The structure of a caterpillar.

The caterpillar head has simple eyes, two short antennae and strong chewing mouth parts. The body is made up of 13 segments. The first three segments behind the head are called the thoracic segments. Each of these three segments has a pair of jointed true legs. These become the legs of the adult butterfly or moth. The remaining 10 segments are the abdominal segments. These usually contain 5 pairs of false legs called prolegs. These are purely to support the growing body. Loopers have no prolegs whcih is why they have the characteristic "inchworm" method of moving. Small hooks called crochets at the bottom of the prolegs act as grippers. Spiracles are circular openings on each side of the body through which air passes in and out.
In order to survive many species have adapted behaviours in defence. Some caterpillars have structures like fierce horns, which when disturbed will adapt an alarming 'stand up and stare' approach. Most swallowtail caterpillars are equipped with a protrudable forked scent gland called an osmeterium which emits a foul odor. Other caterpillars have spines which can cause rashes, these contain toxins, and although not fatal, can be very irritating. Some caterpillars have knobby bits called tubercles.