SAWFLY COCOON: Adult sawflies emerge from their cocoon through a neatly-cut hole at one end.
BRACONID WASP COCOONS: The tiny, white cocoons of parasitic Braconid wasps on hornworm caterpillars are often mistaken for eggs.
The Cecropia Moth’s cocoon actually consists of two cocoons, one inside the other, with a space filled with loose silken threads in between them. The pupa rests inside the inner cocoon.
The aquatic Climacia sp. spongillafly larvae emerge from the water and spin a tiny cocoon (3-7mm). They then cover it with a silken net made up of little hexagons.
It has been estimated that 45 to 60 percent of all animal species on the planet are insects that undergo complete metamorphosis. These insects hatch from eggs as worm-like larvae that eventually enter a pupal stage before emerging as adults. Some of these insects have silk glands and spin silken cases, or cocoons, around themselves just before transforming into pupae inside them.
Cocoons are as varied as the larvae that spin them; they differ in size, durability, structure, material, shape and color, as well as how they are attached to a substrate. Although cocoons are most often associated with moths, other insects, including beetles, fleas, winged members of the order Neuroptera and caddisflies, also create them. Some of these insects’ cocoons can be quite intricate, many incorporate material from the surroundings, and all are well-designed to protect the pupae within.
The aquatic Spongillafly larva emerges from water and spins a tiny, densely woven inner cocoon surrounded by a lacy net. Black flies spin pouch-like, underwater cocoons.
Sawfly larvae spin ¼-inch-long cylindrical cocoons which they attach lengthwise to branches, eventually emerging through a neatly-cut, circular lid at one end of the cocoon. Ichneumon wasp larvae spin cocoons similar to sawflies, but they are marked with blotches of white or gold.
Flea cocoons are found in cracks and crevices on the ground, or among debris. The adults lie motionless, sometimes for months, inside the cocoons until stimulated by the vibrations of a potential host, at which time they spring into action.
One of the smaller, but more familiar, cocoons is that of parasitic Braconid wasps, which most tomato growers have observed. The wasp lays her eggs on the tomato-eating Tobacco and Tomato Hornworm larvae. The young wasp larvae survive by eating the insides of the caterpillar, eventually emerging and pupating inside tiny white cocoons they spin, that often adorn the skin of dying hornworm caterpillars. (If you find these cocoon-laden caterpillars in your garden, let them be, so that the beneficial wasps can mature.)
By far the largest cocoons found in the Northeast are those of giant silk moths, ranging from 1½ inches to 4 inches in length. Even at this size, these cocoons can be very difficult to locate, as they are so well camouflaged. The larvae spin their tough, papery silk cocoons with a spinneret located near their mouths.
Our largest silk moth, the Cecropia Moth, attaches its 3-to-4-inch-long, often spindle-shaped brown cocoon lengthwise to a twig. Inside the outer cocoon is a second cocoon in which the pupa resides. The inner cocoon is separated from the outer one by an air space containing loose silk threads. One end of the cocoon is loosely spun, enabling the adult moth to emerge in the spring.
The Promethea moth larva spins a 2-inch-long cocoon that it wraps in a single leaf and then spins silk around the stem of the leaf so that it remains attached to the twig through the winter. In the spring the adult moth emerges through a valve-like structure at the upper end of the cocoon.
Luna Moth cocoons are much more flimsy than other silk moth cocoons, with very thin walls, making emergence relatively easy for the adult moth. These cocoons are usually enclosed in a leaf, and are found on the ground.
Polyphemus moths cocoons, also about two inches in length, are usually enclosed in one or several leaves, and typically fall to the ground in the fall. Both ends are closed until the spring, when the pupa secretes a fluid that dissolves the gummy substance that binds the silk together.
The purpose of cocoons is to protect the pupae they contain. While some dark-colored cocoons do absorb heat, most cocoons provide little direct insulation. They may, however, reduce the rate at which the temperature of the pupa within them changes. Protection from elements other than the cold is provided in a number of ways. Cocoons are certainly helpful in physically protecting many pupae overwintering inside cocoons that are on the ground, covered by soil and/or snow. They provide a barrier between the pupa and ice crystals that may form on the surface of the cocoon. The silk of some cocoons is so tough that it discourages predators. Other cocoons are more or less waterproof. Last but not least, the silk of many cocoons has anti-bacterial and anti-fungal properties. Were I a pupa given the choice of withstanding a New England winter with or without a cocoon, there is no question that I would choose to be equipped with silk-spinning apparatus.
Mary Holland is the author of “Naturally Curious: A Photographic Field Guide and Month-by-Month Journey Through the Fields, Woods, and Marshes of New England,” “Milkweed Visitors,” and “Ferdinand Fox’s First Summer.” She has a natural history blog which can be found at www.naturallycuriouswithmaryholland.wordpress.com .