biology of a family of insects. His work and endless research provide a detailed summary of the known behavior of the adult stage, the process of egg laying, the time and place of flight, longevity, seasonal distribution, sexual behavior, food preferences, distribution, egg development, larval reactions to physical and chemical environment, the habitat of the larvae, and the classification of the multitudinous number of larval habitats. Despite all this knowledge on the biology of mosquitoes, the author concluded that the detailed and minute information necessary to make clear-cut definitions of habitat characteristics for some 2,000 species of mosquitoes was still lacking.