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Yes, Thelaira nigripes is by far the commonest Thelaira in the UK and they are highly variable in the colour of the abdomen. I have seen a few Thelaira solivaga and they just tend to crop up randomly but infrequently and so any suspected solivaga needs close examination to be sure. Male genitalia can be helpful but an ID usually depends on a number of features and it's best to have the specimen under a microscope to be really sure ... but at least it isn't a Siphona :D