LOWLAND HEATHLAND - SPIDER-HUNTING WASPS (Pompilidae)

There are more than 40 species of spider-hunting wasp in Britain, including a new colonist from the continent discovered in 2005. The group is pretty easy to separate from other aculeate Hymenoptera in the field owing to their long hind legs, but many are hard to identify as individual species without recourse to a microscope. Their behaviour is uniformly fascinating and while heathland is one of the best locations in which to see them, they occupy virtually all environments from coasts to gardens.

One of the best types of habitat is a tree root-plate with light undergrowth nearby offering good hunting areas. The pictured one, on the Greensand Ridge in Surrey, was created by the storm of October 1987 and has hosted at least six species including Anoplius nigerrimus (also pictured).

As the picture below shows, much of the soil on the front of the root-plate fell off after strong winds and heavy rain in the winter of 2006-7, which did no good to those which had nested there the year before, but there were plenty of new colonists from the same species on show the following summer, indicating the resilience of these aculeates.

A few species parasitise other Pompilidae, and a few deal with their spider prey in the latter's webs, but the majority are fossorial, creating burrows in which to store prey or using existing ones made by beetles or other invertebrates.

Unlike a number of solitary wasps, they do not kill their prey but paralyse it. A larva gorges itself on the paralysed spider, pupating after a week or ten days then waiting the best part of a year before emerging as an adult.

Pompilidae also spend more time on the ground than in flight, only a handful actually flying with their prey, which means their navigational skills sometimes are tested to the limit in traversing the woodland floor or sandy slopes. Generally the prey is too substantial and cumbersome to make flight feasible.

Spider-hunting wasps are phenomenal hunters - in the USA one, Pepsis formosa, hunts tarantulas, flicking them on to their backs before stinging them. The only time I have ever seen a spider running from another invertebrate was when Auplopus carbonarius was hunting in Ivy on a rockery - a Lycosid spider apparently detected its presence and high-tailed it out of there.

Considering how well equipped spiders are to defend themselves - and to attack - the success of spider-hunting wasps in catching them, including in the nest, is remarkable, though they have the huge advantage of mobility.

Their sting is also very powerful, not to be sampled lightly by humans let alone invertebrates. On the scale of 0-4 covering strength of sting devised by the American scientist Justin O Schmidt, whereby 0 equals the sensation of being stung by an insect that cannot penetrate human skin, 2 equals a familiar intermediate pain such as from a honey bee, and 4 represents an intensely painful sting, Pepsis formosa (see above) rates 4 and smaller spider-hunting wasps would probably rate 3, the same as the wasp Mutilla europaea (Velvet Ant).

As possible proof of the venom involved, a robber-fly (Machimus atricapillus) caught a small Pompilid (Arachnospila sp) in my view in 2005 and pretty quickly let go of it. It takes something special, or dangerous, to make a robber-fly drop its catch, though for the record, Pompilus cinereus have been known to be taken and eaten.

Images © Jeremy Early. All rights reserved.

In 2013 I published My Side of the Fence - the Natural History of a Surrey Garden. Details may be found, and orders placed, via this hyperlink My Side of the Fence. In November 2015 Surrey Wildlife Trust published the atlas Soldierflies, their allies and Conopidae of Surrey, jointly written by David Baldock and me. Details are on this web page: Atlas.