black flies?
Answers:
All black flies require cool, running water for development and favor sites with cobbled bottoms (pebbles, small rocks) that are largely clear of silt. The black fly larvae attach themselves to rocks or other submerged materials and feed on organic particles they filter from the passing waters. Trailing vegetation or rotted aquatic plants also are attractive to black flies, providing sites for the larvae to attach for feeding. Breeding may also occur in rivulets formed by the flooding of fields.
The black fly life cycle can be rapid, taking about three weeks from egg laying to maturation of the adult. Only the female bites, the blood meal being used to provide protein for egg maturation. Adults live about two weeks. Populations can grow very rapidly. Two to four generations may be produced annually. Individual females may lay several hundred eggs.
Other answers:
On dead things.
On dead things.