Summer World_ A Season of Bounty - Bernd Heinrich [53]
11
Calosamia Collapse
IN 1903, W. J. HOLLAND, THE MOTH MAVEN OF NORTH America, wrote this about the promethean moth, Calosamia promethea, in his “bible” of the American moths: “Every country boy who lives in the Atlantic states is familiar with the cocoons, which in winter he has found hanging from the twigs of the spice-bush, the sassafras, and other trees.”
I am lucky indeed to have made the aquaintance of these giant silk moths and their cocoons. But country living is apparently not what it used to be—I have not met any country boy, and only the very rare biology graduate student, who has even an inkling of what a promethean moth is. Most students don’t know a cocoon from a pupa. Perhaps that’s not surprising, because they haven’t seen any, either in their everyday lives or in school.
Prometheans were still in abundance recently, near my camp in western Maine, where almost every summer their handsome blue-green caterpillars fed on the leaves of ash and cherry trees. Like many other saturniid silk moth caterpillars, these are strikingly beautiful when viewed from up close. They are decorated with four bright crimson tubercles on their front end and have splashes of yellow along their pale bluish green sides. The adult moths are beautiful, too. The females have fiery orange-red wings with white margins—hence the name promethean, for Prometheus, the Greek god who stole fire from the heavens. The males, in contrast, are deep purplish black.
In northern New England, the adults fly only for a week or two in early June, mate, and lay their eggs the next day. The caterpillars hatch and grow to their full size, about the size of one’s little finger, by late July or August. After they stop feeding, they wander; and after their gut has emptied, they stop under a leaf of almost any species of tree and start to make their cocoon by continually exuding silk from their salivary glands. Like all silk moths, they weave, waving their heads back and forth to opposite sides of the leaf as silk is exuded and attaches and pulls the two leaf edges together. After the caterpillars become wrapped in the green leaf, they continue applying silk strands all around themselves, and a solid cocoon takes shape. As they continue extruding their thread of silk, they line the cocoon cavity with more silk, which is then cemented together with fluid from the mouth to produce a tough case.
Caterpillar silk is both tough and flexible, and for centuries it has been the raw material for luxury clothing (although from a different species, Bombyx mori). A cocoon is made from one very long continuous strand of silk. In Calosamia and most other silk moths innumerable strands of silk are cemented together to form a stiff armor. It can be dented, but it is almost impossible to tear into except with scissors. One might assume that once the caterpillar is encased inside the cocoon, nothing else can enter, and that the cocoon would also trap the eventual moth inside, preventing its exit. However, without exception, in all of the many hundreds of Calosamia cocoons I have handled in the last seventeen years, every one had a built-in escape hatch. You can be sure that if it had been up to me to build such a hatch every time, I would have missed several. But the caterpillar does not think ahead—if it did, it too would have failures. Rather, it is behaviorally programmed, and it cannot do otherwise than leave an outward-flanging sleeve at one end of the cocoon. In contrast, the common silk moth, Bombyx mori, and some of the luna and polyphemus moths leave no escape hatch; instead, after molting from the pupa the adult secretes a saliva that contains a silk-digesting enzyme (cocoonase) that dissolves an exit hole out of the cocoon prison.
The Calosamia caterpillars have a curious behavior that is not found in any of the other local silk moth species. They spin an extension of their cocoon along the one-inch-to