Flies in winter

My first winter as a resident of East Devon was marked by great excitement when I went into my chilly loft. Most people would have reached for the fly spray, but to me the huge swathes of cluster flies covering the rafters and gable ends gave me a sort of train-spotter’s pleasure. There were thousands of them, in black splodges a foot across. For many years in my previous job as one of English Nature’s entomological boffins, I’d fielded questions about cluster flies but had never known what the fuss was about. True to the classic accounts, there were only four frequent species. The ‘face fly’, Musca autumnalis was by far the most abundant; there was a smaller proportion, although still large numbers, of the dark green Dasyphora cyanella and dull brown Pollenia rudis. These are all middling-sized ‘house’ flies (some related to Linnaeus’s Musca domestica: THE house fly).  Finally were small numbers of the tiny but attractive yellow and black Thaumatomyia notata. All species muddled in together and did not segregate, apart from some of the chunky Pollenia rudis, which I noted often sat alone. So my loft was a good dry hibernation site for these insects that spend the winter as adults.

I know from accounts by non-entomologist friends that they are common in the Axe Valley, and you will all have seen some of these flies on late autumn days buzzing irritatingly around sunny walls. Among the odd features of this phenomenon is that the requirements of the larvae are so different yet the adults are happy to share a common hibernation site. Dasyphora and Musca have rather dull detritus-feeding larvae that can be in dung or rotting compost. The tiny Thaumatomyia is a predator of root aphids. But the one I like most, Pollenia, is a parasite of worms. It’s a dog’s life being a worm; so many things want to eat you, from badgers and buzzards to flies although, to be fair to flies, few species have this inclination.

Cluster flies sensibly keep their heads down till spring arrives.  But a few active species can be seen on winter days, of which the most conspicuous are the gently dancing swarms of winter gnats. If you see a dull brown gnat in winter, it will almost certainly be one of these. Cold doesn’t worry them – two of our common species fly at 0°C and can still walk about in sub-zero temperatures. Females even get on with laying eggs under snow, although males do need it a little warmer for swarming.

These musings on how some common species cope with our winter remind us that global warming may act in less-than-obvious ways on insect populations, even here in balmy south Devon. Winter gnats may find winter too warm, and retreat northwards as has been proposed for northern and mountain-dwelling species. But even common temperate species may have trouble if winters get too mild.  Many insects survive periods that they find unfavourable by going into diapause – a shut-down mechanism that requires kick-starting by a reliable environmental cue such as the right day length or, in the case of the face flies in my loft, at least four months below 5°C. They may, like Sleeping Beauty, fall asleep and never wake up. I’m not suggesting that face flies will go extinct in a hurry but it may be worth keeping an eye on species with stringent diapause-breaking cues. You might have expected me to have kept some observations going but our loft came with woodworm, and the flies have voted with their wings ever since we gave it the nasty treatment.

Photo of Pollenia Rudis by Richard Bartz (Creative Commons License).