Nick Baker’s Hidden Britain
Getting cosy with cluster flies
S ynanthropic species fascinate me. In a world where natural habitats are in disarray and decline, they buck the trend. If I want some kind of positive affirmation that all is not bad in the world, then I look towards the animals and plants that thrive in man-made spaces.
One such species that gives me this kind of reassurance is found in my attic. Here, separated from the everyday warmth and domesticity chez Baker, is an even stranger world – inhabited by a creature whose enemy is the dehydrating effects and fluctuations of central heating, and which finds sanctuary in this cool but frost-free space lying above us.
Creaking up the loft ladder, I enter the dark. For the most part, this species is widely scattered, so barely registers, but right now I just need to look at the side of the chimney breast. As I shine my torch, I reveal bricks carpeted by a twitching veil of insect life. Hundreds of flies sit silently waiting for spring. The golden fuzz of their collective thoraxes tells me that they are common cluster flies, Pollenia rudis.
During the rest of the year, I hardly see any, apart from on one warm autumn day when they congregate on the sunny side of my chimney stack. Are they queuing to get inside? Somewhere there must be a chink, a crack in the mortar, or a gap in the bargeboards, that acts as a portal to winter safety, because over the next weeks these flies come inside. Insects are sensitive to infinitesimally small changes in microclimate, and cluster flies are no exception. Look closely at the ‘face’ of one of your dozy, domestic denizens and you’ll see two very fine projectiles between its eyes. These are the ‘arista’ – feathery parts of their antennae that are highly sensitive to environmental cues. By using these, cluster flies are able to pick out the perfect spot for their winter sojourn, choosing the same spot every year.
The clustering effect is possibly amplified by the production of aggregation pheromones, which pull individuals into these utopian wintering spots. Here they remain until spring, when the whole mass of them reverse the effect and several hundred of these golden-napped flies let themselves out again.
What they do then is disperse into the garden and surrounds, where they take part in a number of activities. Their scientific name, Pollenia, indicates that, like many other blowflies, they visit flowers and so are important pollinators. But spring is when the really interesting part of their lifecycle happens. After mating, the flies lay their white lozenge eggs on the surface of soil and, when these hatch, the maggots slide down into the soil with the sole aim of finding the mucus trails and burrows of their prey.
On the hunt
They may not look like it, but these little, pallid maggots are predators of earthworms. Using a sharp set of jaws, they burrow in between the soft segments of their host. In some instances, they literally climb into their dinner, with nothing but the tip of their abdomen sticking out.
Over the summer months, these prolific flies can breed several generations, and only when the day length draws in and temperatures drop will they return to our homes.
Cluster flies can be seen returning to the same spots each year, in search of refuge from the harshness of winter.