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.