Hanneton translates to "cockchafer" or "May beetle." It's an interesting and somewhat unusual word to find in song lyrics, especially in a context that's both surreal and darkly humorous.
In the song, Gainsbourg describes training a hanneton on his head, which he calls an "héliport d'héli-coléoptère" (helicopter pad for a heli-coleopteran). This imagery is bizarre and memorable, perfectly capturing the eccentric and slightly unhinged atmosphere of the "Lunatic Asylum" where the narrator finds himself.
"Lunatic Asylum" comes from Serge Gainsbourg's 1976 concept album L'Homme Ă tĂŞte de chou. The story is told from inside a psychiatric clinic, by a man who has lost the woman he loved and is slowly losing himself with her.
He keeps circling back to her name, Marilou. He remembers her as the Playboy bunny, the shampoo girl who rolled him between her fingers, the one who spoke her own baby-talk dialect. The memories are tender and a little absurd, which is the point: they are all he has left.
The middle of the song moves into the ward itself. With nothing to do but wait, he has trained a beetle to land on his head as if it were a helipad, and he sends distress signals out through its golden wings. Nobody answers. Moths intercept the messages, radio lice jam the rest, and he is left with the shrug at the centre of the song: bad luck.
It is a song about madness told very gently, with jokes and wordplay standing in for grief. Gainsbourg lets the narrator sound almost cheerful while he admits the only thing that matters: he was crazy about her.