The pads of various species of Opuntia are sometimes dotted with a white, sticky, cottony mass that turns out to have an interesting natural history.  Dactylopus coccus, the small, red Cochineal Bug, creates a haven for itself by constructing this mass.  Centuries ago human beings found that they could collect the Cochineal Bug from the white mass, crush the bug and obtain a bright red liquid that could be used as a dye.

I used a twig to carefully remove one cottony mass from the above Prickly Pear pad, and I then spread the mass on a rock.  Immediately a bright red liquid oozed out and quickly dried to dark maroon.