Mating Muscid Flies

2015 August 8

I don’t think these are house flies, but they do appear to be among their many relatives. And they are busily engaged in making more. Sam and Rosie found this couple near our front door on April 12, 2015, and when I came out to get pictures the flies hopped onto the back of my left hand and kept right on with what they were doing.

The male is the one on top. This gives us a good opportunity to see how you can tell male flies from female flies. If you look at the eyes of the male, they are large and almost touch across the top of his forehead. The female’s eyes, on the other hand, are smaller and well-separated from each other.

This behavior of continuing to mate even in the face of potential danger is more due to the male than to the female. The female would probably prefer to be rid of him at this point so that she could flee, since she can find another male pretty much any time. The male, on the other hand, is trying to make sure that her eggs are fertilized by him and not by anyone else. He’d literally rather that they both die, than let some other male have a chance at her eggs.

On closer examination (and given when and where we found them), I suspect that these are cluster flies in the genus Pollenia. The male is clearly showing one of the key cluster fly traits; he has yellow hairs on his back. There is a key for identifying them to species in this PDF of an article from the Canadian Journal of Arthropod Identification, but unfortunately the characteristics that they use to separate species mostly aren’t visible in my photographs. From the range maps on BugGuide, though, it looks like the best candidates are either Pollenia pediculata, or the European import Pollenia vagabunda.

Aside from their habit of clustering on the walls of buildings and hibernating under house siding (and frequently getting inside houses in the process), these are innocuous flies as far as we are concerned. Their larvae parasitize earthworms, which are so common that they could actually use a few more predators around here.

One sees flies mating pretty frequently, even though each female supposedly only mates once. It’s just that there are so many flies, and their lifespans as adults are so short [1], that the odds of coming across a couple mating are not actually that bad.

[1] “How long do flies live?” “I don’t know. I think a month, two months.”

3 Responses
  1. August 8, 2015

    Looking at this makes me wonder about the fluid dynamics and plumbing of the situation. It would be best for all involved if they mated like sparrows, in mid-flight, taking only a second or two.

  2. August 11, 2015

    One would think that. After all, a wasp can get enough venom into you in a fraction of a second to give a nasty sting, so they can clearly transfer signifiant amounts of fluid in a hurry when they want to. And yet, I constantly see mating insects of all types where the male hangs around for extended periods of time (sometimes even an hour or more) in a position that denies access to other males[1]. There is clearly a big selective advantage (at least for the males) to not just mate with the females, but also to deny other males the opportunity to do the same. And this advantage to the males obviously over-rides the benefit to the female of having it be quick so that she doesn’t spend so much time hampered and an easy target for predators. Otherwise, we would see quick mating[2], like you say.

    [1] Or sometimes, leaves behind a part of himself to do the same job – when male honey bees mate with a new queen, their genitalia basically explode, leaving part attached to her while the male bee drops to the ground and dies. So, on the one hand, they are mating”like sparrows, in mid-flight, taking only a second or two”, but on the other hand they are managing to block access by other males for at least the amount of time it takes for the next one to remove the ‘plug’.

    [2] Or, maybe, *not* see quick mating. As in, we would never notice it happening because it would be over in an instant. Which means that I’m probably seeing a somewhat biased sample. I’m overwhelmingly more likely to see mating between insects that go in for mate-guarding, simply because they spend so much more time mating than they would otherwise. So while it is obvious that a lot of insects take a long time to mate, it doesn’t necessarily follow that the *majority* take a long time to mate.

  3. August 12, 2015

    Thanks for the excellent tutorial. I’d never thought of the blocking maneuver.

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