Answer: Death's-head moth
After Clarice recovered a moth from behind the soft palate of one of Buffalo Bill's victims, she brought it to the Smithsonian's Museum of Natural History for analysis. This line was said by the entomologist, who identified the species of moth for Clarice as Acherontia styx, or the Death's-head moth, so named because of the naturally occurring skull-shaped pattern on its thorax. Fittingly, the moth's scientific name was derived from the names of two rivers in Hell in Greek mythology - the Acheron and the Styx. Later in the film, the significance of the moth was explained by Lecter to Clarice as being symbolic for change - "caterpillar into chrysalis or pupa, from thence into beauty". Buffalo Bill's pathology was that he was seeking a way to become reborn through his killings because he hated his own identity, and he used the moth as a symbol of the transformation he hoped to undergo. Although the book and the film both indicated the moth used by Buffalo Bill to be Acherontia styx, the species actually depicted in the film and on the movie's poster was really Acherontia atropos, a slightly different species within the same family. The filmmakers must have decided that this species was more photogenic than Acherontia styx.