Insect Anatomy: The Parts of a Caterpillar

Caterpillars are the larval stage of butterflies and moths. They are voracious eaters, considered a major agricultural pest of fruits and produce. On the other hand, if put in an area with a lot of pest plants, they are a benefit to biologically controlling overgrowth.

Caterpillar Anatomy Diagram

Caterpillars come in many colors, shapes, and sizes. Some caterpillars are quite hairy, while others are smooth. Despite these differences, all caterpillars share certain morphological features. These common features are labeled and described in the caterpillar diagram.

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Head

The first section of the cate rpillar body is the head. The head capsule is hard. 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 small and relatively inconspicuous. The labrum is like an upper lip. It is used to hold food in place during chewing by the mandibles.

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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. This section contains three pairs of true legs with hooks on them and a dorsal plate called the prothoracic shield. The prothoracic shield is located in T1, the first segment. The color pattern of this shield is valuable for identifying species of caterpillars.

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Abdomen

The third section of the caterpillar body is the abdomen. The abdomen 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.

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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, silk or other substances. Experts sometimes use the arrangement and the length of the crochets to identify caterpillars to the family level. The number and size of the prolegs can be an identifying characteristic.

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Spiracle

Spiracles are external openings that allow gas exchange (respiration). The caterpillar contracts muscles to open and close the spiracles. One spiracle pair is on the first thoracic segment, T1, and the other eight pairs are 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. They are unlike the fleshy, false prolegs found along the abdominal cavity.

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