Question

how does a Pitcher plant catch insects

Answer 1

Foraging, flying or crawling insects such as flies are attracted to the cavity formed by the cupped leaf, often by visual lures such as anthocyanin pigments, and nectar bribes. The rim of the pitcher(peristome) is slippery, when moistened by condensation or nectar, causing insects to fall into the trap.

Answer 2

If I remember correctly there are two main ways pitcher plants catch their prey.  Some emit an appealing odor to insects which lures them in.  Once the insect lands on the lip, which is slippery, they fall into the bell.

The other I know of have sweet water in the bell and when the insect tries to crawl down for a taste they fall in.