a black fly, Pollenia rudis, slightly larger than the housefly, commonly found hibernating in large numbers in homes in the fall.
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British Dictionary definitions for cluster fly
a dipterous fly, Pollenia rudis, that tends to gather in large numbers in attics in the autumn: family Calliphoridae. The larvae are parasitic in earthworms
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012