Gusano is a term for the larvae “worms” that can be found in bottles of Mezcal.
The worm is actually the larva from a type of moth known as a maguey worm, which lives on the agave plant. The practice of adding a worm began in the 1950’s as a marketing ploy and today, while many consider it to be a gimmick, some brands bottle theirs with them in and on the label you will find the words, Con Gusano.