Unexplainable - What happened to the giant bugs?

Episode Date: September 23, 2026

Once upon a time (around 300 million years ago), Earth was home to massive dragonfly-like insects and two meter long millipede-like creatures. Why did they die off? Guests: Ned Snelling, professor of experimental physiology at the University of Pretoria; Jon Harrison, professor of environmental physiology at Arizona State University For show transcripts, go to ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠vox.com/unxtranscripts⁠ ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠For more, go to ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠vox.com/unexplainable⁠ ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠And please email us! ⁠⁠⁠⁠[e-mail]⁠⁠⁠⁠ We read every email. Support Unexplainable (and get ad-free episodes) by becoming a Vox Member today: ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠vox.com/members⁠ Learn more about your ad choices. Visit podcastchoices.com/adchoices

Anyone who listens to Unexplainable knows that music is really important to me. So I want an amazing sound system. But I also want something that's easy. It just works. Sonos delivers on all fronts. The sound is incredible, and my Sonos system follows me wherever I go. I can mix and match speakers in any room in my apartment, louder in the living room, quieter in the kitchen. And the Sonos app makes it all super easy.

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I mean, I'm a dinosaur guy. That is fair. But I would like to make the case, if you'll allow it, for the Carboniferous instead. Okay. Can you remind me what the Carboniferous is and when it is? Sure. So the age of the dinosaurs is the Mesozoic era. So mezzo, meaning middle. Okay. And then the Carboniferous is in the Paleozoic, so the old era. Got it. Carboniferous ends around 300 million years ago, so 50 million years before the dinosaurs really kick off. And you can imagine it having these huge, swampy, forests. So flowers haven't evolved yet. There are no flowers, but there are tree-sized ferns.

If you've ever seen the plant called Horsetail, which is this sort of segmented, weird-looking plant, there are massive versions of those. There are club mosses, and things are, again, just very swampy. Very swampy, okay. But the thing that I think is really amazing about this pre-dinosaur time is that there were giant bugs. Okay. So we have fossils of huge dragonfly-like bugs. Wait, how huge are we talking here? So we're talking 70-centmeter wingspan.

Give me inches? The height of a medium suitcase. A dragonfly? Like a carry-on dragonfly? You actually can't carry on a medium suitcase. So its wingspan is bigger than a carry-on. And they're carnivorous, by the way. So these are flying, carnivorous, suitcase-sized dragonflies.

There are two meter long millipede-like bugs. So that's more than six foot, longer than I am tall millipede-type things. I'm just imagining, like, my wife yelling from the other room and being like, you've got to get this millipede, it's huge. And I'm like, how big? It's like, two meters. Yeah, it's very hard for me to imagine why this is not as exciting to people as dinosaurs. It seems scarier.

Like, they should do Jurassic Park with two meter log velopedes. Again, a lot of these big bug fossils come from the Carboniferous. That's sort of the heyday of these guys. And then there are some from the Perbian, but by the end of the Permian, that the really giant ones seem to be done so. And so, you know, this is unexplainable. And today on the show, I want to ask, what happened to them? What happened to the bugs?

The really big bugs. This is a big mystery. This is a bug mystery. So, I actually thought that I knew the answer to this question, because I had learned that it basically had to do with oxygen. Yeah. So people have kind of debated aspects of this a bit over the decades, but the thought is that in the carboniferous, in this world, full of plants, oxygen levels might have been higher than they are today. So we are at 21% oxygen. They think the carboniferous is more like 30 to 35% oxygen.

And that's the thing that allows for the big bugs, like more oxygen means bigger bugs? I do feel like I've heard this, like somewhere in the back of my mind. like oxygen equals bug size, but I have no idea why. So I was talking to a few researchers who've sort of looked into this, thought about this, including this guy named Ned Snelling at the University of Pretoria. So he's like an experimental physiologist. He studies how living things work. His main focus is energy.

And he was like, I study metabolism. And metabolism, it defines life. in a way by studying metabolism and the ability of animals to get oxygen and fuels to cells, I'm actually studying the meaning of life and what better pursuit is that. All right, Ned. So he was walking me through sort of some of the basics here and the sort of relationship between oxygen and bugs. So maybe we should start with how insects breathe.

Insects are quite different to humans. So you and I, pretty active breathers. Let's take a breath. Thank you. We're kind of like vacuuming air with oxygen into our lungs right now, right? Yeah. That oxygen sort of diffuses into our blood, but then our blood carries it all around the body, and then it diffuses into the cells of our tissues, like our muscles, our organs, etc.

Insects do things pretty differently. So if you look on the side of the body of an insect, if you look very closely, if you have very good eyesight, you might see that there are little ports on the sides of the insect. Little valves, these are called spiracles. They have many pairs of them distributed along the abdomen and thorax. And these, yeah, these opening ports then give way to. hollow tubes. So they're like breathing all over their body at the same time,

not like through their face like we are? Yes, exactly. Okay. And so that air, you know, it goes through those holes, into the big tubes. And then the tubes as they go deeper into the body, they split up into lots and lots of smaller tubes

until eventually you get these really small tubes in their tissues known as tracheoles. And they are responsible for transferring the oxygen from the air in the tracheal system to the cells of the body. And the kind of classic explanation here is that the process for insects is just less active than our process. So they can pump oxygen around their tissues. They have these air sac type things or almost bellows in parts of their system that can push oxygen to their really important muscles.

I see. But the understanding has been that they seem to have a lot more moments where, they're just letting oxygen seep into the tubes, hopefully it gets to the end, and their cells get oxygen. So I guess if there's, like, more oxygen in the atmosphere, it's, it's like they're, they're just in more oxygen and more oxygen will passively diffuse into their body and let them get bigger, right? The idea is the bigger you get, the sort of further oxygen would need to go. And so if you're too big, the idea is that maybe oxygen wouldn't get to your really, really, really. internal bits. But if there's a lot of oxygen, yeah, okay. Right. Maybe it's just, it's more likely some oxygen gets all the way down to the tube and so you can grow bigger. That makes sense.

Yeah. You're going to tell me that that's wrong. Well, so like this idea that tubes limit insect body size goes back a long ways, right, in 1995. It's a pretty important hypothesis that's published in the journal Nature around this. It points to some research suggesting there was lots of oxygen in the carboniferous, right, and talks about how that oxygen might have affected life forms at the time. And in the paper, they talk specifically about, like, the big dragonfly guys. And they're like, flight requires so much energy, so much oxygen in order for flight to be possible in big insects, they just need more oxygen in the air. Even if they have air pumps, whatever, in a system that relies on diffusion, just need more oxygen, less oxygen, no big bugs.

That's the explanation. I was taught a version of it. It sounds super plausible. But Ned just did a bunch of research and published a paper in nature arguing that it might not be right after all. Huh. And why? I'll tell you what we know. No, no, no, no.

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Sonos. Love the sound. Feel the system. Support for the show comes from Square, the business platform that aims to help sellers become neighborhood favorites. These days, buying stuff is pretty easy with tap to pay. Maybe it's a breakfast sandwich at the corner cafe, a lunch break haircut, or some wall art that you picked up spontaneously on your way home. Square says every day we interact with businesses that rely on Square to make

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slash unexplainable. That is square, S-Q-U-A-R-E dot com slash go-slash-unexplainable. Get started with Square and build a setup that works the way you do. New from Nespresso. Blend wellness into your coffee routine

with a coffee plus range infused with functional benefits. Choose the coffee you love with added B vitamins, like coffee plus B-12 to help support immune function. And coffee plus B-6 to keep your day moving. Or go with the flow and choose ginseng delight. Our new double espresso with ginseng extract. Whatever lies ahead, don't change your morning. Let your morning change you. Discover coffee plus on espresso.com. Ned says he actually started to question things years ago when he was

looking at the oxygen architecture of bugs, essentially. So these tubes and their evolution and and their design. One of the experiments I did was I flew locust in a mini flight chamber to measure their oxygen consumption rate. Hold on. Before you explain this experiment, what is a mini flight chamber? Yeah. Do you know what this looks like?

It's like a tiny wind tunnel. Okay. So I was picturing an F1 wind tunnel. Of course. And he basically said, no. I probably wouldn't describe my flight chamber as formula one. It was a bit more rudimentary, but thanks for the compliment. You told me to picture a little cylinder with air flowing through it and then a locust flying inside it.

I'm imagining like an air treadmill, like air blowing one way and the locust kind of flying against it. So it's just sort of flying in place while the air is. Yeah, that's what I'm picturing. That's just crazy. He was able to, like, measure the air on the back end also, or, like, properties of the air on the back end. And what surprised him in this research was that the locust did not seem winded by the – oh, no pun intended. But, like, the locust didn't seem winded by their exercise. So if humans went for a run, especially if that run was quite energy intensive, say we were sprinting for a period of time,

after we finished exercising, our oxygen consumption rate would remain elevated. We would pant. We stop running and we just go like, and by we, I mean me. Sam. Okay. No, bird when I run a mile, I don't pant at all. But so his locusts, they did not seem to do like the locust version of panting. They recovered from exercise immediately.

And so what that told me, or at least indicated to me, made me suspicious, was that, hey, this insect has no problem getting oxygen to itself. It's so good at getting oxygen to itself that even after flight, which is the most energy intense form of exercise, even after that, it recovers almost immediately. And so he was like, that's weird. The whole basis for sort of thinking that oxygen levels killed off massive dragonflies was insects supposedly are less efficient at getting oxygen to parts of their body than we are. They need more oxygen in the atmosphere in order to get big because they have this passive breathing or whatever. But if in this experiment the insect had no issues getting oxygen to its cells, right?

if it's not even panting after exercise. Right. It does raise the question of, are they actually inefficient at getting oxygen? Yeah. So to investigate this a little bit more, Ned wound up cutting up locust flight muscles to just look at them more closely.

Like in the wings? Yes. So the muscles that power the wings. Imagine a muscle like this is like a city, right? Every day that city sort of needs to be supplied with food, just like the muscle needs oxygen. So the city has trains coming in to deliver supplies. And if the trains are really fast and can just carry huge amounts of supplies, right? If they're true freight trains and this is a really efficient system, maybe you can have just one track come into the city.

And that'll supply the city with everything that it needs, right? High-speed rail. But if your trains are not great at carrying supplies, they're just kind of slow, low capacity. More of the choo-choo-choo variety. Yeah, they're choo-choo-train. Maybe you need a lot of train tracks coming in to make sure that you have enough supplies for the city, right? Maybe you need actually a pretty big percentage of your city just to be given over to trains and train tracks. Yeah.

So basically, looking at these. flight muscles, Ned wanted to see how much space in the muscle was given over to train tracks coming in, to sort of these oxygen tubes bringing in oxygen. And for comparison, our hearts, they have pretty active muscles because they're beating all the time. And there's some different numbers here, but Ned was telling me that something like 10% of the space in our heart muscles is just blood vessels, just trains. carrying in supplies. When he looked at the locust flight muscles,

he found that less than 1% of the flight muscles were given over to, in their case, oxygen supply tubes. It's a very small percentage of the flight muscle dedicated to these oxygen supply structures.

So yeah, you would think that locusts would require lots of train tracks to supply the city with oxygen, that they would not be so efficient. But that's not the case. Yeah. That being said, this is just one bug, right? Who knows if it's going to hold up for other bugs?

Ned knows. Because several years later, he was still intrigued by this idea. And he wanted to look at several insects, but specifically he wanted to look at the flight muscles of really small flying insects and the flight muscles of really big flying insects and everything in between to see as you go up in size,

does something change? Like, even if bugs are very efficient, it still could be, at a larger size, this still could be the reason why they're struggling. Exactly. Yeah. So he works at the University of Pretoria in South Africa.

there are lots of big bugs around apparently, like the Goliath beetle, which, as you can imagine from its name, is big. It's not quite the size of a baseball. And if I placed one on the palm of your hand, you wouldn't be able to see the palm of your hand. Oh, God, that's... So it's big. Really big. I just like don't... I don't love bugs.

Okay. Well, you've come to the wrong place. I mean, I do want to know. I want to know what happened to the bugs. So basically, he's running around his campus. He's trying to catch insects of different sizes. To say that caused some interest in colleagues and students would be an understatement. I had students directing me to different vegetation where I could find different types of insects,

particularly large species of insect. So that would big help. I just, just imagining this, this professor running around, just try to catch bugs. I love it. And he was, you know, he was just trying to get as much sort of diversity in sizes as possible. And so in the end, he got 44 kinds of insects, a whole range of sizes, dissected them to get their flight muscles, did a comparison and basically wanted to see, like, was there a relationship between how big the insect was and sort of how much. of its flight muscles were devoted to oxygen delivery. Were these cities overrun with train tracks as the insects went up in size,

or did the number of train tracks remain relatively consistent? Did it just stay as like one high-speed rail going into the city? And the answer was that, yes, bigger bugs did need a little more space for oxygen tubes. Okay. But not dramatically more space. a bug could actually be 10,000 times bigger than another bug and only need sort of a tiny bit more oxygen tube space in its flight muscles. So it really might not be the oxygen then.

Well, that's why Ned is sort of skeptical. Yeah. He thinks this study shows that we need to find a different explanation for what killed them off. I did talk to a researcher who suggested that it could have been an environmental issue. So as you're transitioning from the Carboniferous into the Permian, the world is just changing a lot. Things got hotter, drier on Earth. And with that, just comes size reduction in animals. We're seeing that with the shrinking birds and climate change.

Yeah. And then another guy said, you know, big animals in general, They're just fewer of them. They're rarer. And so it's easier to kill all of them off. It's just less likely that at least one of them survives. And then a study from 2012 looked at a later moment, and there was actually a lot of oxygen in the air,

but other things had evolved to fly around. So this was this sort of later in dinosaur times. And it showed that insects actually got small, smaller than, which suggests that maybe insects have a hard time being big when they're just sort of other things in the air to take them out. So that might be the thing that's more affecting their body size is if there are things that can hunt you that can also fly and you're bigger, you're just easier to hunt. Yeah, it feels easier to catch a suitcase-sized dragonfly than a tiny little one zipping

around. Sure. But all that being said, there is also still the possibility that we should not throw the oxygen idea out entirely. So as he was doing all this work, Ned ended up connecting with this environmental physiologist whose whole focus is insects. His name is John Harrison. And he has actually been skeptical of the two little oxygen killed off the big bugs hypothesis. at least the 90s. An OG oxygen skeptic.

I thought, well, that would probably just take a year or two to disprove. 30 years later, he is still not sure that they have all the evidence they need to declare it wrong necessarily. So he ended up working with Ned so much on all of this that he was listed as a co-author on this paper. Okay. But he's cautious still because he thinks that stuff like molting might be expected by oxygen. levels. And he also thinks that, like, in this paper, they've shown that flight muscles don't depend on this oxygen stuff so much, but other tissues that are further off in the body or less well supplied, he thinks those tissues might still have an issue. You know, the legs, the muscles

and the legs may be harder to ventilate than the flight muscles, which can be maybe really easily ventilated. So that's a possibility. He and I don't totally agree on this point. Ned feels like they've addressed this in the paper, but John does not think that they have a final answer yet. I do think we've made a lot of progress, but yeah, I don't think it's definitive yet in my own mind. The murderer is still at large. I will say, though, that one thing Ned and John agreed on is that it's just hard to understand bugs in general. John was telling me that there are still questions about their breathing, for example. And then Ned was talking about the questions we have about the insect die-offs we're currently seeing.

So we did an episode a while back where we talked about how we are losing a lot of insects, but people are struggling to work out how many insects we're losing, why that's happening. And this is currently, you know, not 300 million years ago, but, in the 21st century, and we still have a lot of questions around it. Perhaps if we are able to work out what's caused a decline in the size of insects prehistoricly, it also might help us to try and work out what is causing insects to decline currently, because at the moment, we really don't have a good idea of really what's causing that. You know, if we understand what happened prehistoricly, it might actually help us to try and tease apart and work out what's happening or what's not happening at the moment.

Oh, that's so interesting. Okay, bug time is over. Thank you. No more imagining handfuls of Beatles. This episode was produced by me, Bird Pinkerton. It was edited by Joanna Solitaroff and Meredith Haudnot. Meredith runs the show, and Joanna is the editorial director. Christian Ayala did the mixing and the sound design, Melissa Hirsch, check the facts.

Noam Hassefeld does our music. Valerie Schenkman, Jake Reynolds, and Kareem Karea are the fact that houseflies do something like tasting with their feet. And thank you always to Brian Resnick for co-creating the show with me and Noam. Huge thanks also to John and Ned for being so generous with their time and to Conrad Lavenderra for talking me through questions of insect gigantism. If you have thoughts about bugs or honestly thoughts about anything at all, please reach out at Unexplainedable at Vox.com. We love to hear from researchers about what they're working on, from people who have questions, they'd like us to unexplainable at Vox.com.

If you would like to support the show, I cannot tell you how much it would mean to us if you would become a member of Vox. It is very easy to do. Just go to Vox.com slash members. And you'll get access to all of Vox's journalism. but you'll also know that you are supporting all of Vox's journalism. And for those of you who have emailed us to let us know that you signed up because of Unexplainable, you're the best and you're right up there with the people who also have left us nice reviews

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