Small World Discoveries
by Tony Enticknap - tickspics
Taking a close look at insects and other small species that can be found in and around East Dorset and the New Forest ...
HYMENOPTERA | Stinging Wasps
Pompilidae (Spider-hunting Wasps)
These characteristic, long-legged and very active, ground-running solitary wasps are relatively easy to identify in terms of knowing that you’re actually looking at a spider wasp rather than a member of one of the other hymenopteran families, but identifying an individual to species, or even genus, is notoriously difficult.
There are over 40 species of spider wasp on the British List in 14 genera, which are placed into three subfamilies; Ceropalinae, Pepsinae and Pompilinae. Most species have a predominantly black, or black and red body. The pronotum reaches back to the wing bases and, in most species, there are three submarginal cells, which are of considerable value when trying to determine the subfamily and for genus/species identification. The antennae are proportionally longer than those of crabronid digger wasps, often curled at the tip and, when extended, almost as long as the body in some of the species. The identification keys for this family separate females and males, so it’s important to remember that the female’s antennae have 12 segments (scape, pedicel and 10 flagellar) and the smaller males have 13. The females have 6 visible abdominal segments often with a visible sting, whilst the male has 7. The species in some genera have hind tibiae that reach beyond the end of the abdomen.
Females spend much of the time fluttering their wings and waving their antennae around whilst making short rapid movements across bare ground or short vegetation in pursuit of spiders to provision their nest cells. They are a warm-climate, primarily southern, species that are more likely to be active on sunny days. They can be found in various habitats, although heathland is probably the best place to find them. The females dig their burrows in sandy soil, usually finding a suitable spot close to where the prey has been caught. The paralysed spider usually has to be dragged to the nest, rather than carried back in flight, which is why spider wasps generally don’t have a pre-dug burrow like the digger wasps. Once the spider is in the nest cell and an egg laid on it, the burrow is blocked. On hatching, the wasp larvae slowly consume the paralysed spider initially by consuming its internal fluids, until it dies.
Although you’re more likely to find females on the ground, both sexes can often be seen taking nectar from flowers.
Priocnemis spp.
HYMENOPTERA > APOCRITA | Aculeata > Pompilidae > Pepsinae
There are 13 species in the genus, some of which are scarce but, looking at the distribution data of the more frequently recorded, it seems that the most likely ones to be seen locally are Priocnemis exaltata, Priocnemis perturbator and Priocnemis susterai, which are the three larger and more common species, and Priocnemis fennica, Priocnemis parvula and Priocnemis pussilla, which are all widely distributed across southern counties. That doesn’t rule out some of the others, but they’re either regarded as infrequent and/or uncommon or not recorded locally.
A key characteristic of Pepsinae species is that the forewing vein CuA1 is not curved at its start so the second discal cell is not extended, whereas in Pompilinae it’s strongly curved producing a distinct triangular extension in that cell. Other wing venation features, including how Rs meets the wing margin help lead us to Priocnemis, and, as an example, the second submarginal cell having a wider base than the adjacent cell when measured along Rs indicates that the species could well be Priocnemis fennica.
Another observation for this subfamily is that the hind legs are in normal proportion to the front and middle, not over-sized as noted earlier for some species.
All Priocnemis species have a black and dark red abdomen, so identification comes down to small characteristics such as the females having a pale spot towards the tip of the wing. Preferred habitat, normal flight periods and a few other factors may help narrow down options, but any assessments are going to be ‘likely’ at best, hence the use of the cf. qualifier.
Priocnemis cf.fennica (male)
Stour Valley NR, Northbourne, East Dorset | July 21
Priocnemis cf.perturbator (female)
Alners Gorse, Dorset | Aug.21
My assessment that the individual photographed on the edge of woodland adjacent to the River Stour is primarily based on the wing venation, which keys out pretty well. Priocnemis fennica is also described as a slender species that has an association with both trees and watersides, which ticks both of the habitat boxes. It's also a common widespread species that has been recorded locally. It may be seen from late May to early September.
The chunkier and larger-looking female that I found at the Alners Gorse butterfly reserve at the beginning of August is a bit more guess-work. I thought that Priocnemis perturbator was more likely than Priocnemis exaltata based on the open woodland habitat that the species is most frequently associated with. It could be Priconemis exaltata though, or could be neither!
Anoplius nigerrimus
HYMENOPTERA > APOCRITA | Aculeata > Pompilidae > Pompilinae
There are five species in this genus, with all but Anoplius nigerrimus having a black and dark red body. There are also ten or so other black-bodied species in other genera, but my understanding here is that female Anoplius species have bristles on the tip of their abdomens that can be seen in some of my photos.
Anoplius nigerrimus has a body length of 6-12mm, and, in females, an abdomen with silvery pubescence, and is regularly described as the Common Black Spider Wasp. It’s a widespread species that can be seen from May to September in dry habitats such as heathland, grassland, woodland, parks and gardens.
Anoplius nigerrimus (female)
Three Legged Cross, East Dorset | Aug.21
Episyron rufipes
HYMENOPTERA > APOCRITA | Aculeata > Pompilidae > Pompilinae
This particular species can be identified with a lot more certainty by the cream markings on its abdomen and reddish-brown hind tibia and base of femora. The head also has some cream spots near the eyes. Females can have a body length up to 14mm, males are a fair bit smaller. The species is listed as being widespread across lowland Britain where it favours sandy sites, particularly coastal or on inland heaths.
The common name of the Red-legged Spider Wasp is often used when referring to this species.
Episyron rufipes (female)
Hartland Moor, Isle of Purbeck, Dorset | Aug.21 & July 23