Spanish for worm, the gusano is actually the larvae of an insect found within the agave cactus used to make mescal. The gusano is often put into the bottle and sometimes eaten as a sign of machismo.
This is one of the rare instances of historical inaccuracy in the novel. The gusano was not introduced into mescal production until the 1950s after Jacobo Lozano Páez inadvertently discovered that the worm changed the taste of the agave (agave worms are sometimes found in the piña after harvesting). From then on the gusano was put into mescal bottles, mainly as a marketing gimmick.