spiracle
1. A small paired opening that occurs on each side of the head in cartilaginous fish. It is the reduced first gill slit, its small size resulting from adaptations of the skeleton for the firm attachment of the jaws. In modern teleosts (bony fish) the spiracle is closed up. In tetrapods the first gill slit develops into the middle ear cavity.

2. Any of the external openings of the tracheae along the side of the body of an insect.

spiracle
1. Essentially a vestigial gill slit located between the mandibular and hyoid arches in adult sharks, rays, and some primitive bony fish, and visible as an opening behind the eye.

2. One of the pores on the body of an insect at which the tracheae open to the outside. Spiracles are found on the thoracic and abdominal segments, and usually have some mechanism to close them.