While battling it, you use a Pokéball on it. If you are successful, you have caught the Pokémon. However, don't bother. Caterpie sucks and so do both of his evolutions (Butterfree is weak against phsycic, dark, ice and fire, many more weaknesses than any other Pokémon). Bug Pokémon are just not worth the effort. That is why at first all you fight are bug catchers ... and who's ever lost to a bug catcher? that's right. No-one. And you know why? it's because Bug Pokémon suck, as I have previously stated.