Yesterday afternoon my wife called me and pointed out a large insect crawling on the lawn. “Look,” she said, “this is the largest bumblebee I have ever seen.” The insect indeed looked like a big bumblebee as it slowly buzzed, trying to take off. But when I looked closely I immediately realized that it was not a bumblebee, but a fly.

It was not the first time I encountered this particular kind of fly, a member of the family Oesteridae, or a bot fly. These flies are quite pretty as adults, big and fuzzy, similar in their appearance to carpenter bees or bumblebees. They have no functional mouthparts, which means that they cannot bite anybody or feed on anything, and as adults live only for a few days. But earlier in their life they were rather nasty creatures.

The first time I experienced an intimate encounter with a bot fly was a few years ago in Costa Rica, when after a few weeks in the country I found two fat maggots of these parasitic insects embedded deep in my left arm. How they got there makes for quite an interesting story: following the mating a female bot fly had caught a live mosquito, and deposited an egg on its body; a tiny larva hatched almost immediately and waited for the mosquito to land on the host, (me, in this case); once there, the larva disembarked the mosquito, and buried itself into my skin. I only started feeling that something was living in me when the larvae were about two weeks old, and well entrenched in my tissue. This species of bot fly (Dermatobia hominis) attacks people and other primates, but other than it being unpleasant to have a maggot digging in your tissue, usually don’t cause too much damage to their host. This is because we are a very large host for a relatively small parasite.

But in NE North America bot flies attack much smaller hosts – squirrels, mice, and other rodents – and can cause substantial injuries. Often they leave their host severely disfigured, and the wounds they create can cause massive infections. The species we found yesterday, ominously named Cuterebra emasculator, was even implicated in causing sterility of male squirrels by selectively attacking their testicles; luckily for squirrels this turned out not to be the case. Unlike the Neotropical human botfly, North American species do not employ mosquitos to propagate their larvae, but lay eggs near rodent burrows, on rocks or directly on the surface of the soil. The eggs then stick to the fur of the intended hosts, and their body heat stimulates the larvae to hatch.

Dogs often fall unintended victims to bot flies, and last fall our always inquisitive Max contracted one while digging for chipmunks. Thankfully, we found and removed it early on in its development, when the larva was still small and easy to extract. Occasionally, people also get infected with North American bot flies.