A woolly bear hunches along a rock wall, stops, raises its head and starts off in a new direction. Now that it’s late summer, this rusty-brown and black larva of the Isabella tiger moth is searching for food, shelter, or a place to spin a cocoon into which it will weave hairs of its heavy coat.
Caterpillars are so common we pay little attention unless they harm garden and forest. We know them as eating machines, feeding and growing until a metabolic trigger says, “enough!” We think little how dramatic changes halt the caterpillar’s activity and transform this worm-like creature into a pupa in which every organ system is converted into something new, producing a winged creature with a very different role in life.
A caterpillar is a long muscular tube consisting of nerves, tracheal airways and a blood supply driven by five pairs of hearts surrounding an elongated digestive system. The hard chitinous head bears sense organs and a powerful pair of jaws to macerate plant tissue. There is little present in a caterpillar to suggest eventual adulthood as a moth other than a pair of small, undeveloped reproductive organs.
Caterpillars have three pairs of jointed, clawed legs at the front end, the proper number for an insect, but they also have four more stubby pairs of legs well down the body. These fleshy protuberances grip surfaces with a circlet of hard, sharp hooks called crochets. Caterpillars have thin bands of muscle under the skin that maintain constant internal fluid pressure to keep the body inflated, while their larger more complex muscles move them along.
Anyone watching a caterpillar twist and turn, reach for leaves and walk in rippling fashion realizes that muscles are working under precise nerve control. The caterpillar’s small brain sends messages down a trunk line of nerves and nerve segmental ganglia to control body movement. A network of sensory nerves beneath the skin allows a caterpillar to respond instantly to touch or loss of contact with the ground, and that response often is to feign death. Vibrating hairs respond to a wide spectrum of sound waves, so you know that the caterpillar has heard something if it stops crawling and contracts in a wait-and-see attitude.
Caterpillar eyes are clusters of tiny black hemispheres, six to a side. Each is pointed in a slightly different direction; and a fixed-focus lens projects light in a narrow field, so the insect perceives a crude mosaic image. A caterpillar can tell the direction from which light is coming; it can detect color (many caterpillars go toward green), and it can even make out dark vertical shapes that suggest plant stems rising from the ground. A caterpillar turns, adjusts, and tightens its spiral path until a goal is reached. The woolly bear I see raise and waggle its head is augmenting limited vision by altering the angle of perception.
Caterpillars have a specific sense to test the odor and taste of plants. Should one sample a leaf not included in its inherited diet, it spits it out. A larva depending upon cabbage leaves isn’t going to eat rose bushes.
Externally every caterpillar species is distinct from all others. A brightly colored or marked individual is always saying something. Some mimic the natural background, a form of self protection; others use their markings to warn off predators. The monarch butterfly larva, striped in black, yellow and white, is telling predators it is filled with toxins from the milkweed it consumes.
Some caterpillars resemble walking brushes with hairy spikes extending far out from the body. Loosely attached bristles can penetrate human skin or become air-borne to irritate nasal membranes and eyelids. Other shafts may be barbed, hollow and filled with irritating chemicals.
The life of a caterpillar is not an easy one. It must eat constantly, avoid danger, and find a safe place to spin a cocoon. Caterpillars can sicken when attacked by certain bacteria; and parasitic wasps lay eggs in them, so their developing larvae can eat the caterpillars inside-out. After a spider injects paralytic poison to keep a caterpillar alive, its flesh remains fresh until the spider decides it’s time for dinner.
Caterpillars are nourishing fare for predatory birds, toads and other creatures, and for humans. Our hominid ancestors surely popped them into their mouths when hungry. People in Asia, Africa and Asia Minor still eat caterpillars as delicacies, and Australian aborigines once depended upon nutritious giant moth larvae called witchetty grubs. Aztecs in the 16th century liked their corn best when it was infested with caterpillars, and you too can sample candied caterpillars sold by enterprising marketers.
Entomologists study insects, although only a few specialize in larval insects. A pity, for a caterpillar is an extraordinary creature.
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