Caterpillar Anatomy

Excerpted from: Insect Anatomy: The Parts of a Caterpillar.

Eastern Tiger Swallowtail caterpillar, Papilio glaucus

Caterpillars are the larval stage of butterflies and moths. It is the second part of their four-stage life cycle (egg, larva, pupa, adult). Caterpillars have long, worm-like bodies with 6 true legs. They can also have a variable number of stumpy false legs (called prolegs), which help them to move and cling to things.

Caterpillars can change dramatically from when they first hatch to when they're ready to pupate. Some can increase their body mass 10,000-fold in just a few weeks - that's like a baby growing to the size of a sperm whale! Many look very different as they grow.

Caterpillars come in many colors, shapes, and sizes. Some caterpillars are quite hairy, while others are smooth. Despite differences between species, though, all caterpillars share certain morphological features.

Many of these caterpillars are most obvious when they're fully grown and looking for a place to either pupate or settle down for the winter, though some are easily spotted on their favourite food plants. Here are some of the species that you are most like to identify in Wisconsin.

Caterpillar anatomy graphic

Curious about caterpillars? Just for fun, try the:
Caterpillar Quiz


Caterpillar head closeup

The first section of the caterpillar body is the head. It includes six eyes, called 'stemmata,' the mouthparts, the small antennae, and the 'spinnerets', from which the caterpillar produces silk. Antennae are present on either side of the labrum but are small and relatively inconspicuous. The labrum is like an upper lip. It is used to hold food in place while the mandibles do their chewing.


The caterpillar’s tentacles are sensory organs. Caterpillar’s eyesight is poor, and tentacles are tactile. They aid in navigation on the front. They may also play a role in defense/predator confusion on the rear, leading a potential predator to think that the monarch’s rear is its head.


Located in the head section, the mandibles are jaws that are used for chewing leaves.


Caterpillar closeup Spiracles are external openings that allow gas exchange (respiration). The caterpillar contracts muscles to open and close the spiracles. One spiracle pair is found on the first thoracic segment, T1, and the other eight pairs are found on the first eight abdominal segments, A1 through A8.


Caterpillars have six pairs of simple eyes called ocelli. They are small, simple eyes that can detect changes in light intensity, but cannot form an image. Ocelli are composed of photoreceptors and pigments. Ocelli are usually located in two clusters of six eyes on the sides of a larva's head


A segment is a body section of the thorax or abdomen. A caterpillar has three thoracic segments and 10 abdominal segments.


The thorax is the second section of the caterpillar body. It consists of three segments, known as T1, T2, and T3. The thorax contains three pairs of true legs with hooks and a dorsal plate called the 'prothoracic' shield. The prothoracic shield is located on T1, the first segment. The color pattern of this shield is valuable for identifying different species of caterpillars.

Thoracic Legs

Caterpillar closeup of true legs and prolegs

There are three pairs of segmented legs, also known as thoracic legs or true legs, located in pairs on each of the three thoracic segments. Each true leg ends in a tiny claw. These are distinct from the fleshy, false prolegs found along the abdominal cavity.


The third section of the caterpillar body is the abdomen. It is 10 segments long, classified as A1 through A10, and includes the prolegs (false legs), most of the spiracles (breathing holes used for respiration), and the anus (the final stop along the digestive tract)

Caterpillar Digestion

A caterpillar's body is basically a tube for processing and storing food. A set of mouth parts lets the caterpillar chew its food — typically leaves and other plant parts. The mouth empties into a very long intestine with fore and hind parts.

Caterpillars are not very efficient food processors. Caterpillars often utilize less than a 1/4 of the food they ingest. Partly this is due to a mismatch between the food content and the needs of the caterpillar. Caterpillars typically need more protein and less sugar and starch than their host plants provide. The 3/4 of the food that is not utilized is excreted as copious amounts of frass (feces). Caterpillars efficiently remove and absorb the amino acids from the food and excrete excess starch and fiber.

Abdominal Prolegs

Prolegs are fleshy, false, unsegmented legs, usually found in pairs on the third through sixth abdominal segments. The soft prolegs bear hooks on the ends which the caterpillar uses to cling to foliage, bark, and silk. Experts sometimes use the arrangement and the length of these hooks to identify caterpillars at the family level. The number and size of the prolegs can also be identifying characteristics.

Anal Prolegs

Caterpillar setae, bristle-like hairs

The anal prolegs are a pair of unsegmented, false legs that are located on the last abdominal segment. The prolegs on A10 are usually well developed.


Most caterpillars are covered in hair-like bristles called setae. These tiny filaments are often too pale to see. But a number of caterpillars are covered in thick swathes of setae that they mean for us to see and to heed.

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