Sunday, 21 August 2011

Spiders lose out to a special wasp

When you think of spiders, you tend to think of them as having things pretty much their own way. They are top dogs, the fearsome predators that snare, snatch and trap insects and other invertebrates. When it comes to web-producing spiders and flying insects it seems even more one-sided, the insects blundering into the web where they are quickly subdued.

Spiders do not have it all their own way though; there is, for example, a group of wasps that specialises in hunting spiders. Belonging to the family Pompilidae, these spider-hunting wasps are smart species, lightening quick in flight and easily overlooked (despite their size). The wasps favour open ground, with short vegetation and areas of bare sand in which they can excavate a nest. These wasps do not form complex social nests like our more familiar yellow and black vespids, but are solitary in habits. The female hunts spiders, paralysing her prey with her sting and then delivering it back to a prepared burrow into which she will lay a single egg. Each nest chamber (or cell) will contain the egg and one paralysed spider, the latter providing a meal for the wasp larva that will soon emerge from the former.

This group of wasps contains 43 species nationally but, even with such a small number to get to grips with, they represent a serious challenge for the entomologist. Knowledge of their favoured habitats means that finding them is not that difficult. Their quicksilver movements, however, make them particularly hard to catch. Once in the net they have the tendency to sting (from experience I can tell you that it feels like a white hot needle and makes your finger joints ache) and once in a glass tube they can still be exceedingly difficult to identify. Fortunately, some of the more common species can be identified fairly readily, through a combination of body and leg colour plus structural features and wing venation.

One of the first of these wasps that I ever encountered was a species called Episyron rufipes (like many less familiar insects, these wasps lack a common name). It is widely distributed but favours heathland and coastal dunes. Active mainly from June through into August, it targets orb-web spiders (but will also take wolf spiders). Once it has paralysed a spider, Episyron will store it temporarily on a plant while the nest is dug, before retrieving it. Watch a suitable patch of sand and you might be lucky enough to see it deliver the spider to the newly excavated nest. That the orb-web spiders don’t have things all their own way makes me see them in a slightly different light; now they are victim as well as villain.

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