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There is a long-winged form of the short-winged conehead, but the cerci visible by zooming in does not seem to have the curved tip (there is a little brown point at the top of it, but I think that's something else). So, long-winged it is.
Going purely by the photo, I can't see enough detail of the male genetalia to make out which of the two Conocephalus species it would be, even if I zoom in. Both are very very similar and I would like to see a better picture before making a definite ID for this species.
Tachinid Recording Scheme
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Agree that Long-winged Conehead looks to be the likely option, but from this photo alone I also have to agree with Matt that we can't be 100% certain.
Entomologist and biological recorder
I agree that one cannot reliably id this specimen from the image. However using local knowledge I can clarify the situation. I know that C. discolor (long-wing c-h. - now C.fuscus) was first recorded at Wittenham Clumps in 1990 by David Gibbs. Over the 20+ years that I have subsequently worked at this site it has become abundant everywhere. I have never seen C. dorsalis (short-winged c.h.) there. As far as I know the only modern (post 1930's) records for C.dorsalis in Oxfordshire are from the Chiltern Hills (not seen these) and at Otmoor, where I recorded it in early 1990s. My observations are based on male specimens.
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Lat/Lng: 51.633312882412, -1.179840445584
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