These two bowls of thousands of dried worms are called cochineal. Here they look black and white, but while feasting on nopal, they are white in color. However, if you smear these worms with your finger, or grind them with a mortar and pestle, you get a bright red color. This pigment was used as a red dye for tapestries and textiles in prehispanic Mesoamerica and was used by Europeans because at the time they had no dye to create such a color.
This was the major export of Oaxaca starting in the late 1500s up until the middle of the 1900s when a cheaper synthetic substitute for the red dye was created.