Caterpillars, the larval stage of moths and butterflies, do not possess the internal, calcified skeletal structure commonly understood as bones. Instead of a rigid endoskeleton, they rely on a different system for support and movement. This allows them to perform the rapid growth and extreme body deformations needed during their life cycle.
Why Caterpillars Are Invertebrates
A caterpillar is classified as an invertebrate because it lacks a vertebral column, or backbone. This places them within the phylum Arthropoda, alongside insects, crustaceans, and arachnids. Like all arthropods, they have an external structure, the cuticle, which forms a flexible exoskeleton.
This exoskeleton, composed mainly of chitin, provides structural integrity and protection. However, it is soft and pliable compared to the hard shells of beetles or crabs. The internal body cavity is filled with hemolymph, the insect equivalent of blood, which plays a significant role in structural support.
The Hydrostatic Skeleton System
The caterpillar maintains its body shape and rigidity through a hydrostatic skeleton. This system utilizes the incompressible nature of the internal hemolymph, which is held under pressure within the body cavity. The muscular body wall works against this fluid pressure, similar to how a pressurized garden hose maintains its shape.
Contraction of the circular and longitudinal muscles surrounding the body cavity increases the internal pressure, creating stiffness and structural support. This pressurized fluid gives the soft body its turgidity, allowing the caterpillar to resist gravity and maintain a cylindrical form. This system is effective for rapid growth, as caterpillars can increase their body mass by as much as 10,000-fold in a few weeks.
How Caterpillars Move Without Bones
The soft, pressurized structure enables a unique form of locomotion described as crawling or inching. Movement is generated by coordinated, wave-like muscular contractions that travel down the body. This action, known as peristalsis, pushes the internal hemolymph and gut contents forward, creating a ripple effect that moves the entire body.
Caterpillars anchor themselves using specialized appendages while the wave of contraction proceeds. They possess six true legs on the three segments of the thorax, which are jointed and tipped with claws. The rest of the body is supported by pairs of fleshy, unjointed prolegs on the abdomen, which end in small gripping hooks called crochets. By alternately gripping with the prolegs and extending the front of the body, the caterpillar achieves forward movement.