These are fairly rare in collections because the adult is only active for a short time. Many species lack mouthparts because of their short adult life. It flies around, it mates, and it lays eggs on grass stems. The mouse brushes against the egg, the heat of the body causes the egg to hatch, the larva burrows into a natural body opening (open wound, mouth, nose, anus). For 5 days, it crawls under the skin until it reaches a suitable site and builds a little home where it grows. That little home is called a bot or a warble.
Parasites are like hackers, they look for ways to make a system work against itself. The botfly larva causes the body to secrete various wound-fighting serums, which it happily consumes, growing and causing the body to produce more and more serum. A larva can grow 22 mm long in a mouse that's only 65 mm long (ignoring the tail).
In masters study conducted here at KU, a student found that between 20 and 30% of white-footed mice in the study area were infected, though at peak periods, almost 9 out of 10 were infested. A quarter of infested mice had more than one bot. There was no effect of sex, but older mice were more likely to be parasitized than younger mice.
In the next set of images, you can see a warble on a human leg. Burt Humburg, ally of science extraordinaire, kindly provided the video from which I took the screengrabs (warning: slight profanity, disgusting larva, and the site where the video is hosted takes ads from porn sites). The first image shows the warble, a big bump on the leg with a hole. If the video were higher resolution, you'd see the tail of the larva wiggling around. The second image shows the tweezers someone is using to try to drag the larva out, which you can see in the lower left. The remaining images show the horrific beast out of the flesh. (Note: Doctors advise against attempting to yank the larva out as shown above, there are spines that the larva holds itself in with, and pulling is likely to leave parts of the beast in the warble. This is disgusting, and can also cause infection.)
I don't know the origin of the video, so I can't speak to the species of botfly in question. Odds are good that it's a tropical species, since the number of human infestations by North American botflies is vanishingly small. Central American species parasitize humans with greater frequency. One man chronicled his adventures with a bot that grew on his scrotum.
People with farms may be familiar with a very different subfamily of botfly than the one shown above. The genus Gastrophilus (stomach lover) will dive-bomb horses and mules, laying eggs on their fur. The eggs hatch, and as the horse grooms itself, it winds up ingesting the larvae which grow in the gut (hence the name). Horses, not surprisingly, don't enjoy the slow progress of the larva through their digestive tracts, and respond violently when they hear a botfly coming.
Science isn't always pretty, and pretty much anyone who knows anything about botflies thinks that they are the most disgusting things they've ever seen. Indeed, I'm told that the author of a popular book about parasites passed on blogging the video. There's something mildly cute about an isopod taking over as a tongue for a fish, but there's nothing cute about tiny wiggling things poking out of your skin.
But this is what makes biology fun. There are exciting, fascinating and unexpected discoveries awaiting us under barks, rock, and even skin.