A Chestburster is the immature infant form of the Xenomorph which emerges forcibly from the chest of its host, killing it. The chestburster is similar in form to the adult Lurker, but far smaller and pale in color. Its arms are stuck to its chest, forcing it to crawl similar to a snake until it molts, freeing its arms and legs.
A chestburster first develops in its host when an egg is laid in the pharynx by a facehugger. It is speculated that it develops in a manner similar to that of the immature heartworm (Dirofilaria immitis) that migrates through the body absorbing nutrients before establishing itself. Far from the pharynx, it's unknown if the embryo travels to the host's stomach or to the bronchial tubes. Another theory says that the alien begins it's life in the host body not as an embryo but as a tumor that later takes the shape of an immature alien. In this theory, what the Facehugger injects in the host is not an infant Xenomorph but a small egg that inserts its proteins in the host DNA and commands its cells to "create" the chestburster which has both the Alien proteins and the host's.
When fully developed, the creature infiltrates the internal organs of the host and releases enzymes that softens the bone and surrounding tissues, making extraction of the embryo next to impossible. When mature, it will start chewing the surrounding tissue. This causes the victim great pain, shock, and blood loss. Then, true to its namesake, it forcefully penetrates its host's insides, invariably killing the host in the process. If far away from the Hive, it cannot depend on the adults so it actively searches for a nursery or a food locker, something with a large amount of organic or inorganic matter (such as metals) which it softens with its acid fluids and directly converts into body mass. It also will attack and kill humans and feed on them. After a few hours to a few days, it turns into an adult Xenomorph.
The host's characteristics determine the chestburster's features. For example, quadrupedal hosts produce chestbursters that are nearly the size of their host and have forelimbs, whereas chestbursters spawned from humans are small and larvae-like and have no forelimbs.
Behind the ScenesEdit
The chestburster was designed by Alien director Ridley Scott and constructed by special effects artist Roger Dicken. Giger had produced a model of a chestburster that resembled a "degenerate plucked turkey" and was far too large to fit inside a ribcage. Much to Giger's chagrin, his model reduced the production team to fits of laughter on sight. Scott drafted a series of alternate designs for the chestburster based on the philosophy of working "back [from the adult] to the child" and ultimately produced "something phallic." The chestburster in the original Alien was armless but arms were added in Aliens to facilitate the creature crawling its way out of its host's corpse. This concept would be abandoned in Alien Resurrection and subsequent films.