Scathophagidae
It can be quite tricky to decide when you are dealing with a Scathophagid since, in most keys, this family drops out at the end of the key to Calyptrate families if it is not a Muscid, Fanniid or Anthomyiid. They tend to be distinguished by the lack of features that characterise these families, rather than by anything very definite that is unique to the Scathophagidae. In general appearance they can easily be confused with Anthomyiidae and some species (e.g. Leptopa, Delina, Gimnomera) could be mistaken for an acalyptrate on first glance. However, in all British species with the exception of, Leptopa filiformis, the back of the head is noticeably rounded with a patch of fine hair on the bottom part of the occiput, and this does give them quite a distinctive appearance.
Colyer & Hammond (1968)[1] describe the family thus:
"Head more or less rounded in profile; at least four orbitals; ocellar triangle never pointed anteriorly; one or two large vibrissae, sometimes accompanied by smaller bristle posteriorly; proboscis in predatory species strong, labella armed with prostomal teeth for perforating the integument of prey; palpi well developed; arista pubescent or plumose; wings long, often tinged or clouded at cross-veins, in one case spotted with a dark band along the costal margin (Ernoneura); Veins 3 and 4 more or less divergent apically, or very slightly convergent (e.g. Scathophaga scybalaria L.); supplementary cross-veins between Veins 3 and 4 (Pogonota) or recurrent veinlets from Vein 2 (Ernoneura); 2nd basal and anal cells never sharply-pointed at lower, outer corner; Vein 6 may or may not reach margin; Vein 7 always in evidence, even if only as a fold; abdomen, often in profile appearing long and flattened, and with five or six visible segments; male genitalia very prominent."
Hackman (1956)[2] gives the following characters to separate them from other Muscidoidea:
"The eyes in the male are always widely separated and the frons never bears crossed bristles. Wing squama small, costa without spine-like bristles at the end of the sub-costa. Scutellum always bare on the under surface. ... Abdomen with more than 4 pregenital joints and without a dorsal pattern of spots or stripes."
Oosterbroek (2006)[3] describes the characteristics of the family as follows:
"Characters: Small to large (3‑12 mm), usually slender flies. Colour ranging from a dull yellowish brown to lustrous black or yellow, in some species bicolourous. Body and legs often with many bristles, sometimes densely covered by fine hairs. Occiput usually with some to many pale, longhairs; arista bare to plumose; interfrontal bristles absent. Wing usually clear, sometimes distinctly marked or darkened at the tip or along the crossveins; anal vein long, usually reaching the wing margin. Meron without bristles along the hind margin, near the posterior spiracle."