C. said it had come out of the mouse. That was absurd, and I told her so. The thing was as big as the mouse's head! Besides, it looked clean, not covered in zombie slime. (I shot this video with the USB microscope – now you can see the zombie slime, as well as the spiky hairs that secure it inside the host.)
So I went to the internet. As a childhood bugologist, I figured beetle larva. Maybe pine sawyer beetle? We'd found a couple of those big beetles around the place. They're so big they look like hummingbirds when they fly. But wait, beetle grubs are long and narrow and have faces and legs, and they can move pretty quickly. This was faceless and writhing and pulsating, more like... a maggot (ewww No. 2). I hate maggots. I really hate maggots. Gah.
C. said, "Bot fly." I said, "Nah." Hey, I'm the bugologist here. Bot flies are pests on cattle, not little mice. The adult fly lays eggs under a cow's skin, and the nasty maggot grows there and splorts out as a big ol' fly. They leave holes in cow skin and ruin cow hides. I read that someplace, in the olden days, in a book.
So I look up "bot fly larva" and, sure enough, it's a classic rodent bot-fly maggot. They infest mice, squirrels and rabbits. The big, weird fly lays eggs along rodent trails, the rodent comes boobling along and the tiny larvae climb aboard and enter the nose or mouth. They find a cosy site inside, just under the skin, and grow. And GROW. After a month or so, they pop out of their puffy irritated home site (the "warble") on the host, drag themselves off by writhing and hooking their creepy fangs, and pupate into a big weird fly. The host generally recovers, but has nasty dreams. (I've lost count of the ewwws here.)
Cattle, horses, sheep and deer can also host bot flies. People, too, in Central and South America. But the ones that feed on little mice are just as big as the ones that parasitize us large mammals.
Nature, she is a big pervert.