Source : Webster's Revised Unabridged Dictionary (1913)

Spiracle \Spir"a*cle\, n.[L. spiraculum, fr. spirare to breathe:
   cf. F. spiracule. See {Spirit}.]
   1. (Anat.) The nostril, or one of the nostrils, of whales,
      porpoises, and allied animals.

   2. (Zo["o]l.)
      (a) One of the external openings communicating with the
          air tubes or trache[ae] of insects, myriapods, and
          arachnids. They are variable in number, and are
          usually situated on the sides of the thorax and
          abdomen, a pair to a segment. These openings are
          usually elliptical, and capable of being closed. See
          Illust. under {Coleoptera}.
      (a) A tubular orifice communicating with the gill cavity
          of certain ganoid and all elasmobranch fishes. It is
          the modified first gill cleft.

   3. Any small aperture or vent for air or other fluid.

Source : WordNet®

spiracle
     n : a breathing orifice