Facehugger

The Facehuggers are the first part from the Aliens(Xenomorph) life cycle once they are out of the egg they search for a host to impregnate with an embryo(chest burster). Once they find a host they attach to the host's face and then plant the chest burster. Once it has planted the embryo the facehugger will die.

Royal Facehugger?
There is much speculation as to how Queen Xenomorphs are born, many beleive a royal facehugger plants a Queen embryo in it's victim.