Pepsis formosa, the "tarantula hawk", New Mexico's state insect mascot:
[link|http://144.90.137.57/~bfiero/tucsonecology/animals/arth_taha.htm|http://144.90.137.57...als/arth_taha.htm]
Lays its eggs in a live tarantula, with predictable cinematic results.
I could never decide if the idea for Alien derived from this wasp, or as an idea of viral life on a large scale.