The Pepsinae subfamily includes half the British species including the scarce Caliadurgus fasciatellus, which preys on Araneidae spiders sought on the web. The female is about 10mm and is active from June to October. There is dimorphism in the species since the male has a white spot on the tip of abdomen and some red on the legs, which also have whitish spurs.
The pictured female did her hunting and nesting around a particularly prolific root-plate that features in close-up with most of the spider-hunting wasps shown in these pages.
Like all non-endothermic invertebrates, she had to warm up before performing, and her activities made for fascinating watching once in August 2006 when I found a small paralysed Araneus diadematus (Cross spider) hanging from a bramble on top of the root-plate.
The female Caliadurgus fasciatellus was burrowing 15cm away. Her first attempt was abandoned, possibly because the root-plate contains a fair bit of dead root and some small stones which could obstruct excavation. She tried a second burrow and that too was given up. The third time she succeeded.
While this was going on, on what was a windy day the prey became detached by a gust and fell to the ground. Breaking the rules on non-interference in nature, I picked it up and placed it beneath the bramble where it had been pinned. The wasp spent around five minutes going up and down the bramble stem, and aimlessly wandering around on the surface of the root-plate, but she failed to locate the spider despite once passing very close. This perhaps supported the theory that some Pompilidae have poorly developed olfactory and visual acuity. (That leaves open the question of how they locate the prey in the first place.)
Eventually I decided to place the spider on the bramble and within 30 seconds the wasp had located and collected it, laid an egg on the abdomen and started transporting it to the burrow. The latter is dug vertically, with no deviation, and is not very deep.
Priocnemis is the hardest genus to differentiate even allowing for the distinctive whitish mark on the wing which most of them have. One of the commonest and largest (to 12mm) is Priocnemis exaltata, which uses a variety of habitats and catches a variety of prey, all wandering spiders. The prey is pulled backwards by a leg and the pictured wasp had a tough and circuitous ascent of one metre to make before depositing her prize. The process took seven minutes, including leaving the spider once seemingly to check the route, and once to check the diameter of the burrow.
From my observations, Priocnemis exaltata and pusilla are more efficient in transportation, including protection of prey, than two of the Pompilinae group, Anoplius nigerrimus and Arachnospila anceps.
One of the two pictured Priocnemis pusilla (8mm) was tremendously skilful at concealing her spider among wood chippings and leaf litter, and in transporting it under cover, to the extent that despite flattering myself I was watching pretty closely, she escaped my attention when pulling it out from under a twig and into the burrow.
Usually the members of a genus show the same behaviour in transporting prey and with Priocnemis this involves holding the base of a leg. An exception, however, was the wasp in penultimate row of images above, which grasped the tip of the leg. This was also the fastest-moving of those seen, but unfortunately I failed to identify it. Priocnemis parvula (8mm) is almost indistiguishable from Priocnemis pusilla but the wings tend to have no pale markings towards the back edge. Their methods are virtually identical.
Priocnemis fennica (8mm) is not so common as Priocnemis exaltata or pusilla, and it has been claimed that the species is associated with watersides. That may be the case, but not to the exclusion of other habitats. In Holland, Priocnemis fennica is found in heathland and gardens, and the pictured female were in sandy conditions on Surrey heathland.
Something similar applies to Priocnemis schioedtei (9mm), a scarce and very active species which supposedly is associated with broad-leaved woodland. However, seemingly it is found in and around open heathy woodland more than anywhere else, and the pictured wasp was spotted some distance from any trees on the sandy root-plate which has afforded most of my sightings of spider-hunting wasps. The red collar, just about visible in the images, is the easiest way to separate this species from the other Priocnemis ones. However, this wasp also ran with the wings held almost directly above the abdomen, which if typical also distinguishes it from commoner members of the genus.
Last but not least, Priocnemis agilis (10mm), which often has striking red colouration on the legs that can help differentiate it from others in the genus. This one is found widely but remains nationally scarce.
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.