It is a green cricket with a yellow underbelly, little cartoon eyes, and a scuba diver helmet.
Role in series
When SpongeBob tells a bad joke, the cricket appears, and starts chirping. The cricket is used as a joke to say how bored the audience is. SpongeBob tells another bad joke, and the cricket doesn't chirp because that one was even worse.
"To Save A Squirrel"
It scares SpongeBob and Patrick because they think it is a "beast." Sandy then brings the "beast" back to the surface via bubble.