I first heard about bot flies during a college semester abroad in Costa Rica. After having the life cycle of bot flies described to us, my classmates and I lived in semi-constant fear that we would wake up with a bot fly maggot burrowing under our skin (luckily this did not happen to any of the students that year). However, since Costa Rica was where I’d learned about them, I never considered that bot flies could live anywhere other than tropical places. This belief was reinforced by the fact that no one talked about such a thing in New England. It turns out I was wrong. Bot flies are fairly common here, their hosts just tend not to be human, so they are seldom bothersome to us. As it turns out, there are many bot fly species in North America, and some, like the mouse bot fly (Cuterebra fontinella) I came across last week, have very specific hosts; Cuterebra fontinella specializes on using white-footed mice, Peromyscus leucopus, as hosts. 

Adults are about the size and shape of a bumblebee. In fact, with colors that mimic bees, I almost overlooked this individual, dismissing it simply as that: a bumblebee. A second glance, however, revealed various distinguishing characteristics of flies: larger eyes, smaller centrally located antenna, and only two (instead of four) wings. 

Adult bot flies live only a few weeks, during which time they mate and lay eggs. In fact, it is important that they store a large amount of reserved fat as larvae because they can’t feed as adults. Once the female has mated she then goes off to lay her eggs. All said and done, a typical female will lay one to several thousand eggs, in groups of approximately 5 to 15 per site. Within the genus Cuterebra, where exactly the female lays her eggs is dependent on the species. Some species will deposit their eggs on vegetation or other surfaces near the opening to a rodent burrow, while other species actually enter a rodent burrow and lay their eggs within it. Both techniques are designed to give the larvae easy access to a rodent host once they hatch. The larvae are moist and stick to the fur of passing rodents, facilitating this process. The larvae then find a natural body opening (e.g., mouth, nose, eyes, or anus) to enter the host, but will then migrate to a subcutaneous position to carry out their development (which takes approximately 3 to 4 weeks). This produces a swelling under the host’s skin called a warble. Within the last 4 or 5 days of development, the larvae begin to enlarge the pore of the warble to make exit through the skin possible. Once outside the host, the larvae burrow into the soil to pupate. Depending on conditions, it could be anywhere from a month to several years before the adult fly emerges from the pupal case and starts the cycle again. 

I’m very glad I’m not a mouse.