Subject: Weird, weird fly in Wisconsin.
Location: Wisconsin, USA
June 25, 2012 3:24 pm
Hi there,
Do you have any idea what the heck this is? I found it in my window — it’s about 1” long, very hefty. Apparently dipteran. This creature has a weird thing sticking out of the front of its head (mouthparts? emerging parasite?) and a couple of black upright ”fins” on its back just forward of the wing bases.
I’m an amateur entomologist and I’ve never seen the likes of this blighter before.
Thanks, and I’m interested in what you come up with!
Best wishes,
Signature: Rhian

Bot Fly

Dear Rhian,
This is some species of Rodent Bot Fly in the genus
Cuterebra.  Bot Flies are parasitic flies.  BugGuideprovides this graphic description of their life cycle:  “Females typically deposit eggs in the burrows and “runs” of rodent or rabbit hosts. A warm body passing by the eggs causes them to hatch almost instantly and the larvae glom onto the host. The larvae are subcutaneous (under the skin) parasites of the host. Their presence is easily detected as a tumor-like bulge, often in the throat or neck or flanks of the host. The larvae breathe by everting the anal spiracles out a hole (so they are oriented head-down inside the host). They feed on the flesh of the host, but only rarely does the host die as a result.”  We are copying Jeff Boettner to see if he is able to provide a species identification for us.

Bot Fly

Dear Daniel,
Thanks very much for the I.D. on that peculiar creature! The pictures in the link you sent look exactly like it, right down to the “fins” on the back (which I suppose are some kind of halteres?). That’s certainly a bizarre life cycle for a strange looking creature; for some reason, I thought bot flies were mostly tropical.
Thanks again, and keep up the good work with the site! 🙂
Best wishes,

Jeff Boetner replies
Hi Daniel and Rhian,
Great shots. Yes, a Cuterebra botfly, this is one of the Cuterebra fontinella bots. You have two subspecies of this bot in WI, Cuterebra fontinella fontinella, which uses white footed mice as a host, and Cuterebra fontinella grisea, which uses deer mice as a host. The one you photographed is very freshly emerged, the wierd face is from a balloon like structure that inflates to help push the fly out of the pupal case, and then it gets reabsorbed back into the face. These guys don’t feed as adults so have no real mouth parts.
It is hard for me to do this one to species, but if you hung onto it, it might get better coloring after it has been alive for a few days. So if you can keep it alive, (they don’t feed so easy to keep), post another picture once the brown turns to white and black. I don’t see these this fresh, very often, unless I have reared one. Very fun to see.
I am doing dna work on bots, and I would be interested in the specimen. I don’t have dna from WI specimens, and still missing grisea if it turns out to be that one? Yours is female for sure from the spacing between the eyes.
Thanks for posting. And thanks for the forward Daniel. Love you site!
Jeff Boettner
Plant, Soil and Insect Sciences

Location: Wisconsin

2 Responses to Bot Fly

  1. Antikythera says:

    Horse owners can tell you that botflies do exist in temperate places. We used to see their eggs all over our horses’ legs in the summer. They were little tiny yellow nits and they had to be scraped off with a blade.

  2. Troy Meyers says:

    I wonder if you know why the halteres are so big, if they are really halteres, that is. Do they serve a different function in botflies than other flies?

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