"The worm in the mezcal bottle is a marketing gimmick. The worm is actually the caterpillar Hypopta agavis. The originator of this practice was a man named Jacobo Lozano Páez. In 1940, while tasting prepared agave, he and his partner found that the worm changed the taste of the agave. (Agave worms are sometimes found in the piña after harvesting, a sign of badly chosen, infested, agave). The worm soon took on another use as well, as a proof of potency. If the worm was decayed inside the bottle, it could be assumed the mezcal had been watered down. Brands of mezcal that contain the worm include 'Oro de Oaxaca', 'Gusano de Oro', 'Gusano Rojo', 'Monte Alban', and 'Dos Gusanos'.
When a worm is included this is known as 'Con Gusano', which means 'with worm'"