I had been using the key in Fonseca's 1968 RES Handbook to identify a fly of the genus Hebecnema when I came across the following comments by Steven Falk and Howard Bentley in a 2008 thread at the DipteristsForum:
Steven Falk: ... Once you get to genus, remember that a couple of names in the old RES key (if this is what you are using) are now out of date i.e. vespertina (bare eyes, dark halteres) become nigra, affinis (bare eyes, yellow halteres) becomes vespertina. Also beware fumosa, males of which can have the tibiae so dark they almost seem black, and can take you to umbratica (though fumosa averages somewhat larger). ...
Howard Bentley: The difficulties of Hebecnema are compounded by errors in "The Muscidae of Central Europe" by Gregor et al. which many people are now using as their main key. You have to follow "Halteres with yellow knob" to get to vespertina, but the description of the species then gives the haltere colour as "dark brown to black". The latter is wrong. The only British Hebecnema with really black haltere knobs is nigra; in the key, "not black" would be a better description of haltere colour than "yellow" - they can be pretty dark in vespertina, but are certainly not black. Similarly, the key says that vespertina has "hind tibia usually with 2 anteroventrals", while the description says it usually has 1: in my experience the description is more often right here.
The authority for the affinis->vespertina; verspertina->nigra reassignment seems to be Pont, A.C. 1984. A revision of the Fanniidae and Muscidae (Diptera) described by Fallén. Entomologica Scandinavica 15, 277-297. I have not yet seen a copy of that paper, so to help me get things clear in my own mind, I constructed a copy of Fonseca's key interspersed with the comments of Falk and Bentley:
1 (4) Legs largely reddish-yellow, or at least knees and extreme apices of femora conspicuously pale. Halteres yellow. Male: lower squama and its fringe distinctly brownish-yellow. 2 (3) All tibiae and middle and hind femora reddish-yellow. Abdomen tending to be somewhat translucent brownish and shining. Eyes practically bare. 5-6.75mm. Generally distributed. Frequent. ii-x...... 3. nigricolor Fallen. 3 (2) All femora mainly black, the extreme apex conspicuously pale in contrast to rest. Middle and hind tibiae usually obscurely translucent reddish. Facial orbits in profile mainly (Male), or on lower part (Female), invisible. Male: eyes densely hairy; abdomen, seen from behind, rather densely dusted brownish-grey with a hardly discernible median darker area. Female: eyes microscopically but distinctly pubescent; frontal triangle, seen from slightly behind, not extending more than halfway from front ocellus to lunule. 5.5-6.25 mm. Worcs., Middx., Gloucs., Berks., Oxon., Somerset, Hants., Devon and Surrey. Uncommon. iii-x.................... 2. fumosa Meigen. 4 (l) Legs entirely black or brownish-black, at most the knees occasionally obscurely reddish. Facial orbits, in profile, quite distinct throughout their length. Male: lower squama whitish with pale yellow fringe. 5 (6) Eyes densely long. (Male) or short- (Female) haired. Abdomen, seen from behind, with (Male) rather dense grey dusting and a sharply defined narrow median dark line, or (Female) with thin dusting and a broader, less sharply defined dark median area and faint shifting dark spots. Female: thorax, seen from in front, with 4 faint but more or less distinct darker stripes; frontal triangle, seen from slightly behind, usually extending almost to lunule. 3.5-6mm. Generally distributed. Very common. iv-x.... 1. umbratica Meigen. Falk: Also beware fumosa, males of which can have the tibiae so dark they almost seem black, and can take you to umbratica (though fumosa averages somewhat larger). 6 (5) Eyes practically bare. Thorax and abdomen more uniformly dull brownish. black, without distinct markings. 7 (8) Halteres with brownish-black to black knob and paler stem. Hind tibia. with only 1 anteroventral bristle. 4-5.25 mm. Scotland: Morays., Inverness. and Aberdeens. England and Wales: Norfolk, Herefords., Glamorgan. Gloucs., Berks., Oxon., Herts., Somerset, Hants., Devon and Kent. Fairly common. iv-ix...................................... 4. vespertina Fallen. Falk: vespertina (bare eyes, dark halteres) become nigra 8 (7) Halteres entirely yellow. Hind tibia with 2 anteroventrals. 4-5.5 mm. Generally distributed. Common. v-x........... 5. affinis Malloch. Falk: affinis (bare eyes, yellow halteres) becomes vespertina. Bentley: the key [of Gregor et al] says that vespertina has "hind tibia usually with 2 anteroventrals", while the description says it usually has 1: in my experience the description is more often right here.
So my fly, which in Fonseca keys out to affinis but has only 1 anteroventral, is probably what is now known as vespertina.