Insects do not have lungs. Instead, they breathe through a network of tiny tubes called tracheae that deliver oxygen directly to their tissues, bypassing the need for blood to carry it. This system is fundamentally different from how humans and other vertebrates breathe, and it works so efficiently that it powers some of the most metabolically demanding activities in the animal kingdom, including insect flight.
How Insects Actually Breathe
Rather than pulling air into a pair of centralized organs, insects draw air through small openings on the sides of their bodies called spiracles. Most insects have between 14 and 20 of these openings, arranged in pairs along the thorax and abdomen. From each spiracle, air flows into a branching network of tubes (tracheae) that subdivide into increasingly smaller channels called tracheoles. These tracheoles extend throughout the body cavity and reach individual cells directly, where oxygen passes through the thin tube walls into the surrounding tissue.
Carbon dioxide travels the opposite direction, diffusing from the tissues back through the tracheoles and out the spiracles. The entire process can happen passively through simple diffusion, with no need for a respiratory organ or oxygen-carrying blood proteins. This is why insect blood, called hemolymph, is typically not involved in gas exchange at all. The tracheal system is so effective that oxygen-transport proteins in the blood have long been considered unnecessary.
Spiracles Control More Than Airflow
Spiracles aren’t just open holes. They function as valves with muscular controls that open and close in a regulated cycle. Many insects use a three-phase breathing pattern. First, the spiracles close completely. During this closed phase, the insect’s tissues consume the oxygen already inside the tracheal system, and carbon dioxide gets buffered in body fluids rather than building up as a gas. As oxygen levels drop, the spiracles begin to flutter, rapidly opening and closing to let small amounts of fresh air in. Finally, when carbon dioxide reaches a critical threshold, the spiracles open fully, releasing a burst of carbon dioxide and drawing in oxygen.
This cycling pattern appears to serve a critical purpose beyond gas exchange: preventing water loss. Every time the spiracles open, moisture escapes. By keeping them sealed for as long as possible, insects conserve water, which is especially important for small-bodied animals living in dry environments.
Larger Insects Actively Pump Air
For small insects, passive diffusion is enough to move oxygen through the tracheal network. But larger species can’t rely on diffusion alone. Locusts, beetles, and other big-bodied insects actively ventilate their tracheal systems using rhythmic abdominal pumping movements, compressing and expanding sections of their tracheal tubes to force air through. Locusts do this even at rest.
Research using synchrotron X-ray imaging has revealed something even more surprising. Beetles, crickets, and ants undergo rapid cycles of tracheal compression and expansion in the head and thorax that can’t be explained by body movements or hemolymph circulation alone. This pumping action is, in a sense, analogous to the inflation and deflation of vertebrate lungs. So while insects don’t have lungs, some of them have evolved mechanical breathing motions that serve a similar function.
Flight Pushes the System to Its Limits
At rest, insects are remarkably tolerant of low oxygen. A resting locust can maintain normal metabolism even when oxygen levels drop far below what we breathe. But flight changes everything. The oxygen demands of flight are so extreme that locusts need near-normal atmospheric oxygen levels to sustain it. The excess oxygen delivery capacity that insects enjoy at rest essentially disappears during flight, with the tracheal system operating close to its maximum.
This is part of why the tracheal system places a ceiling on insect body size. Oxygen has to travel from the spiracles to the deepest tissues by diffusion, and the longer that pathway gets, the harder it is to supply enough oxygen. Larger bodies mean longer diffusion distances, which is one reason insects today are relatively small compared to vertebrates.
Why Giant Insects Once Existed
During the Carboniferous and Permian periods, roughly 300 million years ago, atmospheric oxygen levels reached 27 to 35 percent, well above today’s 21 percent. That oxygen-rich air allowed diffusion to work over longer distances, and insects grew to extraordinary sizes. Dragonfly relatives had wingspans exceeding two feet. When oxygen levels eventually dropped, those giant species disappeared. The relationship between oxygen availability and insect size remains one of the strongest pieces of evidence that the tracheal system, for all its efficiency, imposes real physical constraints.
How Aquatic Insects Solve the Problem
The tracheal system evolved for breathing air, which creates a challenge for insects that live underwater. Different species have found different solutions. Many aquatic larvae have thin-walled gills that allow dissolved oxygen to pass from the water into their tracheal network. Others, like diving beetles, carry a bubble of air beneath the water’s surface and breathe from it, periodically returning to the surface to refresh the supply.
One of the most elegant solutions belongs to the river bug Aphelocheirus aestivalis. This small aquatic insect carries what’s called a plastron: a permanent, incompressible layer of air held against its body by millions of microscopic hairs. The plastron holds just 0.14 cubic millimeters of air, connected to the tracheal system through the spiracles, and it works as a physical gill. Oxygen from the surrounding water continuously diffuses into the thin air layer, meaning the insect never has to surface to breathe.
A handful of aquatic insects have gone a step further and retained oxygen-carrying proteins in their blood. Larvae of chironomid midges, for example, contain hemoglobin that helps them survive in low-oxygen water. Researchers have also identified a functional hemocyanin (a copper-based oxygen-transport protein common in crustaceans) in the blood of the stonefly Perla marginata. These cases are rare exceptions that prove the rule: for the vast majority of insects, blood-based oxygen transport simply isn’t needed.
Why This Gets Confused With Book Lungs
Part of the confusion around insects and lungs comes from their close relatives. Spiders and scorpions breathe using structures called book lungs, which are layered, page-like folds of tissue where air passes over blood vessels, much closer in concept to how vertebrate lungs work. Crustaceans use gills. Insects took an entirely different evolutionary path, building a system where air goes directly to the cells rather than relying on blood as an intermediary. It’s a distinction that separates insects from nearly every other large group of air-breathing animals.