[This essay appeared in abbreviated form in a recent issue of the Arkansas State Audubon Society Newsletter.]
We easily recognize that this is a bumble bee. It's a large black and yellow insect with a furry yellow thorax with a bald spot in the middle of it. We might feel kindly towards this big bumbling creature visiting our flowers, but if I said "Pick it up, please," you would quickly hide your hand behind your back.
Do you realize instinct has helped to program your response, that you have responded in much the same way a bird or lizard or even many insects might have? What we are responding to is the color and the pattern on its back. Black and yellow are warning colors, they signal to us and other creatures that this thing either tastes bad or bites or stings. Black and yellow bands on a yellow jacket send the same message but not the same--What do I want to say?--trustworthiness. We're wary of yellow jackets; we know the bumble bee is completely inoffensive unless we try to pick it up, and the difference is that round yellow thorax with the black spot in the middle.
Now, lots of harmless insects that are very good eating would like to have the bumble bee's dangerous reputation, as it might discourage predators from attacking. In fact a surprisingly large number of insects imitate the bumble bee as closely as they can manage, for just that purpose. I'll show you here just a few.
Most of you recognize this first bumble bee mimic, the Snowberry Clearwing. It's a very common day-flying hawk moth that hovers before your flowers drinking nectar through its long tongue, its wings an invisible blur, looking like a small hummingbird. When it emerges from the pupa into adulthood it has brown scales covering its wings, and the thorax is covered with yellow fur. But almost instantly it begins shedding scales from its wings until they appear mostly transparent (as on a bumble bee), and the bald spot begins (as here on this fresh individual) to appear on its thorax. The strategy must do it some good; this is one of the commonest creatures in our garden.
You probably don't recognize this creature as a bumble bee mimic. This is an American Carrion Beetle, a common visitor to road-kill corpses (don't confuse its name with the endangered American Burying Beetle). The one in the picture is on the back of a dead opossum. Here on the corpse it doesn't seem much like a bumble bee. It doesn't have a furry thorax, which would just get bloody and messy. But, it has a pretending yellow furry thorax, with a pretending bald spot in the center. I didn't think it was a very good disguise, until one day I saw one flying swiftly to a road kill, following the scent trail. It was buzzing loudly like a bee and sweeping around at about eye height (also like a bee) and looked so much like a bumble bee I had to look and look to be sure it was a beetle, and I have a lot of experience with these mimics.
I've shown you so far a moth and a beetle as bumble bee mimics. This one is a fly, Laphria affinis. In the genus Laphria there are dozens of species, the great majority being bumble bee mimics. They carry the mimicry to a high art, and they are such a successful group that I am sure they get the full advantage of looking like something with a painful sting, which they do not have.
But here the discussion gets complicated. So far I have shown harmless creatures pretending to be dangerous. But this Laphria really is dangerous. It's a robber fly, a powerful killer of other insects, using, not a stinger, but a beak which injects neurotoxins and digestive enzymes that kill quickly and turn the insides of its prey into soup, which it sucks up its hollow beak. If you try to pick this one up he'll stab you and you won't forget it. They wait in an open place, and if a suitable prey insect flies over, they fly up like a falcon and snatch it out of the air, too fast sometimes for you to see.
So is it possible that this one relies on the other part of the bumble bee's reputation, the reputation for inoffensiveness, in order to cosy up to the creatures it is hunting? There is defensive mimicry, and offensive mimicry. Or at least there is a hotly contested theory that some mimicry is predatory in intent.
I was always skeptical of that second theory. For one thing, it is axiomatic that robber flies very seldom visit flowers, and if you see one landed on a flower it is probably a coincidence. So how is it going to cosy up to insects that hang out in the same places that bumble bees do, namely flowers?
Well it was this very species, Laphria affinis, that I one day saw bumbling and buzzing clumsily around some big sprays of flowers, often stopping to hang from them as if he were interested in their pollen.
I happened to have an insect ecology class with me and I was explaining this theory to them, and how skeptical I was of it. A number of orange soldier beetles were visiting the flowers paying no more attention to the big fly than they were to the real bumble bees also there, and before we knew it, he had one.
Here maybe I had better show a pair of mating Laphria thoracica (a different species of robber fly), to prove that at least they can tell each other from bumble bees.
I've led you carefully from step to step to help you see what I see in a remarkable horse fly I found yesterday drinking sap from a tree wound. I saw from a distance what looked to my practiced eye like the old familiar pattern. Here was a rather large stout all black insect with the yellow thorax with the bald spot in the middle. This group of flies in the genus Tabanus have a stereotyped pattern of close-together black stripes on the thorax, perhaps very difficult to alter. But horse flies I'm sure you have noticed have marvelous eyes of all colors and patterns of green or red or green-banded and so on. The males have eyes all the way across the top of their heads with thousands of lenses, and the patterns come from the light refraction from differently shaped deep-down lenses. This is where the different species get a chance to differ from each other, and it may only require a few genes to make the alteration. I think this fly has not only created a pretending bumble bee thorax, it has created a pretending thorax itself, out of its eyes. I have never seen the like before, and I can't seem to find anything like this in my books or in BugGuide (though, interestingly, I found a west-coast Tabanus that had evolved an actual furry-yellow thorax with a bald spot), and I wonder if anyone has seen this fly before and seen the bumble bee pattern, or if it is only my fascination with insect mimicry that has made me imagine it. I looked at it from different angles to see if the pattern held or was just an odd quirk from my angle of vision, and the pattern held.