Thursday, 10 October 2013

Black flies

The other week, sat outside in the dusk of a late summer barbecue, I received a painful bite on my arm that left a truly impressive bruise lasting for many days. It was a sharp reminder (literally) that we provide feeding opportunities for certain insects, many of them small and easily overlooked.

Perhaps the most familiar of these insects are the biting midges, a large family of flies of which just a few – all belonging to the genus Culicoides – bite humans. It is in the north and west of Britain that the impact of midges is most strongly felt, and I remember countless childhood holidays where evening activities were curtailed by the presence of midges. It is the female midges which bite, the small amount of blood taken to aid the development of their eggs. One reason why midges can prove so troublesome is that a feeding midge releases a pheromone, alerting other individuals to the location of a potential meal.

Elsewhere in Britain it is other biting flies that have developed a formidable reputation. The blackfly Simulium posticatum makes a particular nuisance of itself along the River Stour in Dorset, earning itself the local name of Blandford fly and occasionally prompting coverage in the national papers. Blackfly larvae, and those of the various midges, are aquatic and this means that you tend to encounter more of the biting adults in areas close to rivers, lakes and other water features. Some species are associated with fast flowing stretches, others with still or stagnant water.

Some of these biting insects act as vectors for particular diseases. Although not so much of a problem for us here in Britain, elsewhere in the world they can be associated with diseases that are quite serious, including river-blindness and the transmission of certain blood parasites. It is not just humans that are bitten and these flies target many other warm-blooded creatures. Work on owls has, for example, highlighted a high prevalence of blood parasites transmitted by these flies. Owls whose territories contain more waterbodies suffer more bites and carry more parasites, highlighting complex interactions between parasites, hosts and vectors. Of course, knowing this probably provides little consolation if you, like me, have been nursing a sore arm for the last couple of weeks.

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