The Collaborative International Dictionary of English v.0.48:

Bots \Bots\, n. pl. [Cf. Gael. botus belly worm, boiteag
   maggot.] (Zool.)
   The larv[ae] of several species of botfly, especially those
   larv[ae] which infest the stomach, throat, or intestines of
   the horse, and are supposed to be the cause of various
   ailments. [Written also botts.]

   Note: See Illust. of Botfly.
         [1913 Webster]