Sex and Light

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A male Bruce Spanworm / © Bryan Pfeiffer

Relatively few other insects have reason to be flying around New England in winter. These animals need warmth to power their flight muscles.

But my winter moth is an iconoclast. The male is on the wing day or night in the cold. The female does not fly — an unusual forfeiture on her part.

As an entomologist, I’ll admit to going into a bit of a funk in winter. Not entirely from the shortage of light. Not even from the scarcity of insects: I can still find bizarre creatures crawling on six legs atop the snowpack. What I mostly miss is the raw diversity expressed in flight: free-spirit butterflies, fighter-jet dragonflies, work-ethic bumblebees.

A moth in flight in December is like civility in politics or going maskless in an indoor crowd — not impossible, certainly elusive, and mostly a fond memory to be sure. Fluttering in winter, a moth can embody joy in the moment and yet a kind of wistfulness — despite its unbecoming given name: Fall Cankerworm (Alsophila pometaria). (Farther north, but similar in form and function, a moth called Bruce Spanworm [Operophtera bruceata] flies mostly in November and is sometimes called “Hunter’s Moth.” That’s a pair of them in the banner image above.)

By any name, the male, marked like tweed, does not fly to please wistful entomologists like me. Nor does he find much nectar in the naked woods. He flies instead to find and mate with a female. A slender body and powerful muscles combine to lift him into flight when little else but snowflakes float in the winter woods.

The female does not fly to meet him. She has no wings. Flightlessness allows her to devote more of her intrinsic resources to producing eggs, lots of them, more than she might be able to haul around were she equipped to fly.

Mostly alone in the woods, often clinging to a tree trunk, she is plump with eggs, all but invisible to predators, to me, and to her airborne suitors. But the female needs no wings to attract a male in winter. She wafts chemical perfumes, pheromones, into the woods to signal the males. In so doing, she signals me as well.

A moth in flight in December is like civility in politics or going maskless in an indoor crowd…

I tend to locate females by watching for clusters of randy males converging and flapping at some spot in the woods. Or sometimes, when I spy a male resting on the trunk of an oak or a maple, he is already conjoined with a female in their winter coupling (image below).

Having attracted her mate for the nuptials, the female lays eggs in clusters around twigs. Soon after, she and the male die in the cold. In springtime their offspring hatch as “inchworm” caterpillars to feed and grow on fresh vegetation (sometimes in excess, to the point that they are viewed as pests). Then they pupate, transform, and eventually emerge as the adult moths of autumn and winter.

In the cycle and drama of life, females often bear a disproportionate share of the burden of reproduction. In this instance of evolution, a female moth has traded flight for fertility. Those males in flight are more noticeable to songbirds seeking an easy winter meal. So it goes for them in the struggle for existence.

As a hardened New Englander, I am under no illusion that insects mating in December are any early sign of spring — or even a parable for warmth and hope to come. They’re little brown moths going about their business.

Even so, beyond the coming light, my own observance of Dec. 21 also features a performance: Prosaic moths — one defying gravity, the other grounded for life — copulating in the cold on the shortest day of the year.