Caterpillar Anatomy

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

Caterpillar anatomy graphic Caterpillars are the larval stage of butterflies and moths. They are voracious eaters, usually feeding on fresh fruits and vegetables. For this reason, caterpillars are considered major agricultural pests, though some species actually help control overgrowth by feeding on pest plants.

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. These parts are labeled in the diagram above.

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Head

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.

Tentacles

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.

Mandibles

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

Spiracles

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.

Ocelli

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

Segment

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

Thorax

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 head closeup

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.

Abdomen

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)

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

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.

Setae

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.