A Wingless Winter Moth
Flying and crawling in a woods near you
November at dusk with the temperature barely breaking 40 degrees I could see my breath. Yet, all around me moths fluttered through the woods. They were one of two species in the genus Operophtera, visually drab, but physically magical.
Whether these were the native Bruce Spanworm (O. bruceata) or the introduced Winter Moth (O. brumata),devilishly hard to identify by photograph alone, I couldn’t discern. Either way, both of these small, cold weather moths are thermo-conformers. Incredibly, they can fly with air and body temperature ranging from just 27 Fahrenheit up to a balmy 77 degrees Fahrenheit.
Flight is energetically costly, requiring warm, powerful muscles that can create enough wing strokes to stay airborne. How do these tiny moths fly in such cold conditions when muscles simply don’t operate very quickly? Morphology appears to be the key.
The male Bruce Spanworm has one of the lowest wing-loads (total weight divided by wing area) of any moth measured. This reduces the frequency of wing beats necessary to sustain flight and lowers the energetic cost. They also have one of the highest flight muscle to body size ratios. These bulky muscles are able to compensate for low contraction velocity in the cold with a high capacity to generate tension. Strong muscles, combined with fewer necessary wing beats allow them to operate in very low temperatures as they seek the scent of females wafting pheromones.
The female is also well adapted to low autumn temperatures. They are flightless. The Bruce Spanworm has no wings at all while the Winter Moth has just the vestiges of wings on its back visible. When they emerge in October or November, they crawl to the lower trunk of a host tree where they solicit flying males with fine chemistry.
Without bulky flight muscles weighing them down, Bruce Spanworm females fill their body from the edge of the thorax to the tip of the abdomen with eggs, 143 on average. That’s over 60% of their total body weight. A flight model by James Marden, a biologist at Penn State, suggested that if a female were to fly again with even weak flight at optimal temperatures, they would have a 17% reduction in the number of eggs they could carry. The female moth would experience an 82% reduction in fecundity to fly powerfully.
Flying and crawling in the cold was likely a great adaptation to a powerful natural selection force, predation. By late October a large percentage of insectivorous birds have migrated south and bats have migrated or hibernated for the winter. With fewer predators comes great success.