Why Do They Have a Worm in Tequila?

According to Anthony Dias Blue's Complete Book of Spirits, that "worm" is actually a larva from one of two types of moths, known as maguey worms, that live on the agave plant. These larvae are called gusano and bottles of mezcal that contain the lil guys are referred to as con gusano.

What is the purpose of the worm in the tequila bottle?

Larvae began appearing in mezcal bottles in the 1950s, when a mezcal maker discovered a moth larvae in a batch of his liquor and thought the stowaway improved its taste. He started adding “worms” to all his bottles as a marketing strategy.

When did they stop putting worms in tequila?

Once tequila was granted protected status in 1977 (on October 13, if you'd care to celebrate), the worm system was rendered unnecessary.