Parts of a Caterpillar – Head, Thorax, Prolegs, Spiracles

All 10 parts on the diagram

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The caterpillar 8

Head Its head is well developed, with mouthparts that chew.
Thorax Each of its three segments carries one pair of true legs.
Abdomen The abdomen carries the prolegs, up to five pairs of them.
True legs True legs are jointed and tapering and end in a small claw. They become the legs of the adult.
Prolegs Prolegs are thick and fleshy and have no joints. They are shed with the last larval skin.
Spiracles The openings of the air tubes that an insect breathes through. The tubes are called tracheae.
Horn Hornworms, the caterpillars of hawk moths, are named for this horn. Not every caterpillar has one.
Anal prolegs The anal prolegs are on the last segment. An inchworm keeps them though most of its other prolegs are missing.

Close up 2

Simple eyes (stemmata) A caterpillar usually has twelve, six on each side of the head. It has no compound eyes.
Crochets Crochets are gripping hooks. Their arrangement helps tell families of caterpillars apart.

How this list was compiled: Every name checked on 2026-09-30, a name kept only where two of these use it: (1) North Carolina State University, ENT 425 General Entomology, Lepidoptera, https://genent.cals.ncsu.edu/insect-identification/order-lepidoptera/ (head, thorax, thoracic legs, abdomen, prolegs, hornworms); (2) Wikipedia, Caterpillar and Proleg, text only, https://en.wikipedia.org/wiki/Caterpillar and https://en.wikipedia.org/wiki/Proleg (true legs, prolegs, anal prolegs, crochets, spiracles, stemmata or simple eyes, six on each side); (3) J. H. Comstock, A Manual for the Study of Insects, https://archive.org/details/rnanualforstudyof00coms_0 (thoracic or true legs, jointed with a claw; pro-legs, fleshy and without joints, with minute hooks; anal prolegs; spiracles; simple eyes on each side of a larva's head; the caudal horn of hawk-moth larvae); (4) University of Minnesota Extension, Tomato hornworms in home gardens, https://extension.umn.edu/yard-and-garden-insects/tomato-hornworms (the horn on the rear end); (5) University of Florida, Featured Creatures, tomato hornworm, archived copy of http://entnemdept.ufl.edu/creatures/field/hornworm.htm (the horn); (6) a larva identification textbook, Larvae of Insects, passages read through the Internet Archive full-text search, https://archive.org/details/larvaeofinsectsi0000pete_b7a5 (crochets, anal prolegs, spiracles); (7) OpenStax, Biology 2e, 28.6, https://openstax.org/books/biology-2e/pages/28-6-superphylum-ecdysozoa-arthropods (spiracles and tracheae). Licence: own work

The drawing: Drawn for this site on 2026-09-30 from plain geometry; no photograph or plate followed. Licence: own work

How it works

The parts of a caterpillar body are a head, a thorax with six true legs and a long abdomen with fleshy prolegs. The second view shows two small parts close up.

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Front to back, then close up

Numbers 1 to 3 are the three main sections in order: the head, the thorax (the three segments behind it) and the abdomen (the rest). Number 4 is the true legs, one pair under each segment of the thorax. Number 5 is the prolegs, the fleshy stubs under the middle of the abdomen. Number 6 is the spiracles, the row of small openings along the side. Number 7 is the horn on top of the rear end and number 8 the anal prolegs at the very back. Numbers 9 and 10 are close up: the simple eyes on the head, and the crochets, the tiny hooks under a proleg.

True legs are jointed and end in a claw. Prolegs are soft, have no joints and are shed with the last larval skin.
Not every caterpillar has a horn. Hornworms, the caterpillars of hawk moths, are the ones named for it.

Questions and answers

What are the body parts of a caterpillar?

Three main sections: the head, the thorax and the abdomen. The thorax is the three segments behind the head and carries the true legs. The abdomen is the long rear part and carries the prolegs.

How many legs does a caterpillar have?

Six true legs, in three pairs on the thorax. The stubby prolegs on the abdomen come on top of those. A caterpillar has up to five pairs of prolegs, and inchworms have fewer.

What are prolegs?

Small, fleshy stubs on the underside of the abdomen. They have no joints, and each ends in tiny gripping hooks called crochets. The last pair, at the very back, are the anal prolegs.

How does a caterpillar breathe?

Through its spiracles, the small openings along its sides. Each one leads into air tubes called tracheae, which carry air into the body.

How many eyes does a caterpillar have?

Usually twelve: six simple eyes, called stemmata, on each side of the head. A caterpillar has no compound eyes. Those belong to the adult moth or butterfly.