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In my book series, many of the sentient races that exist are anthropomorphized versions of real animals. This has been mostly problem-free with vertebrates, but I have a question regarding invertebrates with hard exoskeletons, such as insects, crabs, shrimp, and lobsters.

Would the chitin-based exoskeleton of these creatures, scaled up to humanoid size, be able to defend them from most late medieval-early renaissance weapons (swords, spears, polearms, axes, arrows, maces, primitive firearms, etc.) or should they wear extra metal armor on top for added protection?

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  • $\begingroup$ May I ask what sort of environment they are using this armour in? $\endgroup$
    – sprout
    Commented Nov 7, 2021 at 20:44
  • $\begingroup$ @sprout These sentients are mostly found on the Planet of Vigam, which is one of the most alien-looking places in the entire galaxy. It's best described as a mix of forests, canyons, deserts, grasslands and massive undergorund cave complexes. $\endgroup$
    – The Weasel Sagas
    Commented Nov 7, 2021 at 20:57
  • $\begingroup$ Ahhh alrighty. So not anywhere armour would get particularly heavy or cumbersome. $\endgroup$
    – sprout
    Commented Nov 7, 2021 at 20:59
  • $\begingroup$ Perhaps they need a skeleton (inner) to carry their weight. $\endgroup$
    – Alchimista
    Commented Nov 8, 2021 at 13:29
  • $\begingroup$ If these bugs have a history of fighting with each other, they would naturally develop weapons to penetrate their opponents shell; so, it only seems logical that they would also develop armor to protect themselves from those weapons. $\endgroup$
    – alexgbelov
    Commented Nov 8, 2021 at 15:16

3 Answers 3

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They would wear extra padding and armor on top.

We can assume that the square cube law isnt going to affect them, but that does not make their exoskeleton stronger than regular bones. Weaker in fact, since the regular bones are one self-supporting mass while the exoskeletons are a thinner outer wall with enough supportive structure to handle the muscle attachments.

Since something like a mace would still be able to break a normal human arm even with the added fleshy shock protection it would stand to reason that an exoskeleton without any protection would break just as well, and even easier due to its thinner hollow tube nature*. Swords, pole arms etc would also have a good chance of cracking and partially cutting the exoskeletons.

So your exoskeleton users will absolutely love themselves padded armors with gambesons and the like.

*I'm not saying the wall has to be very thin, I'm saying that it would be reasonable to assume that the walls are thinner than the same internal bone.

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Pragmatism would suggest that some is almost always better than none.

I would presume a human sized bug would be especially concerned about a cracked carapace, rather than cuts and bruises. This suggests bugs would not be too concerned about cutting and slashing weapons, but quite concerned about blunt and stabbing ones.

Blunt weapons deliver a heavy payload. They depend on sheer, brutish power to pummel targets into mush. It's clear to me that a typical bug soldier could suffer only minor cracks before secondary blows are lethal, therefore, armor that can absorb blunt attacks is desirable.

Piercing/stabbing weapons deliver the entire force of a blow into a minuscule point, the tip. This kind of energy concentration cannot be absorbed, but must be glanced or caught. It's clear to me again that well-placed stabs with enough energy can easily pierce the carapace (and potentially crack it depending on location and combined motions like twisting), therefore a chainmail would be desirable. Chainmail actually works on a catching principal, allowing a non-lethal length of the weapon's tip to penetrate the armor, but then catches the weapon where the force is weaker and spreads the energy out so that it can dissipate broadly.

To fit snugly over the rigid bug form, they should wear padded clothing first, chainmail over the joints and high movement areas, and a broad and sturdy breastplate over it all, with perhaps gauntlets and helmets, etc. Oddly, that's very similar to humans.

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  • $\begingroup$ As for why they'd design pointed weapons in the first place, most of the sentients they'd be fighting against would be humans and other vertebrates with endoskeletons. $\endgroup$
    – The Weasel Sagas
    Commented Nov 8, 2021 at 0:47
  • $\begingroup$ Oh, well they'd use human looking weapons, at least on the killing end. @TheWeaselSagas $\endgroup$
    – user458
    Commented Nov 8, 2021 at 2:49
  • $\begingroup$ But the kinds of weapons they're vulnerable to applies still. $\endgroup$
    – user458
    Commented Nov 8, 2021 at 2:57
  • $\begingroup$ @TheWeaselSagas But the humans would develop weapons against the bugs, ending at the same place. $\endgroup$
    – lidar
    Commented Nov 8, 2021 at 10:45
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I read in this paper that spider crab shells can be 3mm thick: http://meyersgroup.ucsd.edu/papers/journals/Meyers%20303.pdf

This paper talks about the hardness of red king crab shells peeking at around 60 DU: https://www.researchgate.net/figure/Hardness-of-red-king-crab-shells-in-durometer-units-DU-or-kg-at_fig2_250069091

Scaling up a 57cm spider crab to be the size of a 1.7m human is roughly triple the size, so lets say the shell is roughly triple the thickness too, at about 1cm. Roughly as protective as a hardhat.

I think you can imagine the conclusion I'm coming to: crab shells are hard but they are nothing compared to steel. I think a medieval crab creature would do as humans do; protect itself to the best extent that finances and technology can afford. That means in the late medieval period the crab is going to be wearing full plate armor with an arming doublet.

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  • $\begingroup$ "Scaling up a 57cm spider crab to be the size of a 1.7m human is roughly triple the size, so lets say the shell is roughly triple the thickness too, at about 1cm." You'll need more than that, thanks to the square-cube law. At three times the size, you'd scale volume by 27 times, so the cross-sectional area would also need to be scaled up 27 times - and the square root of 27 is 3*sqrt(3), or about 5.1 times as thick. $\endgroup$
    – nick012000
    Commented Nov 8, 2021 at 8:42
  • $\begingroup$ @nick012000 I don't see why. You double the height, you quadruple the volume, sure. But if you double the height, why would you times the skin width by 5? I don't think that's right... $\endgroup$
    – user91320
    Commented Nov 8, 2021 at 9:26
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    $\begingroup$ When you double the height, you increase the volume (and thus mass) by 2^3, or eight times, while the strength (and thus ability to bear weight) of bones (or exoskeletal plates) only increases by a factor of 2^2, or 4. As a result, you'd need to increase the diameter of the bones by a factor or 2sqrt(2) (about 2.8) in order to allow them to bear the increased weight of the doubling in size. en.wikipedia.org/wiki/Square%E2%80%93cube_law $\endgroup$
    – nick012000
    Commented Nov 8, 2021 at 10:16
  • $\begingroup$ @nick012000 If you double the volume then the surface area increases proportionately. For example if you double the radius of a sphere then the surface area is 4x. So you don't need to worry as the skin volume when tripled is already 12x. $\endgroup$
    – user91320
    Commented Nov 8, 2021 at 23:39
  • $\begingroup$ Mass and weight scale by volume. Material strength (and thus the ability to bear that weight) scales by cross-sectional area. The exoskeleton's volume doesn't matter, it's the skin's cross-sectional area that matters here. $\endgroup$
    – nick012000
    Commented Nov 9, 2021 at 1:09

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