Fertility and Flight in a Winter Moth

Blogger’s Note: In case you missed it, here’s a reprise of a post on an LBM (Little Brown Moth) that flies in winter.

Something’s odd about a moth flying through the woods in the angled daylight of November. It’s too cold for adult moths this time of year. Except for the moth Operophtera bruceata, which offers us a lesson in adaptation, sacrifice and evolution.

Operophtera bruceata goes by the common name Bruce Spanworm. As a caterpillar it is one of the inchworms, a member of the large moth family called Geometridae (which means “earth-measuring”). It is also called Hunter’s Moth because, as an adult, it is out flying the woods in the company of deer hunters this time of year.

Bruce Spanworm is not a particularly showy moth. With a wingspan of about 1.5 inches, its forewings are dull gray with dark flecking and its hindwings are even less dramatic. This moth is as drab as November. But who needs pizzazz when you’ve got ingenuity, evolution and a female making some sacrifices in the interest of fertility.

By now virtually all our moths and butterflies no longer live as gossamer-winged adults. Instead, they’re overwintering as either eggs, caterpillars or pupae (a cocoon in the case of a moth). But walk through deciduous woods in November, and Bruce Spanworm (or a relative called Fall Cankerworm, Alsophila pometaria) will most likely be your companion, even fluttering among snowflakes into December.

We normally think of moths as ectothermic (or, more commonly, by the confusing term “cold-blooded”), which actually means they depend on environmental heat sources in order to fire up their metabolism, including their flight muscles. It’s the main reason most insects fly in summer.

So how is it that Bruce Spanworm is such a hearty iconoclast? Why does it fly when warmth and solar energy are in such short supply? Well, for one thing, insectivorous birds are also in short supply, most having left us by now for the tropics. So a moth that can fly when predators are fewer can presumably go about its business breeding (more on that later) with greater success.

Yet a cold-weather sex drive and avoiding getting eaten, while certainly important, aren’t enough for a moth to take flight in freezing temperatures. As it turns out, the male Bruce Spanworm, while dull in appearance, is buff; he’s a muscle-man. And his flight muscles work well in the cold.

One study by James Marden, a biologist at Pennsylvania State University, published in 1995 in The Journal of Experimental Biology, determined that Bruce Spanworm’s flight muscles can generate greater force at lower temperatures than a comparative summer-flying moth. In other words, Bruce’s muscles are adapted to flap well in cold weather.

But physique also plays a role. Bruce Spanworm has more muscle mass as a percentage of his total body weight compared to the summertime moth used in the study. Sure, he may be small, but like a welterweight boxer, ounce-for-ounce he’s formidable. Perhaps more importantly, Bruce Spanworm has a low wing-loading, which is basically the total weight of the moth divided by the surface area of his wings. Our pal Bruce gets a lot of lift for each flap. This lowers the energy he needs to flutter around with the hunters in November, a crucial adaptation for an insect that relies on solar energy for flight. (It should be noted that Vermont biologist Bernd Heinrich’s amazing work on insect thermoregulation, including a 1985 paper on winter-flying moths, contributes greatly to our knowledge of Bruce Spanworm and the thermodynamics of insects in general.)

Note also that I’ve been discussing the flight of the male Bruce spanworm. None of this applies to females. That’s because the females don’t fly. They don’t fly because they lack wings. Females, full of eggs, simply sit there on the trunks of trees and waft pheromones, chemical attractants, into the air in order to lure males for mating. In fact, one of the best ways to find a female Bruce Spanworm is to look for a cluster of a half-dozen or so males fluttering over a spot on the tree trunk. It is a certain sign the boys are competing for a copulation with a flightless female.

The female Bruce Spanworm trades her wings for greater fecundity, a greater ability to reproduce. She is essentially an egg vessel; her body cavity is packed with fertile eggs – an average of 143, according to Marden’s study – rather than large internal organs or tissue. Were she to regain the ability to fly, to develop wings and wing muscles, a female Bruce Spanworm might have to lose some of her eggs. She can’t get something for nothing.

For Bruce Spanworm (and some other members of the geometrid moth family), evolution has progressed so that, while perhaps objectionable to females our species, it is preferable for females of this moth, out there in the cold, to be fertile rather than to be flying.

Reference:

Marden, J.H. 1995. Evolutionary adaptation of contractile performance in muscle of ectothermic winter-flying moths. The Journal of Experimental Biology. 198, 2087–2094.