Take a look at this video of a pair of Phora I observed mating in Central California.
Apart from the humorous aspects of the “action”, including the hilarious way they kick each other to separate, there is actually some good science here.
Notice the movement of the male’s hind legs (how could you miss it!). Males of many species of Phora, and some other phorid genera, have tufts of setae (thick, hairlike processes) at the base of the hind femora. Presumably, the male uses them to contact the female during sperm transfer as a sort of courtship to persuade the female to use his sperm. Female flies often mate with multiple males, and can control to a remarkable degree whose sperm she uses. This is because the sperm from any mating is shunted to a storage organ called the spermatheca, where it is kept until needed. Females have been shown to have remarkable control over the sperm in a spermatheca, shunting unwanted or lesser quality sperm (as judged from mating stimuli received during copulation) to the back, or even expelling it. Thus, males not only have to copulate with females, but also have to convince them to use their sperm when they later lay an egg. Male signals, like the stimulation of hind femoral setae, are what convince her of the worthiness of the sperm.
Also of note is the position of the forelegs of the male, flat on top of the female’s scutum (“back”). Male Phora have thickened, flat foretarsi (the short segments at the end of the leg) that probably have specialized setae as well, but, well, nobody has really looked. It would be an interesting study for someone with spare time and access to a scanning electron micrograph.
The final aspect of the mating is the fastidious cleaning that the female subjects herself to. In insects, as in all animals, close contact with another individual always involves the possible transmission of parasites. For small flies, what we note most frequently, is the presence of mites, which can almost completely coat an unfortunate victim. This female spends a lot of time sitting on a leaf, exposed to predators (and photographers), while cleaning herself. It must be worth it, to keep away those mites!
This post is dedicated to my colleague Erica McAlister at the Natural History Museum in London, to give her something to read on a Sunday morning!