Abstract
The larval instars of four species of black flies were determined. Prosimulium fuscum Syme and Davies and Prosimulium mixtum Syme and Davies had seven instars; however, larvae of these Prosimulium spp. passed through only six instars when development occurred at warmer temperatures. Simulium vittatum Zetterstedt and Stegopterna mutata (Malloch) had seven and six larval instars, respectively. The instars of Cnephia dacotensis (Dyar and Shannon) could not be positively segregated.Stream temperature was the most important physical factor regulating larval black fly population dynamics, determining hatching time and developmental rates. Prosimulium mixtum/fuscum larvae required 240 D °C (day-degrees) above 0 °C for maturation, while St. mutata and C. dacotensis needed 250–275 and 475 D °C, respectively. Stream discharge also influenced the annual dynamics of simuliid populations, as spring floods produced second cohorts of some univoltine species which normally have one cohort of overwintering larvae per generation. Changes in discharge also affected estimates of larval densities by influencing the number of larvae colonizing artificial substrates.
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