Sex and Flight in the Cold

In Celebration of the Winter Solstice

© Bryan Pfeiffer

ON THE SHORTEST DAY of the year, when most of us turn toward the light, let us also celebrate a creature of the dark: a moth. But no ordinary moth. Not only does this one fly around in winter, it performs something else remarkable in the cold: moth sex.

Insects generally have no business flying, let alone mating, in winter. These animals need warmth to power their flight muscles (and lustful intentions). Here in the north, moths airborne and copulating in December are like civility and compromise in politics — not impossible, certainly elusive, and mostly a fond memory.

But there is yet another extraordinary thing about this moth: Only the males fly. The females never fly because they have no wings. Somewhere along the way in her evolution, the female, in losing the gift of flight, has forfeited one of the greatest innovations in the history of life on Earth, something that insects themselves invented more than 300 million years ago.

But Why?

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

So I’ll always welcome a few lonely moths fluttering in the naked woods. Many of you can welcome them as well because this drama of male moth flight and female forfeiture is happening around much of the northern world, largely in forests but also in city parks and perhaps your own backyard — even as far north as the Pacific Northwest and adjoining Canada, southern New England, and the U.K. 

The identity of your own winter moth depends on where you live because, as it turns out, there are a half dozen or so related species whose males now fly and whose females, well, just sit there in the cold and wait. (I list species below — and offer maps for paying subscribers in GO WILD’s Sightings page.) 

Males, whose wings tend to be marked in earthy tweeds, are buff, as in fit, with a body design that can lift off when little else but snowflakes float in the winter woods. This is a crazy thing for an insect — their flight muscles basically can’t function in the cold. But male winter moths (especially those in the family Geometridae) have ultra-light bodies and relatively large wings — a combination (low wing-loading) that allows him to fly at temperatures which ground or even kill other insects. Among the benefits of flying for a mate in winter is a scarcity of predators — no bats and fewer birds.

Although our male moth has mastered flight, the female has relinquished it. (That’s actually a convenient turn of phrasing. She didn’t really have a choice in the matter; evolution by means of natural selection did it.) This is not unusual among insects and birds (emus, penguins and flightless cormorants, for example). And yet for an animal to have lost the extraordinary advantages of flight, there had better be some greater benefit to the species as a result. You don’t give up your wings for no good reason.

© Bryan Pfeiffer

Flight and Fecundity

Among insects, it is the females that more often exchange their wings for the greater good. And besides fertility, another crucial thing a female insect brings to the mating game is fecundity — the ability to produce an abundance of offspring. By relinquishing her wings and flight muscles, and the considerable energy demands of flight, the female can devote more of her body’s resources to producing more eggs and therefore more young. In the many ways that evolution gives rise to the grand diversity of life, a female moth has traded flight for fecundity. So it goes in the struggle for existence.

Now plump with eggs, the female winter moth clings to a tree trunk or twig in the woods, all but invisible to predators, to me, and even to her airborne suitors. But she needs no wings to attract a male in winter. She wafts chemical perfumes, pheromones, into the chilly air to signal the males. In so doing, she inadvertently signals me as well.

I tend to locate female winter moths by watching for clusters of randy males converging and flapping at some spot in the woods — her. 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. (Have another look at the dirty moth picture atop this post. That’s a tan male on the left, his wings folded up over his body, and a dark, plump, wingless female on the right; they are “joined at the tips” in copulation.)

Once the pair has finished its business, the female lays her eggs 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 leaves (sometimes enough to damage trees and be considered a “forest pest.”) Then they pupate, transform, and eventually emerge as the adult moths of autumn and winter.

At year’s end, for hope or reflection or guidance, many of us turn toward holiday and ritual. Along the way, the Winter Solstice here in the north marks a point of inflection, a turning toward the light.

Today, on the solstice, my observance is a celebration of prosaic moths — a male defying thermodynamics, a female grounded for life — making love in the cold on the shortest day of the year.

Postscript (Bonus Evolutionary Biology)

I could write an additional 1,000 words on the forces in play in the evolution of these moths, notably the ecological conditions — selective pressures — that foster or hinder an insect’s evolution toward flightlessness. They include:

• Habitat Stability — Flight allows an animal to more easily move if its habitat becomes inhospitable: escaping a forest fire, for example, or migrating to a warmer place in winter. It beats walking. But relatively stable northern forests, with big oaks and maples and other long-lived trees, make it less essential that a flightless female moth would need to vacate.

• Varied Diet — Flight also helps female moths and butterflies locate and lay eggs on the specific plants that their caterpillars need to eat in order to grow — the host plants. Among winter moths, a caterpillar that for whatever reason finds itself on the wrong tree species can “balloon” on a strand of silk to another tree. Lacking navigation for this maneuver, a caterpillar with a more varied diet (polyphagy) is more likely to land on its proper meal. Also helpful is a diet of longer-lived woody plants, as opposed to more ephemeral herbaceous plants. Whether you’re walking or ballooning, it’s a lot easier to find an oak than an orchid.

• Predation — This one is wonderfully complicated. It certainly appears that adult winter moths encounter fewer predators. But in general, most moths are predated not as adults but as caterpillars (or eggs). A winter moth’s eggs typically hatch in spring so that the caterpillars can feed on tender new tree leaves — and be eaten as warbler food. Songbirds eat a ton of moth caterpillars. And this ballooning business also entails certain risks. Given all this, it helps with survival if a female produces multitudes of offspring.

Some Winter Moths

These moths, in the family Geometridae, might still fly (if they’re male) in December (even rarely into January if it’s not too cold) at northern latitudes (but not too far north). Chasing Nature’s paying subscribers will find maps on our Sighting’s page.

• Fall Cankerworm Moth (Alsophila pometaria) — Flying in the U.S. from southern New England into much of the Eastern Seaboard.

• Winter Moth (Operophtera brumata)  — Still on the wing across much of Europe and into the U.K. (Introduced in North America, it flies in southern New England and the Pacific Northwest into December.)

• Pale Brindled Beauty (Phigalia pilosaria) — Also flying in Europe and into the U.K., with rare January records as far north as Scotland.

• Mottled Umber (Erannis defoliaria) — Perhaps not as common or as far north, but still flying in Europe and the U.K.

• Vancouver Looper (Erannis vancouverensis) — As its name implies, on the wing in the Pacific Northwest in the U.S. and British Columbia in Canada.

• Bruce Spanworm (Operophtera bruceata) — This is the northerner here in the U.S., but it flies (even when it’s below freezing) mostly through November (which is why it is also called Hunter’s Moth). It’s the species you’re seeing in my photos.

• Western Bruce Spanworm (Operophtera occidentalis) — Still flying in the Pacific Northwest (maybe B.C.).

References

Life-history Traits of Forest-inhabiting Flightless Lepidoptera. Pedro Barbosa and Vera Krischik. Am. Midl. Nat. 122:262-274 (1989)

The Evolution of Flightlessness in Insects. Derek A. Roff. Ecological Monographs 60(4): 389-421 (1990)

Brachyptery and Artery in Lepidoptera. J. B. Heppner. Tropical Lepidoptera 2(1): 11-40 (1991).

Evolutionary Adaptation of Contractile Performance in Muscle of Ectothermic Winter-Flying Moths. James H. Marden. The Journal of Experimental Biology, 198: 2087–2094 (1995).

Bat Predation and Flight Timing of Winter Moths, Epirrita and Operophtera Species (Lepidoptera, Geometridae). Mats G. E. Svensson, Jens Rydell and Richard Brown. Oikos, 84: 193-198 (1999).

The Relationship Between the Winter Moth (Operophtera brumata) and its Host Plants in Coastal Maine. Kaitlyn M. O’Donnell. The University of Maine (2015).