'Where do flies go in winter?' is a classic wildlife question, but there are over 7,000 species of flies in the British Isles, and although we may not notice the smallest ones, many of them remain active throughout the year.
It’s the larger flies that attract our attention on sunny winter days when they bask on tree trunks or buzz at our windows. Many of these flies are muscids, which include the blowflies or bluebottles, and their relatives such as the common house fly (Musca domestica).
Most spend the winter as adults in cracks and crannies and wake up in spring to lay their eggs on decaying matter.
House flies delight in unhygienic places, and their maggots feed on rotting food and other human waste. Some of the common bluebottles lay eggs on decaying flesh and, while we may be squeamish about their choice of diet, we should be thankful that they dispose of animal corpses so efficiently.
One winter fly that often attracts attention is the cluster fly. Cluster flies often enter houses in large number and spend the winter in large huddles in the corner of a spare room or loft. They are rather stocky flies, greyish but with a coat of golden hairs on the thorax.
Like all insects they don’t truly hibernate, but enter a state of diapause, which slows down their development and appetite, until temperatures rise and they become active again.
Unlike their relatives, cluster flies don’t lay their eggs on rotten flesh or excrement, but instead have a surprising taste: earthworms.
In autumn, the flies lay their eggs in the soil and, on hatching, the maggots search for a host. Once inside the earthworm, they eat their way from one end to the other and back again, and pupate in the shell of their hollowed-out victim.
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Illustration by Rose Sanderson