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In my world, there was a nuclear apocalypse, and the resulting radiation, along with biochemical pathogens, lead to a dramatic increase in mutation and evolution.

Now, originally in my world I wanted to have giant ants, giant scorpions, giant spiders, and giant praying mantises, but after hearing what you you guys had to say and then doing my own research, I realized…it wasn’t possible.

I still really want them though, and I wanted to ask: Is there anyway to maintain realism while having giant insects? If not, what alternatives could I use? As long as anything you suggest is at least plausible, I’ll be happy.

Insect size

  • The Giant Ants are supposed to be around 1 foot (0.3 meters) long.
  • The Giant Scorpions are supposed to be around 3 feet (1 meter) long.
  • The Giant Mantises are supposed to be around 2.5 feet (0.7 meters) long.
  • Giant Spiders are supposed to be 3 feet (1 meter) long.
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  • $\begingroup$ It all depends on how giant are those giant insects. There is nothing stopping insects from growing very much bigger than most of them are now. On the other hand, if you want them the size of horses, then no. So, how large are those giant insects supposed to be? $\endgroup$ – AlexP Oct 4 '20 at 13:06
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    $\begingroup$ Note that, strictly speaking, spiders are not insects. They have 8 legs, not 6. $\endgroup$ – L.Dutch - Reinstate Monica Oct 4 '20 at 13:17
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    $\begingroup$ @ L.Dutch - Reinstate Monica ♦: Yeah but nobody cares about the difference $\endgroup$ – DT Cooper Oct 4 '20 at 13:19
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    $\begingroup$ @L.Dutch - Reinstate Monica: Same with scorpions. They're 8-legged arthropods, but not insects. $\endgroup$ – jamesqf Oct 4 '20 at 16:40
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    $\begingroup$ Robots, built by underground factories. $\endgroup$ – NomadMaker Oct 5 '20 at 2:48
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Yes, but not from radiation.

The Earth has witnessed plenty of giant, terrifying insects over its 4.6 billion years. During the Caboniferous period, well before the dinosaurs, there was slightly more atmospheric oxygen, because oxygen production was outpacing the capacity for photosynthesis to remove it. As a result, dragonflies grew to be the size of seagulls, and giant millipedes as long as a man scurried through the undergrowth. Giant cockroaches were common, too. And then there were the sea scorpions...

Edit: Oxygen was actually 162% of the current level, at about 32.3% of the atmosphere. That’s crazy.

As others noted, Arthropods are inherently limited in size because of how they’re constructed. Their chitinous shells can only grow so large and so thick. At a certain point the armor either becomes too heavy to carry or too thick to support respiration, from what I understand. Your ballpark estimates of the sizes seem to be reasonable, though.

Still, adding more oxygen to the atmosphere generally makes things grow bigger because it fosters more demanding metabolisms, but it has its drawbacks: At a certain point the atmosphere becomes combustible, or at least more prone to wildfires.

P.S. if you like giant ants and spiders in your sci-fi you should read Children of Time by Adrian Tchaikovsky. His novel involves both a virus that was genetically engineered to accelerate evolution and an oxygen-rich alien atmosphere.

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  • $\begingroup$ Thank you, I will now be checking under my bed every night for giant scorpions. $\endgroup$ – Redwolf Programs Oct 5 '20 at 17:42
  • $\begingroup$ There weren't any giant cockroaches that I know of. $\endgroup$ – Praearcturus Oct 6 '20 at 4:10
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    $\begingroup$ Doesn't photosynthesis produce oxygen instead of removing it? $\endgroup$ – Carsten Oct 6 '20 at 6:41
  • $\begingroup$ Imagine a forrest fire in that environ $\endgroup$ – Pica Oct 6 '20 at 7:41
  • $\begingroup$ Evolution adapts to wildfire. In humid areas, everything (plants in particular) will contain so much water that it won't burn; in dry areas, plant will have protective bark, or have seeds that survive a wildfire (in real life, seeds from most wildfire-adapted plant species even require a wildfire to germinate). $\endgroup$ – toolforger Oct 6 '20 at 8:00
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Those Sizes Sound Manageable

The coconut crab can grow to about one metre.

enter image description here

Seems it has overcome the main limitations like heavy carapace and inefficient respiratory system. Whatever evolutionary tools the coconut crab has, your giant insects have evolved analogous tools.

Realism: Coconut crabs aren't fast. For realism your insects should be much slower than normal sized insects.

Also Realism: Radiation doesn't tend to produce useful mutations. Your insects should be sickly and ridden with malignant tumors.

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    $\begingroup$ That thing looks delicious. And what on earth is in that trash can such that it doesn't tip over from that giant crab hanging off it? A dead body? $\endgroup$ – DKNguyen Oct 4 '20 at 22:15
  • $\begingroup$ Maybe filled with dirt? $\endgroup$ – Tom Oct 5 '20 at 6:18
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    $\begingroup$ Maybe there's a second crab on the other side? $\endgroup$ – John Dvorak Oct 5 '20 at 6:59
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    $\begingroup$ @JohnDvorak Silly me. So obvious! $\endgroup$ – DKNguyen Oct 5 '20 at 14:06
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    $\begingroup$ @DKNguyen They really aren't as heavy as they look, 4.1 kg at the most. $\endgroup$ – Praearcturus Oct 5 '20 at 16:27
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For ants, specifically, plausibility in biology aside, where there's one ant there are thousands more. Plus ants work together. I'd take any of those other giant creepy crawlies over giant ants any day.

I don't think an ecosystem could support that many ants unless they do it the same way people do: take over the land, cultivate, and concentrate resources in a very noticeable manner. If they existed in even a fraction of their current numbers, they would dominate the landscape over everything else, including humans.

A 30cm ant is 200,000 times the mass of a regular ant which is worth several to a few dozen typical colonies of ants so their social structure would have to be quite different, if there is a social structure. Can you still call ants that don't live in colonies ants? Those would basically be giant wasps which are also worse than anything else you listed. Regular wasps are bad enough, but then there are the the parasiticizing egg-laying variety which would make sense for a world full of giant insects. If you though face huggers were bad...

EDIT: Recent genetic studies have apparently shown that ants are more closely related to bees. Bees are friendlier than wasps I guess, but they also come in large numbers like ants.

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    $\begingroup$ "Bees are friendlier than wasps I guess" As someone who has had to deal with infestations from both, I can attest to that. Both are the geese of the insect world, but compared to wasps, bees are totally chill. $\endgroup$ – The Square-Cube Law Oct 5 '20 at 13:53
  • $\begingroup$ This makes me think about massive ants... under ground. Could they build functional cave systems? bigger mandibles. Can bigger mandibles dig dirt effectively? Bigger means bigger holes... will those bigger holes be as stable as the smaller holes? Bigger farms... Ants gather materials to create underground farms (in cases). Bigger ant poop... without septic systems... $\endgroup$ – WernerCD Oct 5 '20 at 15:15
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    $\begingroup$ @WernerCD Naked mole rants seem to do okay, but they are not quite as large. The terrain they burrow in also seems more limited. $\endgroup$ – DKNguyen Oct 5 '20 at 15:15
  • $\begingroup$ @DKNguyen but they dig with... paws... mouths and teeth. Just seems like mandibles and insect legs would be different and less effective. I could be overthinking it but the question and the 1foot insects seem like a good thought game. $\endgroup$ – WernerCD Oct 5 '20 at 15:17
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We have common ancestors with arthropods. The reason why we grow large and they don't is because we evolved ways to scale better to large sizes. We don't molt and we have breathing organs with a much greater ratio of surface area to volume.

Some spiders and scorpions have book lungs, which are not as efficient as our lungs, but which outdo insect tracheas. That's why some south american and australian spiders can grow large enough to justify the human search for other inhabitable planets. Those nopes can grow to be a whole foot wide (~30 cm). That could be a good starting point.

Do pay attention to everyone saying ♡♡♡♡ about the square cube law. They have all the best intentions at heart. But you can justify giant insects if they are less carapacy and more leathery, and if they evolve book lungs and then regular lungs. This evolution in a very short time scale may not be completely realistic in itself, but once you're past that the giant bugs are totally realistic.


Editing to include this comment by Robyn:

A rigid carapace would help giant bugs move, since they don't have skeletons to anchor their muscles to. To optimise strength and weight, I suggest a carapace that is thin over many areas of the body, but with some thick, hard ridges that do the mechanical work of a skeleton.

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    $\begingroup$ A rigid carapace would help giant bugs move, since they don't have skeletons to anchor their muscles to. To optimise strength and weight, I suggest a carapace that is thin over many areas of the body, but with some thick, hard ridges that do the mechanical work of a skeleton. $\endgroup$ – Robyn Oct 5 '20 at 7:08
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    $\begingroup$ A functioning "regular lung" requires constant expenditure of energy to pump air through it. That would change the entire metabolic balance of a (presumably cold blooded) insect. Spiders can remain motionless literally for weeks, with virtually no energy expenditure and no food intake. $\endgroup$ – alephzero Oct 5 '20 at 19:21
  • $\begingroup$ @alephzero snakes and lizards are also cold-blooded, and they have lungs too. $\endgroup$ – The Square-Cube Law Oct 5 '20 at 19:47
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The maximum weight for a terrestrial arthropod is probably somewhere between 228 g and 4.1 kg (the coconut crab can weigh 4.1 kg but has a special respiratory system, the Actaeon beetle larva can weigh 228 g). This means that the maximum size for an ant is between 17 and 62 cm, 37-96 cm for a scorpion, and 26-69 cm for a mantis. For a spider, it depends on the species. A tarantula could only reach 13-34 cm long and 31-86 cm in legspan. A more slender species like a cellar spider could reach 18-48 cm long and 1.4-5.1 m in legspan.

The foot long ants are plausible, but all the others are pushing it.

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  • $\begingroup$ Tarantulas about that size already exist. Goliath Birdeater in South America, and the Huntsman a.k.a. the one animal which is more representative of Australia than Kangaroos and Koalas. $\endgroup$ – The Square-Cube Law Oct 5 '20 at 3:23
  • $\begingroup$ @Renan no spider with a body larger than 11.94 cm or a legspan greater than 28 cm has been found. $\endgroup$ – Praearcturus Oct 5 '20 at 13:33
  • $\begingroup$ Just now I noticed what you meant. I meant to say that spiders slightly below the lower end of that size range exist nowadays. $\endgroup$ – The Square-Cube Law Oct 5 '20 at 13:57
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Insects of such sizes did exist in the distant past, but back then oxygen concentration was larger. The main cause current insects have a size limit that small, is that their respiratory systems are inefficient compared to ours. So don't have radiation cause that growth, or not radiation alone. Maybe there was an attempt to reverse global warming (by seeding the oceans with photosynthetic algae or something like that), but it went too well, and they grew out of control. The result, among others: more oxygen in the atmosphere. This causes the following:

  • insects can grow to larger sizes
  • humans (and most mammals, I would guess) are in a constant state of tipsiness, which can result in interesting social dynamics. Larger than average (but not large enough to be deadly) oxygen concentration has effects very similar to being drunk.
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Yes this is plausible!

There's two main constraints that limit the size of terrestrial animals: Their ability to breath and their ability to walk around. Neither of these prevent arthropods from getting as big as what you describe, which we know because of examples of giant arthropods alive today and in the fossil record.

Insects and arachnids lungs aren't really built to scale up to to large sizes, but some arthropods can. As Daron has already pointed out in his answer, coconut crabs are terrestrial arthropods that can be over a meter long. They have a very unique type of lung - but the point is that it is plausible to have giant arthropods with functional lungs.

The biomechanics might seem problematic, but it actually isn't. Your giant bugs would not move as rapidly as their smaller counterparts, just like you'd expect any giant animal. But arthropods that large and larger are more than capable of moving around on land. Coconut crabs aren't very fast but they are quite mobile. They can climb trees and are known to occasionally kill birds and small mammals.

Perhaps the main reason that arthropods don't get extremely large nowadays is ecological, not anatomical or physiological. It's not that arthropods can't get that large, but it's that vertebrates are better at it, so in general small vertebrates can outcompete giant arthropods for those ecological niches. Looking at the fossil record, before vertebrates got good at moving around, arthropods got truly enormous. For example, the largest known terrestrial arthropod ever is the 2.5-meter-long millipede, Arthropleura. During the same era (the Carboniferous period) there were famously also giant dragonflies over 2 feet long. At that time, there was more oxygen in the atmosphere, but only about 60% more than at present according to wikipedia. That could account partially for the giant insects - but 60% more oxygen by itself wouldn't allow them to be several times larger than they are today. To get arthropods that large in the modern atmosphere, you'd just need a way to get them 60% more air than their Carboniferous counterparts. Since you're not looking at flying insects, their energy needs are not as great as a dragonfly, you can probably get away with less. If you're OK with them not being extremely fast, I would say it is totally plausible for giant bugs to be as large as you describe.

While we're on the subject of large fossil arthropods, aquatic eurypterids are very similar anatomically to scorpions (they are probably closely related to arachnids). They got to lengths upwards of two meters, and fossil trackways show that some species (including the big ones) walked around on land occasionally. The limiting factor on their terrestrial motion was their inability to breath air, not the difficulty of walking itself.

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Realism and giant insects....doesn't go well hand in hand.

Remember the classic: Enlarging something by a scale of x makes its volume grow by x^3 and its cross section grow by x^2. Weight is proportional to volume, strength of legs is proportionate to the cross section of the legs. Thus, larger creatures need thicker legs. Notice how humans have thinner legs, in proportion to the body, than elephants? Notice how ants have thinner legs than humans?

Thus, if you make giant insects, you have to give them legs that are as thick as other living things of the same size.

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