"The Cluster Fly"
Cluster flies invade houses in autumn often collecting in large numbers in windows. They winter in cracks of buildings and appear again in spring. These non-native flies were introduced because their larva were thought to be parasitic on the caterpillars of insect pests. Unfortunately they are parasites of earthworms a beneficial animal. Note the solid black thorax, house flies are striped.
Scanned from a specimen by A. Sinclair
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