Caterpillar Identifier

Caterpillars are the larval stage of butterflies and moths in the order Lepidoptera. With over 180,000 species of Lepidoptera worldwide, caterpillar diversity is enormous. Identifying caterpillars can be challenging because they often look completely different from their adult forms, and many species change appearance as they grow through successive instars. Knowing which caterpillar you have can help you predict whether it will become a butterfly (see our Butterfly Identifier tool) or a moth (see our Moth Identifier tool).

Body color and pattern are primary identification features. Caterpillars display an extraordinary range of colors and markings including stripes, spots, bands, and complex geometric patterns. Many species are green for camouflage among leaves, while others sport bright warning colors like yellow, orange, and red with black. Some have conspicuous eyespots that mimic larger animals to deter predators.

Body hair and spines are important distinguishing characteristics. Some caterpillars are completely smooth, while others are densely covered in fine hairs, tufts, or branching spines. Woolly bear caterpillars are covered in dense fur-like setae. Saddleback caterpillars bear prominent spiny protuberances. The arrangement and color of these structures help narrow identification.

Body shape and size provide useful clues. Most caterpillars are cylindrical, but some are flattened, slug-like, or have unusual humps and projections. Hornworms have a distinctive horn-like projection at the rear. Inchworms loop their bodies in a characteristic motion because they lack prolegs in the middle of the body.

Proleg arrangement is a technical but reliable identification feature. True caterpillars of butterflies and moths have three pairs of true legs near the head and typically five pairs of fleshy prolegs on the abdomen. Sawfly larvae may have six or more pairs of prolegs.

Host plant association is one of the most valuable identification tools. Many caterpillar species feed only on specific plant families or even individual plant species. Finding a caterpillar on a particular plant immediately narrows the list of possible species. Monarch caterpillars, for example, are found almost exclusively on milkweed plants.

Behavior offers additional clues. Some caterpillars roll leaves, others build silk tents, and some feed gregariously in groups while others are solitary.