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The insects of the plant are people-sized, highly intelligent, with a diverse urban culture that brings many species of ant, and inquiline together. Our narrator is a human exchange professor at one of the finest ant universities in the biggest city on the planet.

Of course, insects on earth have rarely grown to be such sizes. From what I understand there are two main constraints:

  1. Exoskeletons present difficulties at larger scales.
  2. Insects do not generally have active lungs, so getting O2 to all parts of the body presents problems as surface area decreases relative to mass in larger bodies.

The ants on the planet of the ants are very similar to earth ants, but this is convergent evolution at an extreme-- distant paths to a similar pattern rather than a familial relation. The exoskeletons of the ants are simply much stronger than the chitin of earth insects, due to a mutation that unleashed huge insects on their planet long ago-- and resulting in a very different evolutionary history. I would love to have some sources or speculation about just how the chemistry and structure of chitin might be altered to make this possible--

Also, these insects have something like lungs.

Are there other biological considerations for huge insects? Could there be constraints on vertebrates on this planet that don't exist on earth? (A reason why all of the animals with backbones are tiny... maybe there's no calcium except deep in the ocean? I really need to learn more about how mineral composition interacts with biology.)

I've considered a lower gravity... but, don't like this much as it over-complicates human visitors. Their planet is like ours but on their planet the insects came out on top.

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  • $\begingroup$ Plain obviously, there are no insects and no vertebrates on an extra-terrestrial world. What there might be is animals which look vaguely like insects and vaguely like vertebrates. And I have no idea what you mean by convergent evolution; what exactly is it that these gigantic, tool-making, art-marking animals have which is so similar to Earth insects to speak of convergent evolution? $\endgroup$
    – AlexP
    May 27, 2023 at 12:05
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    $\begingroup$ @AlexP i think it's very clear that A: vertebrate in his post refers to the clade of animals on his planet that have evolved backbones, rather than the Earth subphylum of vertebrata, and B: convergent evolution in fact refers to evolution that is convergent, resulting in his ant-like creatures have many structures analogous to those found in ants. I honestly do not see the purpose of your comment, it just seems like pointless pedantry. It makes far more sense for him to write "ant" and "vertebrate" than "ant-like creatures" and "creatures that have backbones". $\endgroup$
    – M S
    May 27, 2023 at 12:48
  • $\begingroup$ There are two very distinct questions here - one about considerations for huge insects and one about excluding vertebrates. One question per post is the rule here - given that the answers so far have focused on the first question, I suggest editing to remove the second question and posting it separately to avoid closure. $\endgroup$ May 28, 2023 at 2:14

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They probably don't have nuptial flights

On Earth, most (but not all) ant species have winged sexuals (males and pre-queens) that undertake mass flights wherein they mate. The males die soon after; the females excavate a hole and found a new colony. Given that flying is about the most energetically expensive form of locomotion, that body weight increases faster than flight muscle diameter as you increase in size, and that you are already pushing up against oxygen limits for walking body size, it seems unlikely that Planet Ant 'ants' have winged forms.

What do they eat?

Earth ants maintain their large numbers in part by eating protein sources that are larger than they are, either preying on or scavenging other species of insects or collecting seeds that are heavier than the ants themselves. Termite-raiding ants don't go after larger prey but their quarry is roughly at the same scale as the ant. What do the 'ants' of Planet Ant eat? They might be able to obtain enough calories through large-scale fungus farming or collecting nectar from proportionately-sized trees, but what is the protein source for building their huge exoskeletons and massive muscles? If they molt their exoskeletons like Earth insects do some sort of chemical process to compost and recapture the nitrogen would be very useful, but they still need a protein source to begin with.

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    $\begingroup$ Like many earth ants they are farmers and have livestock. $\endgroup$
    – futurebird
    May 27, 2023 at 23:14
  • $\begingroup$ @futurebird and are their livestock similarly giant-sized 'insects'? $\endgroup$
    – Kirt
    May 28, 2023 at 3:50
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You face a number of problems

Your first problem is oxygen levels, without some kind of active pumping mechanism equivalent to the expansion and contraction of lungs and some kind or internal organelle with a high surface area to volume ratio equivalent to lungs your ants are not going to be able to reach the physical size and more importantly brain size needed to become an intelligent species. Brains (nerves cells) are also very energy intensive organs requiring lots of calories and oxygen to sustain as well as large body sizes (to support the large brains).

As an example insects reached their largest size on Earth during the Carboniferous and early Permian periods when oxygen levels in the Earths atmosphere exceeded approximate 30% of total volume. And even then they never reached the size and mass needed for your intelligent ants. Today that level is about 21%. So absent at least some kind pseudo lung organ/s and extremely high oxygen levels it's not going to happen. Lower gravity would assist your plan as there are physical limitations imposed by the posture/joint structure/locomotion style of insects vs that of mammals. (See the difference in hip structures between lizards and dinosaurs as an example.) As a result a lower gravity than Earth would make it easier for a less mechanically efficient (insect) gate/posture to support a larger body mass.

Footnote: Assuming your human professor has transferred to a planet where the necessary conditions exist for such large insects he or she would need to wear a re-breather to prevent fatal oxygen narcosis!

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