The head (figure 2) bears 6 lateral eyes (the stemmata) that are usually arranged in a crude circle. Other insect larvae resembling caterpillars have only a single lateral eye.

Short antennae are positioned between the mouthparts and the lateral eyes. The labrum, or upper lip, may be cleft, with the notch engaging the leaf while the larva is feeding. A second diagnostic feature shared by all true caterpillars is an inverted Y that runs down the middle of the face (figure 2). The lower arms of the Y delimit the frons, or frontal triangle; the upper portion extends back to the thorax. The 3 thoracic segments each bear a pair of segmented (true) legs with a terminal claw, the thoracic legs. On the dorsal, or upper side, of the first thoracic segment, there is commonly a hardened plate called the prothoracic shield. The abdomen is composed of 10 segments. Most caterpillars possess 4 pairs of fleshy, unsegmented midabdominal prolegs on their third, fourth, fifth, and sixth segments and a pair of anal prolegs on the last segment. Because exceptions are numerous, the number and relative size of the prolegs is often important in the recognition of families. The prolegs bear a series of hooks, called crochets, that are used by the caterpillar to engage the substrate and maintain their purchase (figures 3 and 4). The number, size, and arrangement of the crochets are often useful in identification. Sawfly immatures are caterpillarlike, but differ in the possession of more than 4 pairs of midabdominal prolegs, none of which bear crochets. (3)