If you have to ask, we envy you! Blackflies are small but vicious insects that torment anyone foolish enough to go outside with bites that often leave the victim bleeding. Blackflies emerge from the stream beds in early May as soon as the leaves start unfurling (which is good because after mud season is over, we natives need something to complain about) and stay around until the weather gets too hot- usually mid to late June ( at which point the deer flies and mosquitoes take over).
There are approximately 40 species of blackfly in Vermont. Of these, only 4 or 5 species bite humans. Some blackfly species over winter as eggs, others as larvae. The larvae may molt many times before transforming into a "hump-shaped little pupa inside a spun cocoon." Sometime in late April or early May each year, when the transformation to adulthood is complete, the pupal case splits and the new adult, encased in a bubble of air, spins off downstream and rises to the surface. The bubble bursts and the tiny insect is catapulted into the air. They emerge hungry. They're out for blood.