More than 1700 species of black flies have been described worldwide. Many other species are known but unnamed, and additional species undoubtedly remain to be discovered. The Palearctic Region contains the most described species, about 600, followed by the Neotropical Region, with approximately 400 species. The Nearctic Region has about 255 known species, although not all have been formally described.

Two subfamilies comprise the Simuliidae. The most primitive subfamily Parasimuliinae includes four described and one undescribed species endemic to the Pacific Northwest. The females of these species do not have biting mouthparts. The subfamily Simuliinae contains all remaining species and is divided into two tribes, the Prosimuliini and the Simuliini, the latter including the majority of pest species. The most universally accepted classification system below the tribal level is summarized by Crosskey and Howard (1997), who recognize 23 extant genera in the subfamily Simuii-inae. Eleven genera in the subfamily are found in North America. The largest genus of black flies is Simulium, which contains 41 subgenera and many of the species of economic importance.

The morphological uniformity of black flies creates difficulty for species identification. For this reason, a holistic approach to identification is typically used, relying on characteristics from eggs, larvae, pupae, males, females, and the polytene chromosomes, as well as distributional and ecological information. The need for accurate identification, particularly in programs for pest and vector management, has driven the taxonomy of black flies. As a result, black flies are taxonomically one of the best known groups of arthropods at the species level; for example, about 98% of North American species are known as larvae and pupae.

More than 150 identification keys exist for black flies in various parts of the globe. Crosskey and Howard (1997), in their inventory of the black flies of the world, provide a comprehensive list of identification keys by zoogeo-graphic region. Keys to the genera or subgenera of adults, pupae, and larvae of the Nearctic Region are provided by Peterson (1996). Regional keys for Nearctic species are available for the southeastern United States (Stone and Snoddy, 1969), northeastern North America (Adler and Kim, 1986; Davies etal.} 1962), the western United States (Peterson and Kondratieff, 1995), and western Canada (Currie 1986; Fredeen, 1985a). The most comprehensive treatment of the Palearctic fauna is by Rubtsov (1956). Keys to the supraspecific taxa of the Australasian and Afrotropical Regions are given by Crosskey (1967, 1969, respectively), and of the Neotropical Region by Vargas and Diaz Nájera (1957) and Coscaron (1987). Comprehensive keys for the Oriental Region are lacking, but the key by Takaoka and Davies (1995) provides a useful starting point.

The giant polytene chromosomes (usually »=3), which are best developed in the larva! silk glands, provide a highly useful tool for discovering and identifying species (Fig. 11.1). Giant chromosomes, particularly their banding patterns, often reveal that many black flies regarded as single species are actually complexes of two or more species. These cryptic species, or sibling species, as they often are called, are reproductively isolated and each is biologically unique. Their existence has far-reaching implications for biological studies and population management of pests and vectors. For example, Simulium damnosum, the black fly known for much of the 20th century as a f

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