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.