The web it spins can be considered its second nervous system. It is said that Spinarak determines its prey by the tiny vibrations it feels through the web.
(From Bulbapedia)Spinarak is a green, arachnoid Pokémon. It has black eyes with white pupils, a white horn on its head, red mandibles, and six yellow legs. A greenish-black stripe encircles the middle of each leg and the end of its abdomen. The greenish-black spots on its abdomen resemble a face that has been shown to change expression in the anime.
Spinarak is a patient hunter that can wait motionlessly for several days for unsuspecting prey to become trapped in its thin yet strong web, which it weaves in the forest. The thread it spews is so strong that its webs will not break even when rocks are set on them, and some fishermen weave the thread into nets to catch fish Pokémon. It is even said to be able to identify what kind of prey is in its web just by their vibrations. Its poison is not very strong, but is potent enough to weaken caught prey. Its preferred prey is Cutiefly. Spinarak and its evolution Ariados are the only known Pokémon that can learn Toxic Thread.