A grid bug has an animal body.

These electronically based creatures are not native to this universe. They appear to come from a world whose laws of motion are radically different from ours. Tron looked to his mate and pilot. "I'm going to check on the beam connection, Yori. You two can keep a watch out for grid bugs." Tron paced forward along the slender catwalk that still seemed awfully insubstantial to Flynn, though he knew it to be amazingly sturdy. He gazed after Tron, asking himself what in the world a grid bug was, and hoping that the beam connection-- to which he'd given no thought whatsoever until this moment-- was healthy and sound."

Tron, novel by Brian Daley, story by Steven Lisberger