Hey, so this is pretty gross: Last week my brother-in-law sent me a picture of a larva he had squeezed out of a lump in his dog’s back. I knew it had to be the larva of a bot fly (Oestridae), a member of a family of large flies that parasitize mammals. The thing is, these flies are very host-specific–there are rabbit bots, mouse bots, squirrel bots, sheep bots, cow bots, horse bots–and there is no dog bot. However, the rodent and rabbit bots (Cuterebra) have a roundabout way of getting their offspring into their host animals; they lay their eggs on vegetation where their intended hosts live, and when a warm mammalian body brushes past them, the eggs hatch and the larvae burrow in. Obviously, the warm body that brushes past is not guaranteed to be the right kind.
I didn’t get to see that larva in person because my brother-in-law threw it away in disgust. However, there was a second lump, and when I visited a few days later I got a picture of it:
Yesterday he squeezed the larva out of this second lump, and he brought it to me today.
At around 13 mm long, this one is nowhere near full-grown. See those two little black hooks on the left? Those were moving in and out as I photographed it, clearly trying to chomp the tasty mammal flesh that was no longer within their reach.
And here’s the back end. A bot fly larva maintains a hole in its host’s skin, and it keeps this end near the hole so it can breathe.
When a bot fly larva has finished feeding, it pops out the hole to pupate in the ground. Since this larva was still hungry and had been in the wrong animal to begin with, it was doomed, so I’m giving it to my friend Jeff Boettner at UMass, who as it happens is about to do some bot fly DNA analysis. He says that not much is known about which species get into dogs and cats, but that this is probably Cuterebra fontinella, a mouse bot. Here’s a shot of a recently emerged C. fontinella adult that I found in the woods near my house two summers ago:
That white puddle on the leaf came out of the fly, and I’d bet it’s the only picture you’ll see of bot fly poop. As far as I know, adult bot flies don’t eat anything, so this would be a meconial deposit like what a butterfly leaves after emerging from its chrysalis.
BugTracks 
It looks like a case of you are what you eat. Do the rabbit bot flies have longer ears?
I dunno about ears, but they do have red eyes, just… like… rabbits. Hm.
Great work, Charley. Crazy world we live in! –John Becker
Why are they called bots? Sounds so futuristic.
I’ve often wondered that… Webster’s says “Gaelic botus, a belly worm; boiteag, a maggot.” Whereas “robot” is of Slavic origin.
That nasty little demon sure is a cool bug! Although I feel bad for any animal that has to deal with them.
Fantastic photos, Charlie!
Wonderful photos! I stumbled upon a bot fly last summer and could hardly believe my eyes. It was fun for me to see what the larva is all about.
I had a similar experience with my dog, almost identical. We even found a second a few days later. I argued with friends that it was not a Bot bucause it looked so different from ones I’ve seen.
The experience differers in that my dogs experience seemed to begin a few days before we noticed the hole in his side and SEEMED to be accompanied by a cry of pain and displays of traumatized behavior, shaking, hiding, etc. Can something have caused my dog this trauma by laying an egg in his back?
Or coincidence?
Thanks
That makes perfect sense that your dog was showing signs of discomfort before you noticed the hole. It was clearly unpleasant for my sister & brother-in-law’s dog, and he wouldn’t let anyone near the affected spot. They finally were able to get it out when they distracted him with a big tasty hunk of chicken. These bot flies don’t lay their eggs directly on their host animals, though. My friend who studies them told me a dog or cat can acquire a bot larva by eating a mouse and having the larva chew its way through their stomach and keep burrowing till it gets to the surface where it can make a breathing hole. It’s no wonder that a dog would be a bit traumatized by this.
P.S. Northern California, August
I live on the same ranch as Roger and just extracted 4 maggots from my dog this week. He’s caught several rabbits over the last few months (he did not eat them though) so i imagine he picked them up from them. I’m not sure that they looked like the shot above. Seems like both ends were pointed and i did not look to see beyond that. (omg)
My dog’s symptoms included a productive cough two weeks earlier and a heck of a lot of itching that exposed the warbles this week. These were weeping with little air bubbles coming out. I popped the maggots right out… and i can tell you, both he & I were ‘traumatized’ from the experience. He’ll be going on heartworm prevention as soon as i can manage it- this is not the kind of experience we want to have on a regular basis.
I just found the breathing hole in my dogs skin today. She keeps crying and I don’t know what to do. My daughter told me she saw a large bug next to our dog the other day. She’s 6 so she could not really describe it so I told her to draw it. If you could see her drawing it looks exactly like this fly… Please help I don’t know what to do. My daughter plays in the same yard.
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Found very tiny larvae under a squirrel’s pelt that I shot, I hunted it for a small meal, but didn’t want to eat it but cleaned it and cooked it well on a spit so it wouldn’t go to waste and gave it to the dog. I hope that killed them, but after seeing this I think I”ll get her de-parasited. Me too maybe yuk!
I’m sure cooking them took care of them.
There happens 2 be one of these on my dog right now. We’ve tried squeezing it, but it hurts him. We don’t have the money 2 take him 2 the Vet right now. Please tell me there’s a home remedy !?!
I have no experience with this, but supposedly if you strap a thick chunk of meat over the breathing hole, the larva will chew through it, and then you can remove the meat while the larva is in it.