Identification
Similar to house flies but larger.
Food
Parasitize tent caterpillars.
Life Cycle
Adult flies show up a few weeks before tent caterpillars make their coccoons. The maggots feed on tent caterpillars and drop to the ground where they pupate.
Remarks
Appear in large numbers when tent caterpillars (Malacosoma spp. and Gypsy moths Lymantria dispar) are abundant. They help control tent caterpillar populations.
They are called friendly flies because they land on people or animals in large numbers but don't bite.