Like all groups of wasps, Pompilidae have parasites or cleptoparasites which prey on them and five species which do this are also Pompilids.
Two, Ceropales maculata and Ceropales variegata, are in their own subfamily and uncommon, especially the latter. Both see off 'genuine' spider-hunting wasps with prey, into which they insert an egg. The spider is left, then normally the original captor returns, ferries the prey into her burrow and seals the site. The Ceropales egg hatches, eats the host egg and proceeds to dine off the spider.
The other three cleptoparasites in mainland Britain - there is another in the Channel Islands - are in the Pompilinae subfamily. They are Evagetes crassicornis (8mm), Evagetes dubius (8mm), which is scarce, and Evagetes pectinipes, which is found only on the Kent coast and is a Red Data Book species. All have fairly short and thick antennae with well-developed sensory powers for identifying nests to penetrate, and tarsal combs on the forelegs for digging.
Despite these physiological elements, there has been some confusion as to their biology, with various claims about catching spiders and nesting fossorially. These must have been cases of mistaken identity since their method uniformly is to dig into sealed burrows, eat the host egg and replace it with one of their own.
It is not clear exactly which species they cleptoparasitise, but there is a belief and some evidence that Anoplius nigerrimus, Arachnospila anceps and Arachnospila trivialis are leading prey of Evagetes crassicornis, while Evagetes pectinipes preys on Episyron rufipes. This would fit in with my observations of Evagetes crassicornis on the root-plate on the Greensand Ridge in Surrey where most of the images of spider-hunting wasps in these pages were taken, since all three likely prey species nested there. The pictured female prospected vigorously over the whole root-plate for several minutes before lighting on a spot and digging furiously.
Given that the host could be guaranteed not to return, the haste was not strictly necessary. Equally, it took the wasp seven minutes to get into the burrow fully, and she was in there for 13 minutes. After another four minutes the burrow was sealed. This fitted with the timescale from an observed Evagetes pectinipes in southern France over a century ago, which was in a burrow for 20 minutes.
Presumably the venture at the root-plate was a success, but on leaving the sealed burrow the Evagetes crassicornis proceeded to find another site 30mm away and start digging again. This time the process was faster, lasting ten minutes from initial excavation to departure.
Evagetes dubius can be distinguished from Evagetes crassicornis by the fact that there is only one submarginal cell on the forewing as against two, and its abdomen having darker red colouring, with less of an orange hue than its relative.
The portrait above shows this clearly, and at 11mm this female was larger than the books say for Evagetes dubius. However, the size of solitary wasps, including cleptoparasites, depends on the size of the prey and this female evidently dined well as a larva.
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.