Some kind of MÃ©nage Ã trois or is the lower fly actually being eaten?
There is a lot of this going on. The empids are mating, and the female is feeding on the third insect (maybe a Bibionid). This is common to see. I don't know if the male mounts her as she is distracted by the food, or the food is part of the mating sequence.
In some groups of empidid flies the females won't mate unless they are provided with a 'wedding present' by the male, in the form of a nutritious insect on which to feed. In some cases it seems that this is an essential meal, providing proteins that help the female's eggs develop, while in others it may be that it is not actually necessary for the female's nutrition, but has become part of the mating ritual and is thus still essential to a successful courtship.
In at least one species it has been shown that the time spent in copulation depends on the size of the gift presented to the female - a larger, more nutritious prey item means the female spends longer mating.
Entomologist and biological recorder
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