A Lithophane amanda caterpillar feeds on beaked willow leaves, left. Having 

 emerged in early spring, it is ready to form a cocoon by early June, when 

 many summer caterpillars are just hatching from their eggs. Some cold- 

 weather moths, such as the pair of Japanese Erannis obliquaria below, do not 

 overwinter as adults. They emerge from their cocoons in November to 

 reproduce, but die soon after The female, which mates and lays her eggs on 

 the same tree on which she hatched, has only small, vestigial wings. 



Fukuo lloh; Nature Production 



cated, the moths have evolved two special 

 adaptations. First, their thoraxes are cov- 

 ered with dense fur that cuts their rate of 

 heat loss in half. The fur is formed from 

 greatly elongated scales, similar to those 

 that color butterfly and moth wings. Like 

 other lepidopteran scales, the fur rubs off 

 easily, making the moths slippery in one's 

 fingers and possibly also in the grip of a 

 potential predator. (Winter moths retain 

 the tympanic air sacs used by their ances- 

 tors for listening for bat sonar, but whether 

 or not they still work is uncertain. Never- 

 theless, these air sacs thermally insulate 

 the thorax from the abdomen.) 



Winter moths have also evolved a circu- 

 latory system that reduces heat loss from 

 the muscles in the thorax to both the head 



and the abdomen. As blood flows out of 

 the thorax, it gives up its heat to blood 

 flowing back in. The system conserves the 

 heat in their thorax so efficiently that win- 

 ter moths have lost the ability to use their 

 abdomen as a radiator for dissipating ex- 

 cess heat, which is a necessity for summer 

 moths that would otherwise overheat. 

 Thus, although winter moths fly at temper- 

 atures of 32° F, they fly with thoracic tem- 

 peratures similar to those of their summer 

 relatives (about 86°-95° F). 



Most insects spend the whole winter in 

 a state of toqjor; because the cold greatly 

 reduces their metaboUc rate, they do not 

 need to eat. Winter moths also spend most 

 of their time in toipor. But when they do 

 warm up and fly, they use up their energy 

 reserves very rapidly. Consider a moth at 

 rest at 27° F. Its metabolic rate is so slow 

 that a full stomach of maple sap contain- 

 ing 6 percent sugar would provide it with 

 enough fuel to last the whole winter. A 

 moth in flight, however, must maintain a 

 body temperature of about 86° F. In cold 

 weather, this means raising its metabolic 

 rate to 8,(XX) times the resting rate, which 

 would exhaust the fuel reserves in the 

 maple sap in little more than a half hour. 



To meet their prodigious energy de- 

 mands in late fall, cold-adapted adult 

 moths can still tank up on the few late- 

 blooming flowers such as the witch hazel. 

 After that, however, nectar is not available 

 again until the pussy willows bloom in 

 April. In the interim, the moths must feed 

 exclusively on sweet sap oozing from 

 wounds in, or broken branches of, birches 

 and sugar maples or on the maple syrup 

 that runs from cuts made by red squirrels 

 {see "Nutcracker Sweets," Natural His- 

 tory, February 1991). With this source of 

 sugar, they nearly double their weight in 

 one feeding. 



Sap solves the food problem for the 

 adults, and the reversed winter-for-sum- 

 mer life style protects them from preda- 

 tion, but the switch creates a different food 

 problem for larvae, which normally feed 

 on summer foliage. The caterpillars sur- 

 vive because winter moths lay their eggs 

 before the tree buds open, allowing their 

 larvae to hatch and start feeding the 

 minute the new leaves appear. By this time 

 the first migrant warblers have returned, 

 and some, no doubt, are feeding on these 

 caterpillars. But the caterpillars continue 

 to race through their development cycle, 



47 



