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Cluster Flies
The cluster flies are the genus Pollenia in
the blowfly family Calliphoridae. Unlike
more familiar blow flies, such as the
bluebottle genus Phormia, they do not
present a health hazard because they do
not lay eggs in human food. They are
strictly parasitic on earthworms; the
females lay their eggs near earthworm
burrows, and the larvae then infest the
worms. However, the flies are a nuisance;
when the adults emerge in the late
summer or autumn, they enter houses to
hibernate, often in large numbers; they
are difficult to eradicate because they
reside inaccessible spaces such as roof
and wall cavities. They are often seen on
windows of little-used rooms. They are
also sometimes known as attic flies.
The typical cluster fly Pollenia rudis is
about 7 mm long and can be recognized
by distinct lines or stripes behind the
head, short golden-colored hairs on the
thorax, and irregular light and dark gray
areas on the abdomen. Cluster flies are
typically slow-moving.