glochid

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One of the small, easily detached, barbed spines on a cactus of the subfamily Opuntioideae, such as a prickly pear or cholla.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a form of small, detachable, irritant spine occurring in dense clusters in the areoles of certain cacti such as prickly pear. Typically glochids are short, straight, and microscopically barbed. When stuck into the skin of anyone touching the plant, they detach and remain in the skin when the victim moves away.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a barbed spine or bristle (often tufted on cacti)