How to breathe without lungs

Look down and quickly take a deep breath in.

It’s kind of funny how you get physically bigger. It’s also pretty weird how noisy the constantly necessary act of living can be, and this definitely isn't a great thing if you’re hiding from predators who have better hearing than you. But the weirdest thing is suddenly remembering that most of your chest is taken up by two repeatedly inflating bags each the size of a melon.

We mostly remember why we need these noisy inflatable toys: Every one of your trillions of cells is like a little squidgy petrol engine, burning things with oxygen so it can get energy to do stuff.

Because every cell needs oxygen, which is in the air, not you, you need a way of getting it to every corner of your body. Your lungs are the first bit in a two-part oxygen delivery network. You suck air into them, they’re full of little pipes, and where the pipes come alongside your blood vessels, the oxygen leaches into your bloodstream where it gets pulsed around your body ready to power the cells in your muscles and organs and everything. Like this:

But this two-part system, made out of pumps and closed loops of circulating fluids, and centred around two paper-thin, poppable, sacs of air, is crazy complicated, and can go wrong in so many ways. Choking, cholesterol buildups and strokes are just no fun, right? Wouldn’t it be great if there were a better way to get oxygen to all those inconvenient nooks and crannies inside your body?

Well insects have just the thing. Their oxygen delivery system is so simple and efficient it should make us blush thinking about the contraptions keeping us going. It makes me think of the old (untrue) story about the Russians and Americans in space - the Yanks supposedly spent millions designing a zero-gravity pen while the Russians were happy to just write in pencil.

And here’s the pencil: This insect option is almost the exact opposite of our own. For a start, they don’t just have one entrance for the air tubes, they have a whole bunch. And by not placing them awkwardly next to their food hole like evolution vindictively decided would be a good idea for us, they save themselves at least from the risk of choking.

Where are these throats if not in the mouth, I hear you ask? Like portholes in a ship, the air-holes are all the way down the length of their body. You can sometimes see their vents on a caterpillar with just the naked eye:

Like the airways in our lungs, the pipes these vents start do branch out into smaller pipes, but not for the same reason. As they’re inside the body it’s hard to see them properly - but it turns out that for some larvae with translucent skin, a bright camera flash will reveal the air-pipes lurking just below the skin.

Gross but really interesting picture coming up. I warn anyone grossed out by maggots (which I have to add are just fly larvae after all) that they're about to be grossed out. Also seriously rethink your values. Rethink them seriously. Maggots are just caterpillars for flies. And this one's actually a baby beetle, not a baby fly, anyway. Chill out. Either way. Basically if you want to see some cool branching tubes, scroll on.

Here you can see the vents in the form of orange circles, leading to their networks of air tubes. eventually branching so finely that the camera can no longer pick them up. Picture by Siah St. Clair, who used to run the Springbrook Nature Centre in Minneapolis and now keeps a blog here

They look like aliens. In a way they’re built so differently to us that they might as well be aliens. They don’t have blood vessels. They don’t have bones. They shed their entire skins several times in their life. And instead of a big brain at the front they have dozens of mini brains spaced out around their insides. Totally weird.

But back to the tubes - these alien things aren’t like our own air tubes either. They’re not there to get air into the bloodstream via the lungs, because insects don’t have a bloodstream or lungs! So instead of branching within a small compartment connected to blood which needs oxygenating, the tubes just branch. And branch and branch, until they’ve filled the entire body. Every single cell is within breathing distance of an air pipe which leads all the way to the outside world, even in the very centre of the insect.

Here, the pipes get smaller and smaller, but even the smallest blue lines represent tiny hollow tubes full of air. It’s like every cell is essentially outside, drawing oxygen directly from the atmosphere. In humans, the only part of the body which draws oxygen from the air directly, not from the bloodstream, is the cornea, the transparent outer window of our eye: if it had blood vessels working their way through it, our view of the world wouldn’t be quite so clear.

Turning your entire body into a lung like this opens the doors to plenty of fun party tricks. If it’s dry outside you can save yourself some moisture by closing up the vents. You’re able to sit perfectly silent and motionless if you want to, because there’s no breathing in and out which needs doing. That's pretty useful if you don't want to be spotted by a predator. What’s more, you don’t have the complications of a throat or blood circulation system to deal with, and if you feel like diving you can trap bubbles of air around your air pipe vents as a sort of squidgy scuba tank.

The vents themselves do so many things that their overall design is quite complicated. They’re certainly very intricate to look at:

Electron microscope image of one of the vents, with thanks to Brigette Zacharczenko, who runs the excellent Caterpillar Blog here

Turning your entire body into a lung isn’t all sunshine and rainbows. One minor complication comes about because of the whole skin-shedding thing. The lining of the air tube network is contiguous with the insect’s skin, just like the skin inside our ear canals is contiguous with the rest of our skin.

So when they shrug off their skins, the tube lining goes with it, leaving the raw insides of the tubes exposed to the air. It’s not so good at absorbing oxygen while the lining regrows, and if you measure the amount of oxygen moulting insects use it’s clear they aren’t able to breathe properly for about an hour as they heal up and start hardening their new skin. This snag doesn’t seem so fun.

The main snag though is that the party has to end if you get too big. Air-tubes running into the core of a creature get longer the bigger the creature, and as every row of cells the tube passes uses up a puff of oxygen, at some point there won’t be any left for the cells in the very middle. And no oxygen means no living.

So if you want to be big using this system you either need to actively squeeze air fast through the pipes so the centre cells get some oxygen, or somehow get hold of air with really high oxygen levels. The biggest insects alive today, like the absurdly large weta bug who lives in New Zealand, do the first one:

Mammals never got a proper foothold in New Zealand, leaving the seeds and habitats normally monopolised by mice to insects like the weta bug. You can see the air vents down his side. Picture by Mark Moffett.

But about 300 million years ago, the air really did have more oxygen in it. So the insects who were alive at this time could grow obscenely large without their cores being starved of oxygen. And how they did. We have fossils of a humongous dragonfly which can have a wingspan of almost 70cm. We called it Meganeura.

Because of the centres being so far from the outside, these extinct dragonflies are about the biggest design using air tubes which will actually work without some serious pumping. But the tour of alternative lung designs doesn't just have one membranous stop, and the other odd example we'll look at today is found in spiders. They're not insects (they have eight legs, not six, so are clearly very different), and so are put together very differently. Their lungs are no exception.

Inside you'll find a smart sort-of-lung called a ‘book lung’ which is made up of liquid-filled sheets of tissue interlocked with sheets of air connected straight to the atmosphere by an air hole which punctures their underside. Oxygen diffuses into the liquid-filled sheets, and as the liquid inside the spider's body is constantly being mixed around, the oxygenated liquid gets everywhere which needs an oxygen supply.

There we have it - our familiar lungs aren't only way of getting oxygen out of the air and into your cells. So next time you see an insect sitting perfectly still, remember why nothing seems to be rhythmically rising or falling. And if he’s just landed after a death-defying acrobatic display, have a moment of wonder about how he isn’t even out of breath.