The Marigumo is a Spider-like creature that is in a cuccoon like shape when found, but once a Pikmin is thrown on its net, it will awake and try to kill them, They also act as hazards for Flying Pikmin, as when they are carrying anything back to the onion, and one of these are on the way, they will get stuck, and eventually be eaten.
Strategy
The easiest strategy to defeat one of these is to throw any Pikmin at its net so it can wake, and carefully hit the center of its body with a Rock Pikmin so it dies easily.