From Kingdom Hearts Wiki: A world of information not accessible by Gummiship
A Metal Bug's body is nearly identical to a Metal Bug Blox; it is a metallic, silver cube with patterns resembling computer circuitry covering it. A single, glowing yellow eye is present near the center of the front of its body. Four silver, grey, and black, spider-like legs sprout from its sides.
The Metal Bug's name is a play on its insectoid appearance and its resemblance to a Metal Blox.
Unlike the other bugs, the Metal Bug can turn back into a blox for a short period; as it is based on a Metal Blox, this makes it temporarily invulnerable.