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Insects and other land-based Arthropoda are stuck at the size they are due to how inefficient their respiratory system is. They were able to reach the size they did in the Carboniferous period because of the incredibly high oxygen content in the atmosphere back then.

Let’s say we were to, through genetic engineering, give an insect a more efficient respiratory system, like a pair of mammalian lungs or even avian lungs, then released it into the wild. Then, we left it on its own to evolve.

Now that it has room to grow, how big could it possibly get? Does the arthropod body plan (i.e. an exoskeleton) have certain advantages or disadvantages? Could they get as big as mammals, or would they be stuck at a certain size? What would they look like?

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    $\begingroup$ Inefficient is relative. Arthropoda are an extremely successful phylum, after all. Honestly, size isn't everything! $\endgroup$
    – Starfish Prime
    Commented Jun 15, 2019 at 18:27
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    $\begingroup$ I'm not going to VTC this yet, but if you search for "how big" on this site, you'll see that we've answered a lot of "how big can my creature get?" type questions. Given our discussion about what is a duplicate?, I personally think the specific creature type isn't enough to rationalize this as a non-duplicate, but I'm willing to let the discussion roll forward to see how it turns out. You might want to check out those questions, though. $\endgroup$
    – JBH
    Commented Jun 15, 2019 at 18:54
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    $\begingroup$ (1) Spiders have lungs, crustaceans have gills. Only insects have tracheae. (2) At the size of typical insects their tracheae are very efficient. (3) You may want to consider that insect blood does not carry oxygen; you need to engineer some kind of oxygen carrying blood too. (4) Insects do not have a closed-loop circulatory system -- their blood is not confined in vessels. (5) Mammalian and avian lungs require a dual-loop circulatory system -- you need to engineer the a dual-pump heart. (6) You are in for a major effort of re-engineering, and the end result will not be an insect. $\endgroup$
    – AlexP
    Commented Jun 15, 2019 at 19:14
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    $\begingroup$ "The coconut crab (Birgus latro) is a species of terrestrial hermit crab, also known as the robber crab or palm thief. It is the largest land-living arthropod in the world, and is probably at the upper size limit for terrestrial animals with exoskeletons in recent times, with a weight up to 4.1 kg (9.0 lb). It can grow to up to 1 m (3 ft 3 in) in length from leg to leg." (Wikipedia) Note that B. latro has a kind of lungs for respiration (and will drown if thrown in water). $\endgroup$
    – AlexP
    Commented Jun 15, 2019 at 19:25
  • $\begingroup$ The real reason for its large size is lack of competition. Giant insects were a thing in the oxygen-poor Permian and Triassic periods and didn't start shrinking to their modern sizes until the Cretaceous. $\endgroup$
    – JohnWDailey
    Commented Jun 16, 2019 at 3:26

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Oxygen consumption isn't the only problem.

Their biology is optimized for their current size. With a significant change in it, they should rework almost every major system.

Heat management

With increase in size, their insides (which generate heat) increase more than their surface area (which loses heat). It either requires a better cooling system or a slower activity on their part. Like elephants and rhinos. This (Kurzgesagt) video explains it in more detail, with nice illustrations: https://www.youtube.com/watch?v=MUWUHf-rzks

Weight management

Both inner body support and outside movements had to be reworked or enhanced with size increase. For example, in most arachnids case, their limbs are too far away from their center of gravity. This would significantly increase the required energy to move, as well as the stress on their exoskeleton structure.

Many more, I sadly know even less to discuss.

These are problems, which would have to be addressed. However nature allowed for elephants and for a time, mammoths. They show the possibility for affordable top size.

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  • $\begingroup$ Arthropods don't have bones... $\endgroup$
    – John Dvorak
    Commented Jun 15, 2019 at 19:56
  • $\begingroup$ @JohnDvorak True, thank you. Edited out to exoskeleton. $\endgroup$
    – Lupus
    Commented Jun 15, 2019 at 20:02
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    $\begingroup$ Running hot is a feature, not a bug (badum, tish). Just means you can be active earlier and later in the day, survive shorter seasons, live closer to the poles. $\endgroup$
    – Starfish Prime
    Commented Jun 15, 2019 at 20:21
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    $\begingroup$ @StarfishPrime You have to be able to control it, though. If you can't scale and adjust your heat reduction properly, you will be able to be active ONLY earlier or later in the day, depending on the climate. African and Asian elephants have different ear sizes because of the difference in climate. Bigger ear needed to lose heat in a hotter climate. They can't change their ear size = their cooling system. Only move it, to increase its efficiency somewhat. You won't find a polar bear in the dessert. $\endgroup$
    – Lupus
    Commented Jun 15, 2019 at 20:30
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    $\begingroup$ Then just skip the hot part of the day? That's not a terrible strategy, and certainly it is one used by warm blooded creatures in particularly hot climates. You won't find a polar bear in a desert, but you will find it surviving much closer to the poles, which was one of the other options in my comment, as you may have noticed. $\endgroup$
    – Starfish Prime
    Commented Jun 15, 2019 at 20:38
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Not much bigger than Arthropleura, which was large but also flat.

enter image description here

The other major problem arthropods have besides oxygen is molting, how exoskeletons have to grow. Sooner or later they have to molt to grow larger and when that happens they have no hard exoskeleton to support their body until the new one expands and hardens. Get too large and they will literally be suffocated/crushed by their own bulk while this is taking place. It also slows how fast they can grow since each molt can only be marginally larger than the previous one. Even Arthropleura is probably too large is there are vertebrates around, since they can grow much faster. Arthropleura go away with it because it was the only game in town.

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A few adaptations.

1: No heavy exoskeleton. The old square-cube law you know. Exoskeletons get heavy. These big ones have just enough to hold themselves in - a minimal flexible cuticle like that of a maggot.

  1. Apneustic respirations. Oxygen dissolves directly through the skin. Some maggots can do this, especially water dwelling ones. Amphibians and sea snakes can do this. The thin cuticle on your big bug facilitates this. It would be well suited for an area with minimal gaseous atmosphere because it does not move gas.

3: Huge surface to volume area. This facilitates direct diffusion of oxygen to needy tissues. The big bug is flat. It might be like a big flat pancake.

Now we have a flat, pancake-like maggot. It is too big for legs, so moves by undulation - terrestrial flatworms and nemerteans do that, so no great stretch.

A creature like this is converging on a slime mold except it cannot flow and so must crawl. It could be very, very big. Such a large creature would also be fairly defenseless and so would need to either be unpalatable or to reside in an environment without predators. Maybe the deep earth, where it would itself be the apex predator. I bet they are down there.

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  • $\begingroup$ The assumption is a lungs and closed circulatory system. $\endgroup$
    – John
    Commented Jun 16, 2019 at 1:17
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    $\begingroup$ @John: the assumption is "a more efficient respiratory system"; lungs were a suggestion by the OP. Not sure where you got the closed circulatory system. No arthropods have that. $\endgroup$
    – Willk
    Commented Jun 16, 2019 at 1:22
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    $\begingroup$ A closed circulatory system is necessary for lungs to operate. most of your points ignore the premise of the question. $\endgroup$
    – John
    Commented Jun 16, 2019 at 3:39
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Re: the question on how big an arthropod can get, you have a real example that reached an estimated $2.6\,m$ in the extinct Jaekelopterus. Some relevant data:

  • Not an insect, yet an arthropod as you asked.
  • Aquatic. This probably made growth easier. But I think with a bit of genetic engineering you can solve this, as per the other replies.
  • Apparently, oxygen was not the limiting factor: at least atmospheric oxygen was around 75% modern levels on average during the Devonian
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  • $\begingroup$ I’m aware that aquatic arthropods can get really big cuz they have gills and stuff. I’m asking specifically about land arthropods. $\endgroup$
    – Cobbington
    Commented Jun 16, 2019 at 3:05

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