A caterpillar is considered to be an insect. Why?

Caterpillars are the larval stage of members of the order Lepidoptera (the insect order comprising butterflies and moths).

Caterpillars are also insects, just like their parent butterflies or moths. They have six proper legs,just like all insects, but also up to five pairs of prolegs with little hooks that help them to hang onto things, and to move in a wave-like motion.