Life History

Immature black flies are found in virtually any water that flows, even if only imperceptibly and temporarily, from the smallest trickles to the largest rivers. Most species occupy specific habitats, and some higher taxa are characteristic of particular environments. For example, members of the genus Gymnopais occupy small, icy streams of the Far North, species of Sitmdium (subgenus Hemic-netha) live on the lips of waterfalls and in swift rocky flows, members of the S, noelleri species group are found below impounded waters, and species of Simulium (subgenus Psilozia) usually are found in warm, highly productive streams and rivers with open canopies.

Each species of black fly has a specific pattern of seasonal occurrence. Nearly all species in the tribe Prosimuliini are univoltine, completing a single generation annually. The tribe Simuliini contains both univol-tine and multivoltine species. Some of these multivoltine species can complete seven or more generations per year in areas of North America with mild climates, In certain tropical areas of the world, some species (e.g., members of the S. damnosum complex) might cycle through more than 20 generations each year.

Eggs typically cannot resist desiccation, although some species (e.g., Austrosimulium pestilens) can survive in moist soil of dry streambeds for several years, hatching when streams are inundated. During the summer, eggs of multivoltine species (e.g., S. vittatum, S. damnosum) can hatch in fewer than 4 days. In northern temperate regions, univoltine species (e.g., Prosimulium spp.) often spend the warm months as eggs, whereas multivoltine species spend the cold months as eggs. Accordingly, the potential for long-term survival of eggs must be considered in management programs. Eggs of some species (e.g., S. rostratum) remain viable in the laboratory just above freezing for up to 2 years.

The larval stage lasts from about a week, or even less, to nearly half a year, depending on specics, stream temperature, and food availability. At one extreme are the larvae of some species in the West African S. damnosum complex that complete development in 4 days. At the other extreme are the larvae of many univoltine, temperate species that hatch in the fall, develop during the winter, and pupate in the spring. The number of larval instars varies from 6 to 11, depending on species and environmental conditions, such as food supply.

Final-instar larvae typically move to slower water before pupating in a silk cocoon that is spun on a substrate. Some species (e.g., Prosimulium magnum) pupate in masses, but most pupate individually. The duration of the pupal stage depends largely on temperature and species, lasting from several days to a few weeks. When the adult is ready to emerge, it expels air from its respiratory system, thus splitting the pupal cuticle along the dorsal eclosion line.

The newly emerged adult, partially covered in air, rises to the surface of the water with enough force to break the water—air interface. It then seeks a resting site, often streamside, to tan and harden. Adults generally live less than a month, during which time mating, sugar feeding, host location, blood feeding, and oviposition must be accomplished. Crosskey (1990) provides a full and detailed treatment of simuliid life history and bionomics.

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