From Wikipedia:
The worm
It is a common misconception that some tequilas contain a 'worm' in the bottle. Only certain mezcals, usually from the state of Oaxaca, are ever sold con gusano, and that only began as a marketing gimmick in the 1940s. The worm is actually the larval form of the moth Hypopta agavis that lives on the agave plant. Finding one in the plant during processing indicates an infestation and, correspondingly, a lower quality product. However this misconception continues, and even with all the effort and marketing to represent tequila as a premiumsimilar to the way cognac is viewed in relation to brandythere are some opportunist producers for the shooters-and-fun market who blur these boundaries.
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