Egg development is completed in 2-4 days. The female then locates a river or stream in which to lay them. Depending on the species, this will range from the tiniest spring to the Mississippi River. Female black flies generally ambush their victims from tree-top perches near the edge of an open area. They are active during the day with peak activity in the morning and early evening.
Females live from one to three weeks, depending on the species and weather conditions.
They survive best in cool, wet weather. Studies done by MMCD show that the majority of black flies in the region lay only one egg batch. When the eggs hatch, the tiny larvae begin their life in the stream. They eat by filtering food from the running water with specially adapted mouth parts that resemble grass rakes. This takes from 10 days to several months, depending on the species and the water temperature.