Those particular ones are actually cylindropuntia bigelovii (jumping cholla, teddy bear cholla). Cylindropuntia fulgida are (chain fruit, boxing glove cholla, although they are also called jumping cholla) and don't have as many spines. The fruit on cylindropuntia bigelovii are not viable, they reproduce vegetatively by having easily detachable segments and barbed spines. The segments fall off with a strong wind or get snagged by a passerby and will root wherever they fall. The spines, being barbed, will easily snag just about anything and with a light tug the segment will separate from the plant and seemingly jump on you, hence the name. Cholla give other cactus a bad name. They are actually in the prickly pear family (opuntia).
-RP
