Found Object

Natural History, April, 2001 by Bernd Heinrich

Beautiful winged things come in small packages.

The snow on the steep, south-facing slope of Raven Ridge, near my house in Vermont, was melting quickly in the April sun. I was scrambling up toward the ravens' cliff nest to check on its progress. Climbing the hill up to the base of the cliff is a treat, because here is where I always find the first bare patches of ground after seeing only deep snow for almost six months. The sun reaching this slope cuts through the snow early, revealing a moist layer of fallen oak, maple, and ash leaves. On this day, the first hepatica buds were poking through the leaf mat, and some were opening into startlingly light-blue flowers.

As I searched for a secure footing on some protruding rock, a roundish object caught my eye. It was about the size and shape of a shelled hickory nut but had bits of dead maple leaf adhering to it, as though it had once been wrapped in a leaf that had then withered and partially peeled off. I had on rare occasions found such an object before, and I recognized this one. It was the cocoon of a Saturniidae silk moth, possibly a luna or a polyphemus moth. As adults, both of these moths are stunning aerial creatures, but I had seen them only after raising them from caterpillars and after attracting them to a light at night, when they are active.

Silk moth cocoons are made of a layer of silk that the caterpillar exudes from glands in its mouth. While the silk of Saturniidae moths is considered inferior to that of Asian silk moths, it is tough and durable. The caterpillar, or larva, lays down the silk in one continuous strand, endlessly weaving its head back and forth and frequently changing position. The strands glue themselves together into a resilient fabric that I cannot tear apart with my fingers. Once enshrouded in silk, the larva enters a new stage, that of the pupa. The energy stored in the pupa itself will last the insect the rest of its life: eight months of pupation and a few days of adulthood. (Adult moths have neither feeding mouthparts nor digestive tracts.) While rodents often succeed in opening cocoons and extracting the nutritious pupae, birds rarely invest the time and effort needed to pierce the silken armor. The moth itself can escape from the cocoon only by secreting an enzyme that dissolves some of the silk and creates an exit hole.

This cocoon had no exit. I shook it slightly and from its heft and rattle I knew that it contained a live pupa. I put it in my pocket and continued on to the ravens' nest, thinking that good things often come to one obliquely.

After coming home, I placed my find in a screened cage, which I left outside, on the back porch. The pupa had long been in a state of torpor, but shortly after receiving signals of warmth in spring, its biological clock would restart and its development toward adulthood would resume. If I had taken the cocoon into the house, the premature warmth would have disturbed the insect's timing. If a moth is to find a mate, as well as fresh leaves on which to deposit its eggs (if it is female), then it must emerge and live out its few adult days at the same time as the rest of its population. Up to now the insect's life had progressed at a leisurely pace; indeed, most of it had been spent in torpor. After emerging, though, life would take on urgency.

My moth emerged on the bright sunny afternoon of May 30. I saw it hanging from the ceiling of its cage; it cannot have been out for more than an hour. Its limp, still-soft wings of cream and various browns, with touches of pink, were folded over its thorax and abdomen, which were covered with a light down. This thin cloak acts as insulation when the moth begins to shiver and warm up in preparation for flight. This was a polyphemus moth (named for the Cyclops of Greek mythology); each hind wing bore a large blue-black "eye."

The mission of this moth would be to lay eggs. It was a female, as evidenced by its plump, rounded abdomen and thin antennae. Males have broad, feathery antennae that provide ample space for millions of chemical sensors tuned to detect one kind of molecule: the scent of a female of its species. This fine-tuning of the senses provides the moth with a very narrow window on the world, but it is both sufficient and efficient.

After the moth had dissolved her way out of the tough cocoon, she must have quickly crawled to where she could dangle her wings, pumping blood into them and expanding them to their full length of almost three inches. Over several hours, a hormone released into her blood would serve to harden her wings and the rest of her exoskeleton. Then she would be flight-ready.

I tied a thread around the moth's waist and then attached her loosely to a low branch of the oak tree next to my house. Oak is one of several food plants used by polyphemus caterpillars. As soon as I released my grip on her wings, she shivered, isometrically contracting her wing muscles to increase her thoracic temperature to about 97 [degrees] F, sufficient for takeoff. Attempting to fly, she fluttered for three or four minutes before folding her wings over her back and coming to rest, pendent under a leaf.