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Acrocera orbiculus. Taken as half of what appeared to be a pair in copula. Back at the ranch I cannot distinguish between them. Should I be able to see an obvious external difference in the terminalia? Do both sexes have holoptic eyes? Isolated elder bush on Bedfordshire, UK chalk. 29 July 2021.

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In Acrocera, it is dead simple

scutellum yellow -> male
scutellum black ->. female

wait for the forthcoming fieldguide to flies with three pulvilli (yours truely & Andre Schulten)


Theo

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Thank you Theo. In that case both individuals are male. Apparent mating just the result of over-readiness I guess.

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So can also check the genitalia, but is tricky. Female have long cerci, often taken for male genitalia.
Most Acroceridae are sexed vice versa !
If you have both genders, it should be obvious.
But if you do not, this is of little help

Theo

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https://www.researchgate.net/publication/331954794_Evolution_of_fossil_and_living_spider_flies_based_on_morphological_and_molecular_data_Diptera_Acroceridae/figures?lo=1

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Thanks again Theo.