Thursday, 27 July 2006

Robber of the dunes

At first glance, Norfolk’s coastal dunes seem an inhospitable environment in which to live. Shaped by the wind and baked by the summer sun, there seems little opportunity for wildlife. However, a diverse community of plants and invertebrates makes a living from the dunes, overlooked by those who come in search of sun, sea and sand. A weekend trip to Winterton Dunes highlighted the popularity of the beach; the car park quickly filled and the shrieks of children soon settled in with the rhythm of the sea. Intent on gaining a spot on the beach, few tourists venture into the dunes that stretch inland. These particular dunes are a Site of Special Scientific Interest, reflecting some of the special animals to be found here. Among these is a group of flies that require the hot, dry conditions that the dunes provide. Known as robberflies, virtually all of the 28 species found in Britain have a strong southerly or coastal distribution. These robust flies depend on the warmth to carry out rapid attacks on unsuspecting victims.

Dune Robberfly

On Sunday I was lucky enough to watch what I believe to be a dune robberfly (aptly named, I know) as it pounced on a smaller fly. The robberfly had been sitting alert, seemingly scanning its surroundings. The large compound eyes curve around almost to the back of the head providing excellent all round vision and allowing the fly to spot potential prey. The facets that make up most of the compound eye are fairly small and it is thought that this gives the fly great visual resolution. Unusually for a fly, the facets at the very front of the eye are much larger, and the front of the eye itself is rather flat, and this provides excellent binocular vision, something that is essential if the fly is to make an accurate strike at potential prey. There was an instant where the fly seemed to tense, before springing from the ground to make a darting flight at its victim. The victim was caught unawares and, held in the sharp piercing mouthparts, hardly managed a struggle before succumbing to the poisonous saliva that the robberfly had injected. The robberfly returned to its perch and I was able to take several photographs which, I hope, will enable me to confirm my tentative identification.

At about 25mm in length, the robberfly was impressive. Despite its size and the sharpness of its mouthparts, the species is harmless and rarely bites, even when handled.  But it is exactly the sort of fly which would not be well received by those on the beach. It is a good job that they choose to ignore the dunes and the wildlife they contain.

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