(Anat.) The nostril, or one of the nostrils, of whales, porpoises, and allied animals.
2.
(Zool.)
(a)
One of the external openings communicating with the air tubes or tracheae 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.
(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.
... while from the aperture project the tips of the mouth-hooks (fig. 21 e, f), formidable, black, claw-like structures, articulated to the strong pharyngeal sclerites and moved by powerful muscles, tearing up the fibres of the flesh. On either side of the prothorax is an anterior spiracle, a curious branching or fan-like outgrowth (fig. 21 b), with a variable number of tiny openings which are probably of little use for the admission of air to the tubes. In many maggots the mouth-hooks and the front spiracles become more and more complex in form in the successive instars. The cuticle, ... — The Life-Story of Insects • Geo. H. Carpenter
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