The Xenomorphs- that is, the aliens from the movie Alien and its sequels- are partially so named because they pass through an insectile variety of forms during their life cycle. Having just finished watching the movie, I decided to node what I now know. And so, it all starts like this...

Egg: The Xenomorph starts out as an egg. It does, in fact, bear some resemblance to a terrestrial egg, being oval in shape, but instead of cracking open, a sort of seal at the top opens into four 'petals'. These eggs are very durable, and the embryo inside can remain viable for indefinite amounts of time.

Facehugger: The first stage of the newly-hatched Xenomorph is the facehugger. This name is rather self-explanatory, since the first stage involves leaping from the egg to latch onto a host's face, wrapping itself around him and inserting a tube into his throat. The host (if human, at least) enters a coma, in which he stays until time for stage two.

(Unnamed?): This stage, stage two, is a very brief sort of commensalistic symbiosis. The facehugger has fallen off and died, leaving a sort of embryo inside the host. During this stage, the host awakens from his coma and eats as much as possible until the Xenomorph has enough energy and nutrients for stage three.

Chestburster: During stage three, the Xenomorph 'hatches' from its host and begins to grow into its adult form. Like stage two, it spends relatively little time in this stage, having already gathered most of the requirements for full growth. Stage three begins with the Xenomorph erupting rather violently from within its host, then attempting to find a safe place to grow.

Adult: This is the stage that most commonly comes to mind when thinking of a Xenomorph. It's basically a humanoid, taller than most humans, with an elongated, eyeless head (though perhaps containing some sensory organs), claw-tipped limbs, and a long prehensile tail.