How Do Insects Breathe? Through Air Tubes, Not Lungs

Where does the air go in?

Spiracles are valve-like openings on the sides of the thorax and abdomen. From them, a network of cuticle-lined tubes, the tracheae, branches through the body, as Lehnert and Myers describe it in a 2026 study of antlion larvae. A 2025 study of seal lice by Preuss and colleagues adds that spiracles can be opened and closed, and that spiral thickenings called taenidia keep the tubes from collapsing.

A spiracle is more than a hole. Dunn and colleagues imaged the abdominal spiracles of a last-instar larva of the firefly Photinus pyralis in 2022. Each has a two-chambered slit fringed with branched bars, a branched filter behind it, a bulb-shaped chamber and a closing muscle at its base. In their figure of one of them, the opening is about 48 µm long [10−5 m], measured for this page against the 50 µm scale bar. That is about half the roughly 0.1 mm the naked eye can make out.

This is where insects part ways with lungs. In a lung, oxygen crosses from inhaled air into the blood of the capillaries, Maina writes in a 2012 comparison of lungs and tracheal systems. In insects the circulatory and tracheal systems are "totally disengaged", he writes, and oxygen diffuses from the air in the tubes to the tissues and cells.

How fine do the tubes get?

Behind the spiracle the tubes branch and narrow. In x-ray video of living insects they stand out as air-filled tubes. Socha and colleagues give a trachea 100 µm across as their example and describe earlier work that followed tubes down to 10 µm in diameter (2007). The rungs from 1 to 10 µm, between those tubes and the finest ones, are where the spider silk page places its threads.

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The tubes end in tracheoles. Their tips are filled with liquid, and they deliver oxygen directly to the tissues, Lehnert and Myers write. In the gut wall of the katydid Gampsocleis gratiosa, Li and colleagues found tracheoles less than 1 µm in diameter reaching into the gut cells themselves (2018).

In their electron micrograph, two labelled cross-sections measure about 0.45–0.5 µm on the short axis and 0.6–0.7 µm on the long one. Both were measured for this page against the 1 µm scale bars. That is about two to three and a half times the roughly 0.2 µm a light microscope can resolve, the second dashed line on the ladder.

Some tracheoles go inside cells. Each fruit-fly flight muscle is about 1 mm long and 100 µm wide, by the account of Sauerwald and colleagues (2019). They describe tracheal branches that wrap its surface and then grow into it, shortening the path oxygen must diffuse to the mitochondria. The indirect flight muscles of flying insects have "the highest known metabolic rates in the animal kingdom", they write, citing work from 1964; the bumblebee flight page looks at what such muscles drive.

In many tissues, Maina adds, the tracheoles may contact practically every cell, and the distance still shows. In the brains of fruit-fly larvae, Baccino-Calace and colleagues found that a cell's oxygen supply, read through a genetically built sensor, fell off exponentially with its distance from the nearest tracheole (2020). As of that study, they knew of no experiment that had measured oxygen diffusing from inside a tracheole into the cells around it.

How far can oxygen diffuse?

Diffusion is why the air goes so far in. Oxygen is nearly 10,000 times more diffusive in air than in water. Somerville and Proctor give its diffusion coefficient as 2.1 × 10⁻⁵ cm² per second in water at 25 °C and about 0.2 cm² per second in air, citing a standard handbook (2013). A 2019 lung model by Park and colleagues uses the same air value.

The time diffusion needs grows with the square of the distance, t ≈ x²/2D, which follows from how a diffusing cloud spreads, as set out by Vestergaard and colleagues in 2012. Computed for this page with those coefficients, oxygen crosses 1 cm of air in about 2.5 s but 1 cm of water in about 6.6 hours. Across 10 µm of water it needs about 24 milliseconds.

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So the long haul happens in air, and only the last stretch is liquid. As Wikipedia's article on the insect respiratory system puts it (read 2026-10-24), oxygen in the tube first dissolves in the liquid of the tracheole. Then it diffuses across the membrane of the neighbouring cell. The physics is the same for an insect and for a person; what changes is how far oxygen must travel through liquid.

Single cells face the same arithmetic. A cultured Escherichia coli cell, like those on the human body's bacteria page, is 1.0–1.5 µm wide, by Volkmer and Heinemann's measurements (2011). Oxygen crosses that width of water at 25 °C in about 0.2–0.5 ms (computed). The same square law runs through How Big Is a Cell. A bead 1 µm across in water at 20 °C, by contrast, wanders 1 µm in about 1.2 s (computed), the jiggle described in Brownian motion.

How do insects breathe underwater?

Water makes the problem harder. It holds less oxygen than air, and oxygen diffuses through it more slowly, as Verberk and Bilton note for insects with aquatic larvae (2011). By the figures Preuss and colleagues cite, only about 75,000 of 1 million named insect species can survive in water.

Those that do have several workarounds, as the same study summarizes. Some keep a closed tracheal system and take oxygen through the skin, as larvae of the midge Chironomus do, or through tracheal gills, as caddisfly larvae and damselfly nymphs do. Some fly larvae breathe through a siphon, a tube to the surface, and some shore flies and crane flies pierce plants to reach the air spaces inside them.

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Corixid bugs carry a bubble that works as a gill for a while. As the insect uses its oxygen, more diffuses in from the water and nitrogen diffuses out, so the bubble shrinks and the insect must surface to renew it.

A plastron is a thin air film held by water-repellent hairs that can number more than 6 million per square millimetre, under about 0.4 µm apart on a square grid (computed). The film keeps its volume, so it can draw oxygen from the water for long periods. The water strider page shows hair-held air at work on top of the surface.

The seal louse Echinophthirius horridus rides on seals through dives of 20–35 minutes to depths of up to 600 m. Preuss and colleagues found no plastron on it; instead, a closing apparatus at the spiracles keeps oxygen stored in its tracheal system (2025).

Do bigger insects need more tubes?

Diffusion alone has a reach, and many insects add pumping. Beetles, crickets and ants squeeze and re-expand tracheae in the head and thorax in rapid cycles, Westneat and colleagues reported in 2003 (abstract). They likened it to the inflation and deflation of vertebrate lungs. In x-ray video of a live ground beetle, Platynus decentis, Socha and colleagues timed one collapse and reinflation at 1.0 s (2007).

Size changes which mode matters. In the grasshopper Schistocerca americana, the youngest stages lack air sacs, rely more on diffusion and are the most sensitive to low oxygen. Larger individuals tend to move gas by bulk flow, Verberk and colleagues write in a 2021 review.

Bigger bodies also carry more tubing. Across four darkling beetle species spanning about three orders of magnitude in mass, Kaiser and colleagues found that tracheal volume scaled with mass to the power 1.29 (2007, abstract). Over a thousandfold range of mass, that makes the share of the body given to tubes about 7.4 times larger (computed for this page).

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In the legs, tracheal cross-section in the leg opening scaled with mass to the 1.02 power, the opening's own to the 0.77 power. Their share of the opening therefore grows about 5.6 times over the same range (computed), and the authors suggest that leg space may limit how big living beetles can get.

Larger species seem to pay in tubing rather than in oxygen shortage, Verberk and colleagues suggest. Across a range of insect species, the critical oxygen level for metabolism does not depend on adult size, though limits to tracheal expansion may still set an upper size. In a growing tobacco hornworm caterpillar, Manduca sexta, the tracheae can only be enlarged at a molt, so oxygen demand rises between molts while supply cannot.

How big did insects get?

The largest fossil insects lived around 300 million years ago. Wingspans reached as much as 70 cm in the Carboniferous and Permian, Clapham and Karr write in a study of more than 10,500 fossil wing lengths (2012, abstract). That is seven times the 10 cm hand at the top of the Zoom ladder.

The best known is Meganeura monyi. Wikipedia's article places it in the Late Carboniferous coal measures of Commentry, France (read 2026-10-24). The article gives a wing up to 32 cm long and a wingspan of about 65–75 cm, and describes griffenflies, the extinct order Meganisoptera, as resembling dragonflies. The GBIF Backbone lists the species under its original genus, as Dictyoneura monyi.

Did extra oxygen make giant insects?

The giants lived when there was probably more oxygen in the air. Three models broadly agree that oxygen peaked at around 25–30% of the atmosphere about 300 million years ago and stayed above about 18% afterwards, Krause and colleagues write (2018). Dry air at sea level holds 20.946% today, by Somerville and Proctor's figure, so the peak was about 1.2–1.4 times today's level (computed).

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Illustration

Dudley argued in 1998 that the oxygen-rich Carboniferous air helped arthropods grow giant and that a late Permian fall in oxygen ended it (abstract). Harrison and colleagues' 2010 review adds support from experiments: most insects develop smaller in low oxygen, and some develop or evolve larger in high oxygen. Larger insects also invest more of the body in tracheae (abstract).

The record does not line up perfectly. Maximum insect size followed oxygen only for the first 150 million years of insect history, Clapham and Karr found. In the Early Cretaceous it fell even as oxygen rose, after early birds evolved and spread (abstract). Large griffenflies also lived on into the late Permian, when oxygen was putatively lower, Verberk and Bilton note (2011).

Verberk and Bilton look at the larvae instead. Many Paleozoic giants apparently had aquatic larvae, though larval habits are hard to read from fossils.

In their test, active larvae of the stonefly Dinocras cephalotes were warmed in water from 10 °C by 0.25 °C a minute until they stopped moving. They stopped 2.9 °C lower in oxygen-poor water (14 kPa) and 1.5 °C higher in oxygen-rich water (36 kPa), and recovered in cool water. In water, the authors suggest, oxygen supply set the tighter limit on larval size, and too much oxygen may even have favoured larger larvae, less at risk of oxygen poisoning.

Why is the question still open?

The size side of the debate rests on fossil wings. Charles Brongniart described the type specimen of Meganeura monyi in 1884, Wikipedia notes.

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The explanations in play, side by side:

The full table, with qualifiers and reading levels, is in how-insects-breathe-giant-insect-hypotheses.csv (sources, version and license, JSON) Data: Dudley R 1998, Journal of Experimental Biology 201(8):1043-1050, https://doi.org/10.1242/jeb.201.8.1043 (abstract), retrieved 2026-10-24; Harrison JF, Kaiser A, VandenBrooks JM 2010, Proceedings of the Royal Society B 277(1690):1937-1946, https://doi.org/10.1098/rspb.2010.0001 (abstract), retrieved 2026-10-24; Kaiser A et al. 2007, PNAS 104(32):13198-13203, https://doi.org/10.1073/pnas.0611544104 (abstract), retrieved 2026-10-24; Clapham ME, Karr JA 2012, PNAS 109(27):10927-10930, https://doi.org/10.1073/pnas.1204026109 (abstract), retrieved 2026-10-24; Verberk WCEP, Bilton DT 2011, PLoS ONE 6(7):e22610, CC BY 4.0, https://doi.org/10.1371/journal.pone.0022610, retrieved 2026-10-24; Verberk WCEP et al. 2021, Biological Reviews 96(1):247-268, CC BY 4.0, https://doi.org/10.1111/brv.12653, retrieved 2026-10-24.

With the fossil record as it is, Harrison and colleagues call it impossible to definitively accept or reject the oxygen-size link (abstract). One side of the comparison is measured: more than 10,500 fossil wings. The other comes from models that agree on the peak near 300 million years ago but not on the early Paleozoic, by Krause and colleagues' account. And the tube measurements that would join the two, like the 1.29 exponent, come from four living beetle species, not from a griffenfly.

Sources

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Ants, Cells & Atoms. “How Do Insects Breathe? Through Air Tubes, Not Lungs.” Published 2026-10-25. https://antscellsandatoms.com/insects/how-insects-breathe/. Accessed (add the date you read it).

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