Sepsidae

Black Scavenger Flies

DIPTERA > BRACHYCERA | Higher Diptera

MUSCOMORPHA > SCHIZOPHORA > Acalypratae > Sciomyzoidea > Sepsidae (29 species)

(subfamilies: currently none, but previously Orygmatinae and Sepsinae)

With the exception of the infrequently recorded seashore species Orygma luctuosum, which is somewhat of a misfit, the family as a whole are generally described as small, slender, dark-coloured, ant-like flies. They can often be seen running around on vegetation whilst repeatedly waving their wings; a curious habit that is particularly apparent in the species of the common Sepsis genus, as the motion is emphasised by the flickering dark spots at the wing tips. This wing-waving behaviour is believed to be part of their courtship display that has been likened to flag-signalling, which in turn has given rise to the alternative common name of Ensign Flies.

Orygma luctuosum is a relatively large, brown, seaweed-breeding species that was formerly classified with the Coelopidae kelp flies, but is now considered as the basal genus of the Sepsidae. It is unique in respect of both habitat and its bristly appearance in comparison with the other members of the family and, as such, is often referred to as a small coastal dungfly rather than a sepsid. It is found on the strandline in and around beds of rotting seaweed, particularly where a mixture of Fucus seaweed, such as bladder wrack, and Laminaria kelp is present. In this habitat it will usually be found living in close association with the likes of Heterocheila buccata, the kelp fly species Coelopa frigida, Coelopa pilipes and Malacomyia sciomyzina, the Fucellia strandline anthomyiids and, very likely, the predatory scathophagids Ceratinosoma ostiorum and Scathophaga litorea. It is probably under-recorded as there are very few confirmed records locally either from Dorset (VC9) or South Hampshire (VC11) and, although described as widespread, records elsewhere are very localised and scattered around the coastline

The more typical sepsid species are generally found in damp habitats where the larvae feed on various organic matter such as rotting vegetation, fungi and dung. They are small to medium-sized (2-6mm) flies with relatively few bristles or hairs; body colour generally black, in some cases lustrous and/or with a distinct pattern of silvery pruinosity on the side of the thorax; head rounded; wings clear or tinged, often with a dark spot or clouding at the wing tip; costa continuous; vein Sc complete; crossvein BM-Cu present; cell cup closed; in the male, the femur and tibia of the forelegs often with extrusions, spurs or teeth; tibiae usually with dorsal preapical bristle.

There are five species that seem to be particularly common; Sepsis cynipsea, Sepsis fulgens, Sepsis punctum, Sepsis orthocnemis and Nemopoda nitidula. A little further down the rankings are Sepsis violacea and Sepsis flavimana, followed by some of the eleven Themira species.

Although individual species can be determined from a 'key', it is almost impossible to reliably identify them without microscopic examination. However, with a few well-detailed lateral views, experts who are familiar with the species can sometimes identify males by the spines along the underside of the front femora. That was the case with at least one of my photos; the image captioned Sepsis cynipsea shows the characteristic ‘nick’ that identifies the species. Another individual photographed a couple of weeks later is also likely to be the same species. Similarly with some others I saw at around the same time, but there's no certainty, which is why they're simply captioned as Sepsis sp. All twelve members of this genus have the characteristic dark markings at the wing tips, whereas Nemopoda nitidula and the various Themira species have clear wings.