aculeus

from WordNet (r) 3.0 (2006)

aculeus
    n 1: a stiff sharp-pointed plant process
    2: a sharp-pointed process especially a sting of a hymenopterous
       insect

from The Collaborative International Dictionary of English v.0.48

Aculeus \A*cu"le*us\, n.; pl. {Aculei}. [L., dim. of acus
   needle.]
   1. (Bot.) A prickle growing on the bark, as in some brambles
      and roses. --Lindley.
      [1913 Webster]

   2. (Zool.) A sting.
      [1913 Webster]