Cluster fly

Cluster flies (a.k.a. grass flies or attic flies) are flies of the genus Pollenia in the family Polleniidae. Unlike the more familiar blow flies, such as the bluebottle genus Phormia they do not lay eggs in human food. They parasitise earthworms; the females lay their eggs near earthworm burrows, and the larvae then feed on the worms. But the biology of this group is relatively poorly known and a few have been recorded from other hosts including caterpillars and bees.

Cluster flies seek refuge in cold weather and find their way into attic spaces and similar areas indoors. They often emerge on warm days, and cluster at windows attempting to exit (hence the name).

The typical grass fly Pollenia rudis is about 7 mm long and can be recognised by distinct lines or stripes behind the head, short golden-coloured hairs on the thorax, and irregular light and dark grey areas on the abdomen. Cluster flies are typically slow-moving.

Cluster flies have a widespread distribution. Eight species are found in Britain and 31 in Europe. Pollenia species are also numerous in Australia and New Zealand (over 30 species); they are common in North America. P. rudis has spread widely in association with humans.