A Chestburster is the infant form of the Xenomorph, which are impregnated into living hosts by the Facehugger. Earning its' namesake, the Chestburster emerges forcibly from the chest of its host, killing it. The Chestburster is 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. Different chestburster variants vary in size and appearance.
A Chestburster first develops when a living host is incapacitated by a Facehugger by latching itself onto the host's face, forcing the host to ingest an embryo inserted via the pharynx. Once the embryo is successfully ingested, it moves far from the pharynx further within the body, establishing itself within the host's upper torso all the while absorbing nutrients from the body until it matures. As displayed by Kane aboard the Nostromo, the host shows little to no considerable outward negative symptoms after being impregnated, the most common being sore throat, slight nausea, increased congestion, and feeling hunger.
Unbeknownst to the host, however, is that the Chestburster is laying dormant until it reaches it's maturity. Over the course of one to 24 hours—indeterminable in some cases, and sometimes up to a week, the Chestburster began to emerge violently from the body. The unsuspecting host first experiences a shortness of breath, followed by great pain in the chest area, causing shock. The host would continue to experience extreme pain until the Chestburster rips through the chest cavity, causing massive hemorrhaging and invariably killing the host in the process. Upon emerging from the body, its first instinct is to flee and hide until it fully matures. Within the span of a few hours, the Chestburster would undergo a dramatic growth spurt, molting its skin before eventually reaching the adult form of the Xenomorph; the Drone.
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.
Appearances in other Media
Dark Horse Comics
The following information follows the continuity from Dark Horse Comics' Aliens comics and all related media, therefore unrelated to the primary continuity.
The Chestbursters appeared in numerous stories from Dark Horse Comics' Aliens comic book series, set on an alternate continuity following the events of Aliens. One particular Chestburster served as the main antagonist of Aliens: Kidnapped. The egg that contained this individual's embryo had contracted a mysterious virus, turning it pink. A group of mercenaries retrieved a clutch of Alien eggs from a ship wreckage and had them delivered to Gaianet researcher Jeremy Rumford. Rumford rejected the pink egg and advised the mercenaries to dispose of it in fear of it being infected by a deadly micro organism. Instead of listening to Rumford's warning, the mercenaries decided to sell the egg to the Penga Bar. Media sensation Darryl Zither visited the Penga Bar to order one of its exotic menus. When he decided to taste the pink egg, a Facehugger suddenly hatches and impregnates him. After being dropped off to his home by his friend, he was picked up by billionaire Ivy Derringer who invited him to visit her pleasure planet, Celeste. While on their way to Celeste using Derringer's private ship, the Chestburster violently emerges out of Zither and proceed to devour Derringer. After escaping the ship, it begins a killing spree on Celeste. After killing dozens of Celeste's residence and employees, the Chestburster eventually died and its body was discovered by Celeste's police force that was hunting it. Though the Chestburster had died, its reign of terror was not over as it was revealed that the Chestburster had been infected with a class ten virus. The pink-colored slime trail it left behind starts a viral outbreak that causes people to go delirious and detonate, causing Celeste to be sterilized via nuclear missiles.
Behind the Scenes
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.