Fig. 1—Diagram of a caterpillar.  Fig. 2—Frontal view of caterpillar head.  

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. (1)
 

The orange-colored oblongs visible between the tufts of setae (bristly hairs) are the lateral spiracles—the external openings to the caterpillar's respiratory system. They can serve as reliable landmarks. All caterpillars have 9 pairs. There is 1 pair on the first thoracic segment; the remaining pairs are located on the first 8 abdominal segments. The thoracic and last abdominal spiracles are often twice the size of those in between.

Many adults and larvae of the tiger moth family are diurnal, that is, they are active during the daytime. The woolly bear caterpillars are familiar to many because of this habit; they can often be seen traveling along the ground or crawling amidst low vegetation. These encounters are often the first many children have with the moths, and the "woolly" aspect of these creatures should be used to illustrate a key difference with caterpillars of many common butterflies (which are more properly called larvae).