This one was my favorite, the bullet ant;
It's a full inch long, it lives in trees and thus can and will fall on you to scare you away from its hive--the one you didn't know was there, because it's in a fucking tree. Before it does this, it shrieks at you. This ant, you see, can shriek.
It's called a Bullet Ant because its 'unusually severe' sting feels like getting shot. On the Schmidt Sting Index, Bullet Ants rate as the number one most try-not-to-shit-out-your-spine painful in the entirety of the Kingdom Arthropoda.
Also--and we do feel the need to stress this--they fucking shriek at you before they attack.