Wurmple is a small, caterpillar-like Pokémon. Its body is mainly red with a cream underside and face. It has large yellow eyes with dark pupils. It has one sharp yellow stinger on its head and two of them on its rear. It has a tall, erect, red spine behind its forehead and a similar, smaller spine behind that. It has small spikes running down the top portions of its body and five pairs of stubby white limbs.

Wurmple has suction pads at the tips of its feet, which allow it to cling to glass or tree branches without slipping. It can spit white silk that turns gooey when exposed to the air. To defend itself, it can release poison from its tail spikes, which can weaken foes. Wurmple's diet consist of leaves and sap, the latter of which it gets by pulling bark off trees. Wurmple are abundant and live in forests. It is unknown why Wurmple evolves into either Silcoon or Cascoon, and not even Wurmple's appearance gives a hint to which species it will evolve into. Taillow, Swellow, and Starly are natural predators of Wurmple.