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. (Note: for more information on how tequila is made, see mezcal.) However this misconception continues, and even with all the effort and marking to represent Tequila as a premium -- similar to the way Cognac is viewed in relation to brandy -- there are some opportunist producers for the shooters and fun market who blur these boundaries.
In the 2000s, a distributor known for their Tequilas, launched Villa Lobos, a vodka which had the unique selling point that it too featured the "agave worm". The marketing of this product highlighted the drink's links with Tequilas and said that it was developed in reaction to the Tequila crisis of the previous years
The best Tequila comes from a region just a bit outside of Guadalajara
in the state of Jalisco. No worms in that one, though.
From Region's link, I can believe that Tequila with a lesser quality would
have a worm in it. Makes sense, doesn't it? I wish all poor quality liquor could be identified that way.