Hairy

This cactus never gets infected with mealy bug.  I guess the insects just can't penetrate the dense mat of hairs.  The hairy covering presumably also protects the plant from an excess of UV radiation.  Like most other plants, cacti use chlorophyll to extract energy from the sun to grow, so there must be a trade-off here.  However, light is probably not limiting in the deserts where these plants live.