Closeup of a pair of prolegs. Note the "velcro" hooks that help it
hang on. Prolegs aren't real legs; just muscular extensions of the
abdominal body wall without segments. Insects only ever have 3 pair
of true legs and they're on the thorax (just behind the head in caterpillars).
The fundamental definition of an insect is 3 body regions (head, thorax,
and abdomen), 3 pair of legs on the thorax, and in adults two pair
of wings (on the 2nd and 3rd thoracic segments).