Spiracles

Spiracles are external openings that serve as entry points to an animal’s respiratory system. The term is used for two main, related ideas: the air-exchange openings of terrestrial arthropods and the gill-access openings found in some fishes.

In insects and many other arthropods, spiracles are paired openings on the thorax and abdomen. They connect to a network of tracheae and tracheoles that deliver oxygen directly to body tissues. Spiracles can be opened or closed by valves or sphincters, a mechanism that helps regulate airflow and reduce water loss in dry conditions. Gas exchange is largely achieved through the tracheal system, though some small or juvenile insects also exchange gas across the cuticle.

In spiders and other arachnids, spiracle-like openings connect to respiratory structures such as book lungs or tracheae. The specific arrangement varies among groups, but spiracles generally function as gateways to internal gas-exchange organs.

In cartilaginous and some bony fishes, the spiracle is a small opening located behind the eye that leads to the pharynx and, ultimately, to the gills. This allows water to be drawn in for respiration when the animal’s mouth is closed or obstructed, such as when the animal is resting on the seabed. In some species the spiracle is large and prominent, while in others it is reduced or absent.

Etymology traces to Latin spiraculum, meaning a small breath or opening. Spiracles illustrate how diverse respiratory adaptations support life in air or water across different animal groups.