Sialis cornuta is being studied in a series of abandoned beaver ponds in the Rocky Mountains 80 km west of Calgary, Alberta. The species has a 2-year life cycle in this area. Eggs are laid mainly on the undersides of uprooted trees; each egg mass contains a mean of 585 eggs which are susceptible to heavy parasitism by Trichogramma semblidis. Most eggs are laid during late June, they hatch in 7 to 21 days, and most larvae attain the fifth or sixth instar by the first winter. In the laboratory there are between 7 and 10 instars before pupation, but in the field the number appears to be more constant at 10. Sexual dimorphism and possibly a larval diapause complicate the separation of instars and the translation of laboratory data to the field. Between May and mid-June of their second year larvae leave the water and pupate in the soil a short distance from the water's edge and adults emerge through June and the first half of July.Larvae feed on benthic invertebrates in approximate proportion to their abundance provided they are not too large. Chironomid larvae, tubificid worms, and ostracods form the bulk of the food.Information is given on mating, oviposition, and feeding behavior, and comparisons are made with the life history features of S. californica and S. rotunda in Oregon.
