Spiracles are small external openings that allow air or water to enter an animal’s respiratory system. They appear most famously in insects, where they serve as the entry points for a network of breathing tubes that deliver oxygen directly to tissues. But spiracles also appear in spiders, scorpions, sharks, and rays, each time serving a slightly different respiratory purpose.
How Insects Breathe Without Lungs
Insects don’t have lungs. Instead, they rely on a system of branching internal tubes called tracheae that carry oxygen directly from the body surface to individual cells. Spiracles are the openings along the body wall where air enters and exits this tube network. In the most common arrangement, an insect has 10 pairs of spiracles: two pairs on the thorax (the middle body section) and eight pairs running along the abdomen. This layout appears in beetles, caterpillars, and many other species, though the exact number can vary between species and even between life stages of the same insect.
Each spiracle connects to a small chamber that branches into progressively smaller tubes. The finest tubes, called tracheoles, are so thin they deliver oxygen directly to muscle fibers and organs without the need for blood to carry it. Carbon dioxide travels back out through the same route. This system is remarkably efficient for small bodies, allowing insects to keep their spiracles closed for long stretches and only open them briefly to exchange gases.
The Valve System That Keeps Insects Alive
Spiracles aren’t just passive holes. Each one has a muscular valve that can open and close with precision. In locusts, the valve on the second thoracic spiracle is controlled by a small muscle receiving signals from two nerve fibers: one “fast” fiber that triggers a quick contraction to snap the valve shut, and one “slow” fiber that produces a gentler, weaker contraction. When the fast nerve signal stops firing, elastic hinges in the valve spring it back open. The timing of opening and closing is coordinated by nerve signals originating in the abdomen.
This level of control matters enormously for survival. The tracheal system is the insect’s main route for water loss, and keeping spiracles sealed is the primary defense against drying out. When researchers forced insects to hold their spiracles open by exposing them to high concentrations of carbon dioxide, water loss increased two- to tenfold. One drought-resistant species, a blood-feeding bug called Rhodnius, died within three days under those conditions.
Many insects use a strategy called spiracle fluttering to balance these competing needs. By keeping the spiracles closed most of the time and rapidly flickering them open at high frequency, an insect can pull in plenty of oxygen while losing very little water. The key is the ratio: a spiracle that’s open only a small fraction of the time but flutters rapidly achieves high oxygen intake with minimal moisture escaping. This decouples two challenges that would otherwise be in direct conflict.
Spiracles in Spiders and Scorpions
Arachnids have spiracles too, though they connect to different respiratory structures. In scorpions, spiracles are located on the underside of the abdomen, near segments four through seven. Each spiracle opens into a sac-like chamber called an atrium, which leads to the book lungs, layered structures where gas exchange takes place across thin, stacked sheets of tissue (the “pages” of the book lung). During embryonic development, these spiracles form as inward folds of the outer body wall, with cells migrating inward to build the highly ordered layers of the book lung behind them.
Spiders follow a similar pattern, with spiracles on the underside of the abdomen leading to book lungs, tracheal tubes, or both, depending on the species. Some spiders have two pairs of book lungs, while others have replaced the rear pair with a tracheal system more like an insect’s, retaining a spiracle as the external opening for each.
Spiracles in Sharks and Rays
In the ocean, spiracles serve an entirely different purpose. Sharks and rays have a spiracle on each side of the head, just behind the eye. These are modified gill slits, specifically a remnant of the first gill cleft. In bottom-dwelling species like skates and rays, spiracles are critical for breathing. Because these animals rest on the seafloor with their mouths pressed against sediment, they can’t easily draw clean water in through the mouth the way free-swimming sharks do. Instead, water enters through the spiracles on top of the head, flows over the gills, and exits through the gill slits below.
Spiracles in these animals also house a sensory organ derived from the lateral line system. This spiracular organ contains hair cells embedded in a gel-like structure and sits inside the spiracle’s canal. It detects movement of the hyomandibular cartilage, the structural element that helps suspend the jaw from the skull. When the jaw protrudes, the cartilage flexes and physically stretches or compresses the spiracular organ, giving the animal feedback about its own jaw position. In skates, the organ is anchored directly between the skull and the jaw cartilage, making it especially sensitive to this motion.
Active, fast-swimming sharks like great whites have reduced or absent spiracles because they breathe by ram ventilation, continuously swimming forward with their mouths open to push water over their gills.
Aquatic Insects and Underwater Breathing
Some insects have adapted their spiracles for life underwater, and the solutions are ingenious. Certain aquatic bugs carry a thin film of air trapped against their body by a dense mat of tiny hairs. This air film stays in direct contact with the spiracles. As the insect consumes oxygen from the film, the slight drop in oxygen concentration causes dissolved oxygen from the surrounding water to diffuse inward, replenishing the supply. This structure, called a plastron, works so well that the insect never needs to surface. Under normal conditions, it can breathe indefinitely underwater.
Other aquatic insects use a less permanent version of the same idea. The saucer bug, for example, surfaces periodically to trap a bubble of air under its wing covers and along its belly. It inhales through spiracles on its back and along its abdomen, and exhales through spiracles on its chest. While submerged, the stored air functions as a temporary physical gill, with oxygen diffusing in from the water to extend the air supply between trips to the surface.
An Evolutionary Link to Your Own Ears
The fish spiracle has a surprising connection to the anatomy of land vertebrates. In ancient fish, one of the first gill slits narrowed into a tube running just above the rear of the jaw joint. This was the spiracle. As vertebrates moved onto land and evolved over hundreds of millions of years, this region of the skull was repurposed. In reptiles and birds, the old spiracular space became the middle ear cavity, the air-filled chamber that houses the tiny bones transmitting sound to the inner ear.
In mammals, the story took a different turn. The middle ear evolved from a slightly different region of the skull, lower in the head than in other land vertebrates. The Eustachian tubes connecting your middle ear to your throat, while they look similar to the spiracular connections in reptiles, actually evolved independently. They are not direct descendants of the fish spiracle, even though they occupy a similar anatomical neighborhood. It’s a case of evolution arriving at a similar solution through a different path.