The Chigs received their name because they allegedly look like Earth chiggers (which are small, six-legged larvae of mites). The Chigs are an alien race, who, according to reports, decimated the two Earth colonies Vesta? and Tellus?, thus beginning the galactic war in which the 58th currently find themselves fighting.
Chigs appear humanoid, though they are rarely (if ever) seen without their armoured environment suits. They have an extremely different physiology based on sulfur dioxide. Exposure to water (and water vapour) causes a chemical reaction creating sulfuric acid, and thus leading to death. The Chigs smell like sulfur, and their suits emit a sulfur signature. The scent of humans is repellent to the aliens. They refer to humans as Red Stink Creatures.
The Chigs have apparently allied themselves with the A.I.'s; however the true nature of this alliance is yet unknown, and is possibly one of mutual necessity.
It has been determined that the Chigs, having heard of Earth religions and their belief in the 'resurrection of the dead' and the immortal soul, are scared silly that soldiers whom they've just killed are going to rise up and begin attacking again. Therefore, to forestall such a thing, Chigs chop off their victim's head and hack apart the body, apparently theorizing that a body which has been so disassembled cannot possibly rise and/or resurrect.