Insect Anatomy: The Parts of a Caterpillar

Caterpillar diagram
A. M. Liosi/Wikimedia Commons (CC by SA license), modified by Debbie Hadley, WILD Jersey

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.

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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.

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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).

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Segment

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

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Horn

The horn is a dorsal projection present on some caterpillars such as hornworms. The horn may help camouflage the larva. It may also be used to frighten away predators.

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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 on 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.

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Spiracles

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.

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True Legs

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.

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Mandibles

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

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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.