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I am trying to get to ID the genus Hebecnema and would be grateful if anyone could tell me if all our UK species have 4 post-dc
I believe they all do. Once you get to genus, remember that a couple of names in the old RES key (if this is what you are using) are now out of date i.e. vespertina (bare eyes, dark halteres) become nigra, affinis (bare eyes, yellow halteres) becomes vespertina.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). Not the easiest muscid genus, esp if you are only using photos. Some Lophosceles can look similar in the field, but they have a strong PD on the hind tibiae.
Hope this is helpful
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.
Hope this helps to sort out any confusion.
Thank you both for the useful advice, this is where experience really shows
I'm trying to understand the genera charcteristics in the hope that it will be easier to identify the species, as many of the muscidae look the same on first glance!