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One of the most important aspects getting giant arthropod creatures in earthlike conditions is the requirement of a more efficient respiratory system; the current system is not able to support massive animals because it is an open respiratory system. Oxygen is diffused directly to the tissues and absorbs it from the atmosphere passively, in the case of insects, and some actively via book lungs in the case of arachnids and crustaceans.

The biggest arthropods ever had their boom in a period in which atmospheric oxygen was approximately 10% more than today. And the nearest group to arthropods which evolved a closed respiratory system were cephalopods, indicating that the common ancestor of these groups had an open respiratory system. It is still hard to determine and think how this would be possible.

So, from the current structures that different arthropods have, how could they evolve to meet the requirements of a larger creature (take air from the atmosphere to bring oxygen to the internal tissues)?

Basically this question is searching for the series of mechanisms and process that must evolve and appear and how they would work to make it possible.

I also think that this question can be divided into another, which would be about the cardiovascular system.

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    $\begingroup$ Possibly related (but not a duplicate): worldbuilding.stackexchange.com/q/186750/21222 $\endgroup$ Aug 7 at 10:54
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    $\begingroup$ Also, how big would your giant arthropods be? For me, for example, a 30cm spider is already justification for weapons of mass destruction and those do exist. $\endgroup$ Aug 7 at 10:55
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    $\begingroup$ May it be the case that you are confusing arthropods (the phylum) with insects (one the classes in that phylum)? Most arthropods have gills or lungs which exchange gases in the usual way and oxygenate the blood; only insects have a respiratory system which oxygenates the tissues directly, with their hemolymph playing no part in oxygen transport. (And I have no idea what a "closed respiratory system" would be. A respiratory system by definition must exchange gases with the environment. That is what it does. Are you thinking of a closed circulatory system?) $\endgroup$
    – AlexP
    Aug 7 at 11:27
  • $\begingroup$ Arthropods can get quite large; the Tasmanian giant crab can grow to 17 kg (47 lb); Japanese spider crabs can grow to 19 kg (42 lb); old lobsters can grow to 20 kg (44 lb). The major limtation to arthropod growth is their exoskeleton, which needs to be shed and rebuilt from scratch when the animal grows, consuming energy and nutrients, and making the animal vulnerable. $\endgroup$
    – AlexP
    Aug 7 at 11:33
  • $\begingroup$ @AlexP . Maybe you have reason, I should have separed the questions, marking more the difference between insects and the other arthropods, and maybe yeah, I confused closed cardiovascular system with closed respiratory system, for be strecthly related. $\endgroup$
    – Drakio-X
    Aug 8 at 2:49
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Most insects have spiracles but dragonfly nymphs have the closest thing to an internal lung.

The Odonata, which includes dragonflies and damselflies are insects that go through a complete metamorphosis. Dragonfly nymphs respire through an internal gills in their rectum called the anal pyramid (this orifice also allows them to propel themselves underwater). These nymphs like many other species are known to breathe air during their final instar, when they leave the water to metamorphose into dragonfly adults. The late-final instar nymphs developed functional mesothoracic spiracles, allowing them to breathe air by positioning their head and thorax above the water’s surface. While breathing air in this position, the nymphs could also ventilate their submerged rectal gill.

This is the most likely candidate to develop into an "insect lung", and would mean giant arthropods that breathe through their butts, while the spiracles become what pores are to us.

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  • $\begingroup$ I never knew there were creatures that breathed through their a**. I love this SE site! Things you learn!!!!, $\endgroup$
    – Stilez
    Aug 13 at 7:15
  • $\begingroup$ As it turns out sea turtles also breathe through their bums. The tissue lining their bug has a lot veins that allows oxygen to dilute through. Thanks to this some turtles can stay submerged for days. $\endgroup$ Aug 13 at 7:23
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Gills.

The closest thing to arthropods that have colonized land is not cephalopods or molluscs. They are crustaceans.

tree

https://evolution.berkeley.edu/evolibrary/article/_0_0/arthropods_02

And an awesomely land adapted crustacean is the pillbug, which is an isopod.

https://www.pbs.org/newshour/science/pill-bugs-emerged-sea-conquer-earth

pillbug

A Different Way to Breathe

“Like their ocean ancestors, pill bugs have gills,” said Wright. Gills work great in the water. They’re basically exposed mucous membranes that absorb oxygen out of the water and into the blood that feeds the rest of the body. But on land, gills are a liability.

If the pill bug dries out, its gills won’t function properly and the pill bug can suffocate. That’s why you usually only find them in damp areas, like under a dead log. If they start to overheat and dry out, pill bugs will even roll into a ball to protect the remaining moisture on their gills.

Here is where being big helps. Pillbugs need to keep gills moist, so they stay where it is moist. We need to keep our lungs moist too but we truck around great reservoirs of moisture in the form of blood, with addtional water produced on demand by the oxidation of fat (CHx + O2 -> CO2 + H2O).

Your giants use gills to maximize surface area. Because they are giants with large volumes they are full of water - or better, fat or oil because it is lighter. They can keep their gills wet because of these onboard supplies. Their exoskeletons are thin or vestigial and I here assert that the reasons isopods have not displaced insects on land is because the carapace is difficult for them to evolve around and it gets too heavy and bulky as it gets larger. I was interested to read that one of the largest truly terrestrial (not littoral) isopods (Porcellio magnificus) prefers low humidity - which it can get away with because of its size and onboard water supply.


from comment: /Interesting, but how could this evolve in other groups of arthropods?/

You could start with insects that had already evolved gills. Behold the hellgrammite!

hellgrammite source

The hellgrammite is the large predatory larva of the dobsonfly. They have external gills to facilitate water breathing.

hellgrammmite gills

http://nathistoc.bio.uci.edu/neuropt/Corydalis.htm

Just as the ancestors of whales left the water, got adapted to the air, then brought those adaptations back to rule the waters you could have an insect adapted to air, move to the water, then back to the air with its respiratory adaptations. I like the idea of giant hellgrammite as a crocodile-like ambush predator.

The molluscs have done things like this - terrestrial snails that go back to freshwater and evolve secondary gills.

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  • $\begingroup$ Interesting, but how could this evolve in other groups of arthropods? And thanks to a comment on the original question it makes me think that I should separate the insects from the whole, if you can provide a way for insects to evolve in this way? $\endgroup$
    – Drakio-X
    Aug 8 at 2:54
  • $\begingroup$ @Drakio-X I think you may be asking for a little too much detail for a hypothetical evolution question, we can't give the genetic changes needed to make this happen. $\endgroup$
    – John
    Aug 10 at 13:47
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    $\begingroup$ I don't think he was asking for what kind of respiratory system it ought to get; rather, what evolutionary pathway could lead to a more efficient respiratory system emerging? $\endgroup$
    – JDługosz
    Aug 10 at 23:04
  • $\begingroup$ Thank you very much for add the explanation for insects $\endgroup$
    – Drakio-X
    Aug 11 at 0:06

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