How does a young stick insect regrow a lost leg?

Answer: A new leg forms under its skin and appears when it molts

It carries spare legs stored inside its body — Stick insects don't carry spare legs inside them. The new leg is built from the cells at the stump, then revealed when the insect sheds its skin.

A new leg forms under its skin and appears when it molts ✓ — A young stick insect grows by molting, shedding its outer skeleton several times. After losing a leg, it builds a new one under the old skin, and the leg appears, often smaller at first, at the next molt; adults no longer molt, so they can't regrow legs.

The stump grows a full new leg within a few days — A lost leg doesn't regrow in days; the new one shows up only at the next molt, which can be weeks away. It may take more than one molt to reach full size.