Caterpillar Anatomy: Digestion, Respiration, and Locomotion

Though a caterpillar’s anatomy is somewhat simpler than that of a butterfly, there’s more to a caterpillar’s body than meets the eye. While much of the body is taken up by the digestive tract, appropriate for a creature whose main task in life is eating, a caterpillar’s body also needs to breathe, move, sense the world around it, and defend itself. (Note that caterpillars lack reproductive organs; mating takes place only as adult butterflies.) This week we’ll take a closer look at caterpillar anatomy; look for posts later this week on Caterpillars and Silk and Caterpillar Senses and Defenses.

 
Digestive Tract:
A caterpillar’s body is basically a long tube. The mouth, with mandibles for chewing, is at one end. The digestive tract runs the length of the body, ending in the anus, where the caterpillar excretes waste. (Learn more about caterpillar waste, called frass, by clicking here.)

This digestive tract is especially easy to see on the Canna Skipper caterpillar (Calpodes ethlius), as it is essentially transparent. The green color on the caterpillar to the right is actually the plant material inside its gut. As it travels through the gut, the caterpillar extracts the nutrients, expelling only that which it cannot use. Caterpillars are extremely efficient digesters.

Legs:
While caterpillars may appear to have many sets of legs, they actually only have six true legs, like all insects. The legs on the three segments of the thorax, toward the front of the body, are the true legs, jointed and ending in hooks. On the abdomen, the caterpillar has sets of prolegs, which are stumpy and end in sticky suction-cups called crochets. These prolegs allow the caterpillar to cling to surfaces even when upside-down.

In the photo of a Black Swallowtail (Papilio polyxenes) caterpillar to the left, note the true legs held close to the head, pulling the parsley stem close to the mouth. The prolegs are further down the body, clasping the stem.

Respiration:
Caterpillars also need to breathe. They take in air by means of spiracles, tiny holes along the sides of their bodies that deliver oxygen directly to their body tissues via tracheae, eliminating the need for lungs. Spiracles can be opened and closed to control water entering or exiting the body. Once again, spiracles and their web-like tracheae are easy to see on the transparent body of the Canna Skipper caterpillar below.