Infants infected with the Necromorph contagion develop swollen torsos as their internal organs are liquefied and converted into a highly volatile explosive pus. The torso is so morbidly swollen that the host cannot crawl on its front, and so its shoulders are shifted so that it can crawl with its back to the ground, hence why a Crawler's head always appears to be upside down. The host infant's legs are also fused together into a tail-like appendage, making it move much like a caterpillar.
Numerous human babies were killed and turned into Crawlers during the Necromorph infestation of The Sprawl and Isaac Clarke would encounter alien Crawlers years later on Tau Volantis. The alien Crawlers differ little from the human ones, the only real difference being appearance and the sounds they make.
Being made from babies, Crawlers are incapable of moving with any significant level of speed, but they are relentless in their approach and they always appear in large groups. They are capable of crawling over walls and ceilings and will not be deterred by any obstacles or threats. When they approach an intended victim, their swollen torsos will rupture and detonate, killing themselves and their targets. Luckily, Crawlers are slow and very weak; a single shot from any weapon is enough to kill one, or several if they are packed closely together when one detonates. Crawlers can also be killed without detonating them by shooting off the head or limbs.