Nineteen Ontario species were studied and five from outside Ontario examined. Newly emerged females were of five types: with (1) mature eggs and reduced mouth parts (Gymnopaïs spp., Cnephia spp.); (2) eggs half developed, much stored nutrient, and reduced mouth parts (C. emergens, P. alpestre); (3) eggs half developed, much stored nutrient, and piercing mouth parts (C. mutata); (4) eggs one-fifth to one-third developed, usually much stored nutrient, and piercing mouth parts (Simulium spp.); (5) eggs less than one-quarter developed, little stored nutrient, and piercing mouth parts (Prosimulium spp., Simulium spp.). In Cnephia spp. (type 1) mating occurred shortly after emergence with little flying. In C. mutata parthenogenesis is usual. Male mating flights occurred in species of Prosimulium and Simulium. But S. decorum mated on a support shortly after emergence and also just before oviposition. C. mutata fed infrequently on deer. Occasional females of S. decorum and S. vittatum (type 4) fed on mammals. Species of type 5, S. croxtoni, S. euryadminiculum, S. latipes, and S. rugglesi fed only on birds, S. venustum naturally on birds and mammals, and others naturally only on mammals. Ovarian development in species of types 4 and 5 took live or more days. In those of type 5 it began only after a blood meal that may take two weeks to acquire. Females of Prosimulium, Cnephia, and some Simulium species dropped eggs into the water while flying. Other Simulium species oviposited on water-covered surfaces either while flying or alighted, depending on conditions. Oviposition was greatest in the evening but occurred at other times when populations were large, and conditions favorable.