1) Mezcal is a Mexican distilled spirit made from the agave plant.
Tequila is a mezcal made only from the blue agave plant in the region around Tequila, Jalisco.
Many mezcals will contain a "worm" (sometimes more than one) in the bottle.
http://empresarios.mundoejecutivo.com.mx/images/upload/mezcal-y-gusano.gifThe Mexican standards authority, NOM, prohibits adding insects or larvae to tequila.
