At dusk the temperature hovered around 40 degrees and was falling fast. I could see my breath, but all around me fluttered moths through the woods. Moths flying on a cold November night? I snapped a few photos of one and headed home to find out what this was all about.
My colleague Hugh McGuinness, a regional moth coordinator for the Butterflies and Moths of North America database (www.butterfliesandmoths.org), identified it as one of two species in the genus Operophtera, either the native bruceata (Bruce Spanworm) or the introduced brumata (Winter Moth), devilishly hard to identify by photograph alone.
Either way, both of these small cold weather moths are thermoconformers and can fly with air and body temperature ranging from about 27 to 77 degrees Fahrenheit. Flight is energetically costly, requiring warm, powerful muscles that can create enough wing strokes to sustain flight. How do these tiny moths fly in such cold conditions when muscles don’t operate quickly?
Morphology appears to be the key. The Bruce Spanworm male has one of the lowest wing-loadings (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 ratios and these muscles are able to compensate for low contraction velocity in the cold with a high capacity to generate tension. Strong muscles, combined with few wing beats allow them to operate in very low temperatures as they seek the scent of female emitted pheromones.
The female is also well adapted to low 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. When they emerge in October or November, they crawl to the lower trunk of a host tree where they solicit flying males by wafting pheromones into the air.
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, suggests that if a female were to fly again with even weak flight at optimal temperatures, it would have a 17% reduction in the number of eggs it could carry. It would experience an 82% reduction in fecundity to fly powerfully.
Flying and crawling in the cold was probably 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.
Source: Marden, J.H. 1995. Evolutionary adaptation of contractile performance in muscle of ectothermic winter-flying moths. Journal of Experimental Biology 198, 2087-2094.