Archytas Tachinid Fly

2024 June 30

Sam has been house-sitting for a friend, and on June 26, 2024, she texted me this picture, and said “I found this gigantic sinister-looking fly on the porch. He’s almost the size of the end of my pinky”

My first idea was that it might be a Bot Fly, which are parasitic flies that infect mammals[1]. So, I told Sam that, and asked her if she could catch it for me and pop it in the freezer for later. To which she replied, “Yup, I can catch your horrible parasite fly for you”. What a trouper.

So the next day, Sandy picked it up and brought it home so I could get pictures.

And with a bit more time to look it over and search online, I came to the conclusion that I was mistaken. It is not a bot fly. But, it is a parasitic fly. It appears to be one of the Tachinid flies in the genus Archytas.

The wings were in rough shape[2], indicating that this was an old fly and was probably about to die of natural causes when Sam found it. And while it is parasitic, its normal hosts are large caterpillars, particularly cutworms and armyworms. Not mammals. Whew!

They are kind of bristly, and so are sometimes called “Bristle Flies”. The face of this particular genus is pretty distinctive, almost bone-white.

I don’t think the adults bite, their mouthparts look more suitable for things like flower nectar. And possibly other nutrient-rich liquids that I’m not going to dwell on here. It is, after all, a fly.

There doesn’t seem to be a lot of information about these flies specifically, because they are a member of the Tachinidae family, which has thousands of species. They are all believed to have very similar lifestyles, with the adults sticking their eggs to various type of caterpillars. The eggs then hatch, and the maggots then burrow into their hosts to devour them from the insides. Parasitic insects like this tend to diversify into a lot of different species, because they usually specialize onto particular hosts. Tachinid flies aren’t generally as specialized as the parastitoid wasps are, they tend to parasitize general types of caterpillars rather than a particular species, but they do have their preferences. And one point is that they can’t actually be larger than their hosts, because how would that even work? So a big parasitic fly like this has to target only the bigger (but still reasonably common) caterpillar species.

[1] Bot flies are kind of horrifying. They are enormous, ugly flies that use a variety of methods to get their eggs into their mammalian hosts. The eggs then hatch, and one way or another the larvae generally end up in a cyst under the skin of their host, where they cut a little breathing hole and settle down to eat the pus and lymph and blood that leaks into the cyst. Although, sometimes they live other places, like in the nasal passages or intestines. Eventually they turn into this enormous maggot that pops out of the breathing hole, burrows into the ground, and pupates into a new fly to start it over again. But, this isn’t a bot fly. Maybe someday I will catch one, though, and then I will have an excuse to give all the gory details. Something to look forward to, right?

[2] Insect wings don’t heal, they progressively accumulate damage all through the insect’s adult life. So the condition of the wings is a pretty good indicator of whether a mature insect is near the beginning of its adult life, or near the end.

One Response leave one →
  1. July 13, 2024

    Unlike many tachinids, Archytas does not actually stick its eggs onto its hosts; it instead gives live birth, dropping large numbers of maggots somewhat arbitrarily onto vegetation (the maggots then wait for passing caterpillars).

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