Sex and the Single Entomologist
by Philip Johns
Whence Wings?
A Look at the Mayfly
Until recently, I lived in Philadelphia, a city whose claims to fame include cheesesteaks and rowing. If Philadelphia isn't the heart of American rowing, it isn't far off. In April, the US National Team's first selection regatta of the year happened near Philly, and I took advantage of this. How often do you get to watch national team try-outs? A quick drive in rush hour traffic, my usual unplanned detour, and I was at the finish line -- but on the wrong side of the lake. I was half an hour late, but the powers that be had blessed this boating event as they did Noah's and water poured from the heavens, delaying everything.
I found myself standing next to a national team coach and coxswain how cool am I? They smiled nervously and edged away from me when I told them I wasn't a spouse but a crew groupie. What fool would walk through mud and rain on a Friday evening to see strangers race in an arcane sport? I wondered what they would have done if I had told them I was an entomologist a claim that strictly speaking isn't true. For years my favorite joke has been, "I'm not a real entomologist but I play one on TV". Then I taught an entomology course and had to stop telling that joke.
But by then the weather was lovely and soon rowers barreled down the course. Men and women, absurdly tall and lithe, went by one at a time in the muted light. I noticed dimples on the water although it had stopped raining, and felt the almost damp fluttering of some buggy kind. These turned out to be mayflies, insects in the Order Ephemeroptera, or "ephemeral wing". Mayflies lack working mouths and may live only a day or two, sometimes only a few hours. Ephemeroptera is one of the most beautiful insect names, I think. They are also ephemeral winged in that some kinds of mayflies molt after they have developed wings. No other insect does.
The next morning, at the next round of races, I noticed something crawling near my feet. What appeared to be stoneflies (Order Plecoptera) were crawling over my shoes, over the gravel on the bank, some were coupling. I knelt to watch them, thus not only earning the sidelong glances from the other half dozen spectators but missing a race I had come to see.
Mayflies and stoneflies are ancient bugs, early branches on the evolutionary tree of insects. Some of the evidence for this lies in the structure of their wings. Over time, insect wings have become less and less veiny, in general. The wings of mayflies and stoneflies, for example, look like fine lace. In contrast, more recently evolved insects like flies and bees have only a few veins in their wings.
One of the big buggy questions is, why do insects fly? Whence wings? The ancestors of modern flying insects (silverfish, bristletails) don't have wings and they do fine. Where did wings come from? And why did bugs up and evolve them? Why bother?
Turns out, the ancient kinds of insects that don't fly also don't live in water as juveniles. A guy named Jim Marden has made a good case that insect wings evolved from larval gills which some mayfly and stonefly larvae beat under water [Warning: 6.9 meg file of gill-flapping but oh, so worth it]. In other words, these insects had flappy structures (gills) underwater already, and were thus pre-adapted to have flappy structures (wings) above water, too. But why do that? Marden outlined a series of steps that mayflies and stoneflies went through, skimming on six, then four, then two legs, and using their flappy wings to blow them around the surface like little wind boats. It's kind of cute. If you live in the right place, you can see winter stoneflies do this in February or March right after they emerge. The fewer legs mayflies use to skim around faster they can go. Eventually, it wasn't much of a leap to fly.
But why? Insects don't sit around in committees deciding how to evolve. No mayfly nymph strained at making wings. From the point of view of a water-bound nymph, flight isn't even a possibility. What advantage allowed those fast and faster skimmers to leave more offspring and push insects towards flight? Why fly?
If you guessed sex, you're probably right. These guys literally rush to newly emerged virgin females, and the guys who can rush fastest win. Skimmers outmaneuvered slower swimmers, four-leg skimmers out-mated six-leg skimmers, and eventually fliers were the ones who got the girls. Love literally spurred insects to travel in new dimensions.
I mused about all this as I watched the stoneflies crawling at my feet, and looked up to see strapping young men and women squeezed into indecently small lycra costumes, skimming along the flat waters, trying with everything they had to go just a little faster than their peers. What spurs them?
Then I took a closer look at the bugs at my feet. Hmmm. Something was wrong with their wings. Wait, those weren't stoneflies at all. They were alderflies, Megalopterans. That's a different kind of bug completely. Did I mention I'm not a real entomologist? I left to console myself with a cheesesteak.
Review - Rowing to Latitude: Journeys Along the Arctic's Edge 3/02
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