Opuntioid cacti include the prickly pears (Opuntia) and chollas (Cylindropuntia). Prickly pears have flattened stems and chollas have cylindrical stems. Opuntioid cacti are distinguished from other cacti by four characteristics:
- the stems of opuntioid cacti grow as jointed segments;
- the areoles produce small, barbed spines known as glochids;
- new joints produce rudimentary leaves; and
- the seeds have a pale covering called an aril.
Opuntioid cacti are historically an important reason for the Spanish conquest of the New World. Initially, the Spanish sought gold until the discovery of a small insect that feeds on the opuntioid cacti. Cochineal is a scale insect; the males are winged and lack mouthparts, but the females are wingless and have short antennae and legs. They are covered with white waxy plates.
The female cochineal insect (Dactylopius coccus) is important as the source of a bright crimson dye. Dried and ground- up cochineal insects were used by the Native Americans for dying their textiles a rich red or purplish color. In Europe, reds and purples was so rare that only royalty could afford it. Eventually royal crimson from cochineal became reserved for the kings by law. The cultivation and export of cochineal became a major economic activity, and the source of the dye was kept secret. Cochineal insects were commercially important until about 1875, when less expensive aniline dyes were introduced.