“Oh wow, look at that fly!”
This is not an uncommon expression from a camper in the “Forest
of Mysteries” summer camp, where we explore whatever we find outdoors. But this fly was more than uncommon: it was a
once-in-a-lifetime encounter with one of the most bizarre creatures to fly through
Vermont’s forests: Cuterebra emasculator (the
Squirrel Bot Fly).
One of the first
things you notice as you closely examine the face of this bumblebee-like fly is
that it has no mouth. In fact, the bot
fly’s mouth implodes upon emerging from its pupa. Adult flies do not eat, and only live for
around a week. Within this short adult
life, they quickly go about mating and laying eggs, and are rarely
encountered. The bulk of C. emasculator’s
life is spent in its larval stage, just under the skin of a chipmunk.
Eggs of C.
emasculator are laid just above the entrance to chipmunk burrows, or above
their frequently-traveled trails. As a
chipmunk passes under an egg, its body heat causes the egg to hatch and the
freshly-emerged larva drops onto the mammal’s back. The larva quickly finds its way into the body
(through mouth, nose, etc.) and wanders until eventually settling down (often
near the testes). There, the larva
drills a snorkel through the skin and the chipmunk’s tissues develop an
isolated chamber (warble) for the larva.
The larva often emerges from the chipmunk without causing
life-threatening injury. This is to the fly’s advantage. After all, if a parasite kills off its host, it
may put itself “out of business”.
Allowing the host to survive helps sustain the species. And thus this parasite and its chipmunk host
manage to coexist.
This is just a small taste of why I always look forward to
the next time a camper says,
“Oh, wow, look at that!”
