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Pfeiffer: Winter Moth

12/14/11 5:55PM By Bryan Pfeiffer
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(Host) Females endure much in order to bear children. But commentator Bryan Pfeiffer notes that one kind of moth takes sacrifice to extremes.

(Pfeiffer) In the angled light of December, in a forest of naked trees dusted with snow, a moth crossed my path the other day. Wait! A moth? In December? We might as well have orchids blooming or warblers singing.

Moths aren’t supposed to fly when it’s cold. They need heat to power their wings. No, the warm November wasn’t a factor here. Neither is climate change. This moth flies every fall. This moth is Operophtera bruceata. Bruce Spanworm is the common name. The guys in the woods with rifles call it Hunter’s Moth.

So how is it that Bruce Spanworm flies in the freeze? And more important … what does all this have to do with sex?

First … some physics.

It turns out that Bruce Spanworm, plain and gray, is rather buff. He’s got a slender physique and broad wings. His flight muscles generate greater force at lower temperatures than similar summer-flying moths. It means Bruce gets lots of lift for each stroke of those wings. How do we know? Well, actually, a guy did an experiment on this.

Bruce is like a welterweight boxer; pound for pound he’s tougher than you and me. But he’s not angling for a fight. His angle is, well, you know – to make more moths. In December! Even when it’s cold out there, Bruce is hot to trot.

By the way, none of the physics applies to the females. That’s because females don’t fly. They can’t fly. They can’t fly because they don’t have wings.

The female is like a tiny, grounded dirigible: a chubby, brown body out in the woods. She a couch-potato. She's a flightless moth. But she’s not lonely out there. Her sluggishness is all about survival.

As Darwin’s law of evolution would explain it, the female Hunter’s Moth has become a listless egg factory. Having given up her wings and the muscles and organs necessary for flight, she can devote more of her body weight to eggs. Her cavity is packed with more eggs than a similar summer moth.

So there she sits, wafting pheromones into the chilly woods to summon Brucie drifting around and on the make. It works. She often attracts a cluster of eager males. I’ve seen them, hovering over her, jockeying, competing to be the fittest moth in the winter woods, to mate with a female whose option for survival is simply to lay lots of eggs.

The onset of colder days brings an end to this ritual. By now, the females have laid their eggs in the barren woods. And this year’s generation of Bruce and his wingless women has since died off. But next fall a new generation will rise and resume the cycle.

It’s one thing to see a moth flying in December. It’s quite another to see a moth with no wings. In the struggle for existence, a female has traded flight for fertility. Nature is nothing if not surprising.
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