Butterfly caterpillars can vary in size, colour, and shape. They may have spines, bristles, or soft body extensions. All caterpillars have 13 body segments. The first three segments make up the thorax. The thorax has three pairs of legs. These legs are called true legs. The other 10 segments make up the abdomen. The abdomen has five pairs of soft legs called prolegs. The prolegs have tiny hooks at the end of each of the foot. They are used to hold on to things. The hooks are called crochets.