Chu Worms are insects that fight in synergy with Shell Jellies. Upon spotting Link, they will hop into a nearby Shell Jelly to defend themselves, making them invulnerable to attacks. To defeat them, Link must either first drag them out using the Clawshot, or blast their Shell Jellies with a Bomb Arrow. After getting separated, Chu Worms will attempt to hop back into their Shell Jellies.
↑"Chu Worm These two-legged insect monsters hide inside Shell Jelly, using their protective shells to crush Link and absorb sword and arrow attacks. A Chu Worm can only be hurt by first dragging it out of the jelly or by blasting it with Bomb Arrows." (Encyclopedia (Dark Horse Books) pg. 170)