A caterpillar is the larval stage of a moth or butterfly, and like all insects, it possesses six legs, but it also has a variable number of other appendages that function like legs. This creature’s body plan is a specialized adaptation for consuming plant material before transformation. To count the caterpillar’s feet, one must distinguish between the permanent, jointed structures common to all insects and the temporary, fleshy supports unique to the larval form.
The Two Types of Appendages
The number of appendages a caterpillar uses for locomotion typically totals sixteen, though this count can vary depending on the species. These appendages are divided into two distinct groups: true legs and prolegs. True legs are permanent structures found on the three thoracic segments immediately behind the head. All caterpillars possess three pairs of true legs, totaling six, which are retained and modified in the adult insect. Prolegs are temporary, unjointed supports located further back on the abdominal segments. Most species have five pairs of prolegs, but some groups, such as the Geometridae family (inchworms), have a reduced number, forcing them to adopt their characteristic looping gait.
True Legs vs. Prolegs: Structure and Function
The two types of appendages differ significantly in structure and function. True legs are jointed, hard, and possess a chitinous exoskeleton, similar to the legs of adult insects. Each of these six true legs ends in a small, sharp claw, which is primarily used for manipulating food and grasping the substrate near the head.
In contrast, the prolegs are unsegmented, fleshy, and sac-like extensions of the abdomen. They lack the internal skeletal structure of true legs, relying instead on hydraulic power for extension and rigidity. The movement of internal body fluid, or hemolymph, creates hydrostatic pressure that allows the caterpillar to extend and stiffen the prolegs for walking and support.
A specialized feature allows the prolegs to anchor firmly to surfaces such as leaves and silk strands. At the base of each proleg is a pad equipped with microscopic, curved hooks called crochets. These hooks provide a secure hold that prevents the soft-bodied larva from being dislodged by wind or gravity. This combination of hydrostatic extension and crochet-based anchoring makes the prolegs the primary structures for the caterpillar’s characteristic crawling movement.
The Fate of Caterpillar Legs
The temporary nature of the prolegs means they undergo a breakdown during the pupal stage of the insect’s life cycle. When the caterpillar enters the chrysalis or cocoon, these fleshy, abdominal supports are dissolved through a process known as histolysis. The cellular material from the prolegs is broken down and recycled, with the structures entirely disappearing before the final adult form emerges.
The six true legs, however, follow a different path, undergoing a transformation called histogenesis. The larval true legs are retained and reorganized using specialized groups of cells. These structures are modified and developed into the six permanent, jointed legs of the adult moth or butterfly. These final insect legs are used for walking, perching, and the minute chemoreceptors on the tarsi allow the insect to “taste” a substrate simply by standing on it.