The small black soldierflies in subfamily Pachygastrinae

Within the family Stratiomyidae, the subfamily Pachygastrinae contains five small, dark and rather dumpy little soldierflies. Three of the species have wings that are entirely transparent, with the other two having wings that are darkened in their basal half (that is the half of the wing nearest the body). This page focuses on the two dark-winged species.

A close look is needed to tell if the wings are in fact darkened, since when at rest the wings are held over the dark abdomen of the insect, and it isn't easy to tell whether you are seeing a darkened wing or a clear wing with a dark abdomen showing through. One way to check is to look closely at the leading edge (costa) of the wing, where the costal vein that forms the edge of the wing should be clearly darkened in the two dark-winged species.

At a recent FSC BioLinks workshop we recorded both of the dark-winged species: the widespread (in England at least) and common Pachygaster atra (Dark-winged Black) and the much scarcer and restricted Eupachygaster tarsalis (Scarce Black). These were found by sweeping the foliage of oak trees - in fact, both species were caught in a single sweep of the net, allowing close comparison, and providing a chance to take the photographs shown below. These are female flies - for the males, the difference in the edge of the scutellum will be the same, but the differences in width of the occiput are less pronounced.

Comparison of Eupachygaster tarsalis and Pachygaster atra

Eupachygaster tarsalis has only about 30 records on the recording scheme database, but it seems likely to have been under-recorded, given its small size and close similarity to the much commoner Pachygaster atra. Both species have larvae associated with decaying wood, and for Eupachygaster tarsalis there may be a link to rot-holes that are reltively high up in trees. Sweeping tree foliage seems to be the best way to record the adults, so keep a close eye out for the rarer one and send in your records of both species please!

The two individuals photographed above were found at Dinton Pastures, near Reading in Berkshire, on 27 June 2018. Record details are available on iRecord: