The death's-head hawk moth is a famous symbol from Hannibal Lecter (Anthony Hopkins) for help in understanding Bill's psyche so she can catch him. Despite Hannibal's somewhat overpowering presence throughout the film, the character himself only appears onscreen for around sixteen minutes.
One of Buffalo Bill's MO's in The Silence of the Lambs is leaving a peculiar staple behind within the throats of his victims; a death's-head hawk moth. This is first discovered during an unnerving autopsy scene, and the symbolism behind the moth becomes a key part of understanding Bill's motivations. As Lecter himself later imparts to Clarice, the signification of the moth itself is change. "Caterpillar into chrysalis, or pupa, and from thence into beauty." Lecter is referring to the process of metamorphosis, and this concept runs deep throughout Clarice's journey too.
Of course, in relation to this, there's the problematic depiction of Bill's seeming gender dysphoria and how he deals with it. In Clarice
The moth symbolism extends to Clarice's story too, with The Silence of the Lambs opening with her still in training at the FBI academy before she soon undergoes her own transformation and is baptized into a world of darkness. While Clarice begins the narrative as an inexperienced agent who needs help understanding the psychology of killers, the movie ends with her and Lecter being the two people who understand Bill best. Any semblance of innocence has been eroded and under Lecter's tutelage, she's grown on both a personal and professional level.
There's also the glaring symbolism behind the death's-head hawk moth itself, beyond metamorphosis. The moth derives its foreboding name from the fact it has a design resembling a human skull on its upper body. Even after he's finished with his victims, Bill leaves a symbol of death within them; most likely left in their throats because these moths can squeak, thus mocking the unlucky souls' fearful screams. Just like Buffalo Bill and The Silence of the Lambs as a whole, the moth theme is layered in disturbing subtext.
