by Carl Strang
There are mimics, and there are mimics. A couple years ago at Mayslake Forest Preserve I found a syrphid fly that mimics bumblebees.
It was comparable in size to a small worker bumblebee, though none of our local bumblebee species match this color pattern, and it doesn’t really pass for more than a few seconds’ examination.
But there are others, like the one I saw yesterday:
This is a robber fly, a predator known to catch bees as well as other relatively large flying insect prey. There are a couple species in the East which are very similar to one another. Their larvae tunnel in rotten logs, preying even then on other larvae they encounter. After maturing, they perch in sun flecks on leaves or on more solid perches. The more common one in DuPage County appears to be Laphria thoracica.
Note the long black hairs on the top, front and sides of the head. If those were a dense solid yellow this would be a different species, Laphria grossa. I encountered those in south central Pennsylvania, and the impression was much the same. That beak probably could deliver a nasty bite if you grabbed this critter, but why would you want to? These take a longer, closer look to distinguish them from bumblebees. That beak is one giveaway. Another is the single pair of wings rather than two. Also, the perched fly frequently snaps its head to different angles, tracking possible prey. Bumblebees don’t do that. These robber flies are on my short list of niftiest local mimics.
Why the mimicry? One has to think it provides some protection from vertebrate predators. Also, these are among the largest of the robber flies, most of which are not mimics. That large size seems to slow them down, as they don’t fly nearly as fast as their smaller relatives. Perhaps the bumblebee coloration and speed leads prey to ignore them until it’s too late.