Distinguishing species of Thelaira

In my experience a lot of people have trouble with this genus because the traditional features used in keys are quite variable. In particular it is difficult to separate the common Thelaira nigripes from the rare solivaga.

All keys use the size of the outer-vertical bristles and the anterodorsal bristles on the middle tibia, which are a bit variable, and Belshaw (1993) uses the colour of the abdomen, which is one of the worst features to rely on – it is a confirmatory feature at best. This leads to a lot of Thelaira nigripes being wrongly classified as Thelaira solivaga – the 2 species are very similar but they can be distinguished if the correct features are used.

Belshaw is a little ambiguous when he talks of the middle leg  only having “2 long bristles on its anterodorsal surface”. What constitutes “long”? All Thelaira have both long and short bristles on the ad surface of the mid tibia but I prefer to ignore the smaller ones at the top and bottom of the tibia and focus on the rest, in the middle. Here are some photos of Thelaira nigripes tibiae:

Here is a photo of a Thelaira solivaga mid tibia:

Also, a very useful approach is to look at the male genitalia – here is a nice set of figures from an article I saw a while ago:

43Thelaira leucozona (Panzer, 1809); 44Thelaira solivaga (Harris, 1780); 45Thelaira nigripes (Fabricius, 1794); 46Phenicellia haematodes (Meigen, 1824) (not British).

Pay particular attention to the segment just before the genitalia themselves and the way that the surstylus curves. The distinction between nigripes and solivaga is still very fine but in conjunction with the other features you should be able to make a more confident determination. Later I will add some closeup photos of the male genitalia.

2 comments to Distinguishing species of Thelaira

  • John Coldwell

    Hi Chris/Matt

    I’ve found a female Thelaira this year that has 4 long mid-tibial ad bristles, just like your photo.
    Is the length of the penultimate tarsal segment as stated in Belshaw relevant? If so, it serves to confirm my id as solivaga. Does it matter which leg? or are we to assume he’s still referring to the middle leg?

    Cheers,

    John

    • Apologies for the delay in getting back to you – I don’t check comments on the website very often – emails are a faster option 🙂

      I would say that yours sounds like a solivaga – they are much rarer than nigripes but they do crop up fairly regularly. The main issue is the variability of the bristles on the mid-tibiae and it can be quite difficult to judge which bristles are long enough to qualify.

      If you bring the specimen to any of our workshops or an exhibition then I’d be very happy to look at it for you 🙂