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this, probably, Calliphoridae pair was photographed in May 2007, Warsaw. Posiible ID?

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sodomy?

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very interesting photo indeed. I think that Calliphoridae is correct but I don't recognize the species. I haven't met every genus and species. Did you collect these? do you have other photos? I cannot see enough details to even identify a genus. I think that Calliphora is not possible based upon the wing alone. Onesia also have reddish-gold hairs at the back of the head but it doesn't look like an Onesia. Any other angles will help.

Since you are from Poland, you could try to ask Krzysztof Szpila from Katedra Ekologii i Biogeografii. He may be able to recognize the species and better explain what you are seeing.

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I see that you have posted a similar photo in the past:
https://diptera.info/forum/viewthread.php?thread_id=29897

I am not an expert but it is known that size of flies can vary based upon availability of food. For example, Pollenia rudis can measure between 6 and 10mm. If a 10mm male meets a 6mm female, then you have a strange photo. Perhaps this is a case of a smaller female meeting a larger male.

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Yes indeed. This entrance had no response, and I returned to it after I had gone through my archives. You suggest natural selection behind this sticking dimorphism. So, could it be that a female was such bigger than a male in an option?

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Females can be bigger too indeed.
Here are two examples.

Ben

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Another one.

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Beautiful posts Ben. Excellent photos to illustrate a point. :)

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nice!!!