They will be particularly abundant near culverts under roads, attached to plants trailing in the water. Most black fly larvae are filter feeders, with the larvae feeding on nutrients in the water as it flows by. Larvae pass through six stages before reaching the pupal stage. Pupae are encased in a silken cocoon attached to vegetation or other objects in the stream. Adults emerge from the pupal case through a slit and float to the surface on a bubble of air.
Some species mate as soon as adults emerge.
The length of the cycle from egg to adult is variable, depending on the black fly species and water temperature. The time interval for S. slossonae is estimated to be from three to four weeks. Emerging adults live from two to three weeks, to as long as 85 days. However with many black fly species, high populations are regulated more by suitable water for the developing larvae than anything else. The high water levels in 1998 have produced the large numbers of flies seen.