There is something odd about a moth flying through the woods in the angled daylight of November. Moths and November ordinarily mix about as well as fire and water. It’s basically too cold for adult moths at this time of year. Except, of course, for Operophtera bruceata, a moth that offers a lesson in adaptation and evolution.
Operophtera bruceata goes by the common name Bruce spanworm. As a caterpillar, feeding on various shrub and tree leaves in spring and summer, it is one of the inchworms, a member of the large moth family called Geometridae, meaning “earth-measuring.” It is also informally called hunter’s moth because, as an adult, it flies in the woods in the company of deer hunters at this time of year.
Bruce spanworm is as drab as November. With a span of about 1.5 inches, its wings are dull gray with dark flecking. It’s certainly not attractive, but at least it’s still flying.
By now virtually all our once-gossamer-winged moths and butterflies have begun overwintering in different forms—as eggs, caterpillars or pupae. But walk through deciduous woods this month, and you are likely to see Bruce spanworm in flight. Sometimes they even flutter among snowflakes in December.
Bruce spanworm, like all moths, are ectothermic, which means they must depend on environmental heat sources to fire up their metabolism, including their flight muscles. As it turns out, the male Bruce spanworm is quite a specimen, quite a muscle man. Physically, he is better equipped than other moths at dealing with the cold.
A study by James H. Marden, a biologist at Pennsylvania State University, published in The Journal of Experimental Biology in 1995, found that the Bruce spanworm has more muscle mass as a percentage of total body weight than did the study’s summertime moth. Bruce spanworm also has a low ‘wing-loading,’ which means his total weight is low in comparison to the surface of his wings. Therefore, this moth gets more lift from each flap of the wing, which in turn means he needs less energy to fly.
There’s still another reason Bruce spanworm flies in late autumn, and that reason has nothing to do with anatomy. We see lots of them because their predators, insectivorous birds, have left us by now for the tropics. The moths can go about their main business—breeding—without being eaten.
While the males flutter and flex their muscles, the females, full of eggs, simply sit around, usually on a tree trunk. They must be forgiven for their indolence: They don’t have wings. They just sit and waft their pheromones, chemical attractants, into the air to lure the males. 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 flying over a particular spot. The boys are there competing to copulate with a flightless female.
The female Bruce spanworm, over the course of her evolution, has traded wings for improved ability to reproduce. She is essentially an egg vessel; her body cavity is packed with eggs – an average of 143, according to Marden’s study—rather than large internal organs or tissue. She lays those eggs in bark crevices, where they overwinter until hatching in early spring.
Were the female Bruce spanworm to evolve another way, to develop wings and wing muscles to fly, she would have to lose weight by carrying fewer eggs. Marden calculated her egg load would have to be reduced by as much as 80 percent for her to fly in cold weather.
Over the course of her evolution, the female Bruce spanworm traded in her wings for the ability to produce more offspring out there in the cold. A November walk in the woods shows that this strategy has paid off.