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Although I agree with what Chris and Adrian have said, wing venation can still be a very useful tool in identifying genera and (sometimes) species. One of the early keys to British Tachinidae genera (by C.D.Day) was largely based on wing venation, and although it wasn't 100% accurate, it was MUCH easier to use than the subsequent key (by van Emden). One of the reasons that wing venation is not used more is that describing the relative lengths and angles of the veins can be very tedious - often there are other characters that are easier to describe using words in a key!