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"For big sizes and masses arthropod legs are a bad option" is what many people say, but nevertheless, this always looks cool and scary.

So based on the first solution for an Anatomically correct Arachne, we're going to assume a weight of 120 kg with 15 kg distributed for each leg (with variations), but even this solution the general consensus was that the biomechanics of the matter would not allow support that weight.

Add the fact of arthropod chitinous legs, because, yes, this probably can be solved using normal bones that just look like spider legs, but would be contradictory with the Arachne solutions.

Searching solutions for this, the best thing that I could found were these spider trucks:

Obviously, biomechanics are very different to technology mechanics (you can see those hydraulic pistons and motors), nevertheless, this gave me some hope to think that the design is viable.

Also, the most representations that can be found shows very long and thin legs, something that looks completely implausible and unrealistic

Are a few which shows shorter, stronger and thicker legs, like the showed by the first answer to the Arachne or this

So, maybe we will lose some of the height and for this some of the imposing posture as it rises, but if but the disturbing movements and form that terrify people so much remain enough will be enough.

What changes and implements should get the legs for resist the weight and walk?

How thick and short should be the legs for make physically viable?

Is the chitin enough for resist the weight?

Or it's completly impossible biological very segmented legs able to carry and move those sizes and masses?

Remember all the previous, some based on the arachne answer.

I thought were enough questions, other things like are very hard to think and more like magic, so is unnecessary

The_CIA
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Drakio-X
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    I'm afraid I don't understand what you understand by "arthropod legs". In particular, the legs of the "spider trucks" in the first two pictures are much more similar to human linbs (= articulated rods) than to arthropod legs (= articulated tubes). (Fun factoid: human limbs have the same number of joints as insect legs. The difference is that insect legs are articulated tubes, while human limbs are articulated rods; that is, exo- vs. endo-skeleton.) Human-made machines rarely use the articulated tube approach, mainly because we have no way of making strong flexible joints between two tubes. – AlexP Dec 07 '20 at 19:43
  • @AlexP. With arthropod legs I understand what looks like arthropod legs, even if not have the same function. But also happens that I ned more knowledge about the arthropod locomotion. – Drakio-X Dec 07 '20 at 19:51
  • Chitin is unsuited for large creatures (too soft compared to calcium skeleton), and the tubular breathung system (tracheas) too ineffective for large land creatures. Carboniferous saw the biggest possible ones, in an atmopshere with much higher oygene partial pressure. –  Dec 07 '20 at 22:59
  • @a_donda there are creatures with chitin that do not use trachea. Chitin and lungs are completely possible. – John Dec 07 '20 at 23:36
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    the big limitation is how chitin grows. Chitin is shed then secreted in the larger size, this means the organism has to be able to support itself with only part of the exoskeleton while molting. – John Dec 07 '20 at 23:39
  • @a_donda. "I am not sure how resilin and other flexible proteins would act on a larger scale. But if you play around with the alpha, beta and gamma chitin ratios in both mobile and immobile parts of the exoskeleton insects should be able to support a large weight while maintainaning mobility", is this true and how resistant could be? – Drakio-X Dec 08 '20 at 00:53
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    @AlexP: Why do you think there's no way of making flexible joints between tubes? At the simplest level, close the ends of the tube and use the same joints as for rods (or I-beams). Then there are for instance the joints in space suits, and a couple of minutes with Google will give you many more instances. – jamesqf Dec 08 '20 at 02:49
  • @jamesqf: Strong joints. We can of course make jointed tubes, in may ways. Load bearing joints, not so much. (And no cheating -- arthropod legs are continuous jointed tubes, not tubular segments jointed in the manner of beams.) – AlexP Dec 08 '20 at 03:06
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    @Drakio-X: creature size limits can be discussed. Some argue that even large athropods (1m and larger) of the carboniferous were too large to support their weight, meaning there may be solutions to keep ventilation intact that we're currently not aware of or can speculate about. I am not self-confident enough to judge exact sizes and limits, but sure you can find work an the matter. In worldbuilding, some handwaving won't harm :-) –  Dec 08 '20 at 09:42
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    Another thing that comes to my mind: creating a vertebrate / arthropod chimera combines two completely different "blueprints" with mutually exclusive functionality not just concerning breathing system, but all of metabolism and life cycle. E.g. how can it build a calcium skeleton and at the same time molt it's exoskeleton, or blood and hemoglobin and open ventilation, or digestion outside and inside of body, and all that. Body functions would be really ... messy. One would need a lot of hand (or whatever applies :-)) waving ... –  Dec 08 '20 at 09:58
  • @a_donda. Oh yea, about something more chimerical, the answer to the anatomycally correct arachne, the answer from pluckedkiwi, solves many problems, even the looking like a human, except for the problem for what I am asking now. – Drakio-X Dec 08 '20 at 13:41
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    @AlexP: I don't see how tubular segments are "cheating". You might consider the hollow tubular booms used for pumping concrete, though. – jamesqf Dec 09 '20 at 03:21
  • @jamesqf. That sounds interesting, could a possible solution. – Drakio-X Dec 09 '20 at 04:39
  • This is strange I consider my question was answered with just comments. – Drakio-X Dec 13 '20 at 07:21
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    Related question/possible answer: Are 'ambiskeletons' viable?. – rek Jul 13 '21 at 04:14

3 Answers3

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Pseudo-legs:

I was reviewing your question, and there is one way I can think of this that makes sense. The muscles that operate the legs aren't in the legs at all. The whole thing is powered by opposing tendon-like structures with the muscles in the torso. The legs are worked like a puppeteer.

The tendons, being thin, don't need appreciable air or circulation supplied to them, since they do very little work. The limbs might not even need to be be proper limbs, but grown or made structures strapped into place like artificial legs and regrown/replaced when they wear out/break. If the species is fully intelligent, they may be woven together out of chitin strips and spider silk, with the silk providing the "strings" that get pulled to and fro. When another dies, their extra limbs can be shared out with others.

By eliminating the need for all or much of the living tissue in the legs, they can be extremely thin. They would fit onto or grow out of a stump-limb with extensive internal muscle and be moved about like a puppeteer would operate them. The limbs would be able to be beaten up or cut off, and if they were manufactured, who cares? Go get your spare set after the fight so you can walk normally again. If they were spring-loaded, the movements could even have a fair a mount of power behind them.

If the limbs are artificial, you have some really fun alternatives available. They can have spikes, hooks, and saws attached. Limbs can be completely replaced with weapons, or tools. Made of metal, the arachne could look cybernetic or clockwork-like.

Not to mention that your arachne are likely to be master puppeteers with legs designed like this. Envision an arachne concealed in a ceiling web using actual puppets run via invisible spider silk to interact or fight with the players. Fake skeletons, "animated" armor, or dancing weapons could make entire encounters with the players never interacting directly with the arachne!

DWKraus
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    that still leaves the biggest problem, a thin walled hollow tube of chitin just is not strong enough, and because of how it has to grow you can't make it thick enough to hold the weight. making the legs thin just makes it worse. – John Jul 13 '21 at 02:44
  • @John That's part of the idea here. I suggest that the limbs don't need to be all that thin-walled. They don't even need to be alive, like hair, or they could be completely manufactured. They are like the limbs of puppets, only the strings would likely be internal to the limbs (but wouldn't even need to be). They'd work mechanically (even if they were biologically derived by whatever means they were produced) – DWKraus Jul 13 '21 at 02:59
  • What are you going to attach the strings too, you will run into similar problems with whatever limbs they attach too. – John Jul 13 '21 at 03:19
  • @John I'm imagining not much more than a muscle attached to a hook/loop secured to a silk string or a long tendon-like thread that extends inside the faux limb. Possibly even a soft limb like a caterpillar inside/attached to the main carapace. No joints to speak of. The "tendon" could even be external, exactly like a puppet if desired. – DWKraus Jul 13 '21 at 03:34
  • You need it to move and exert enough force to support the limbs, the weight of the body is supported by muscle tension either way. so you need joints for movement and it needs to be strong enough to withstand the forces involved. a puppets weight is supported externally by the puppeteer, you are building something more like a prothesis. – John Jul 13 '21 at 11:45
  • It's an interesting concept, but how the creature's body would produce the legs? – Drakio-X Aug 04 '21 at 07:22
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    @Drakio-X There are a couple of ways you could do this. One, the limbs simply have really long tendon-like structures and are grown like normal limbs. Circulation requirements will be minimal without muscle inside the limb. The "tendons" might even be external, made of silk, and the limb nothing more than a grown exoskeleton moved like a puppet. Otherwise the limbs can be artificial (but with the body evolved for artificial limbs) and completely manufactured. – DWKraus Aug 07 '21 at 02:20
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You can make something that looks sort of like spider legs at least.

You need a replacement for chitin or at least a completely different way to grow it. Something that can be grown thick and still be jointed and still support the weight. Nothing excreted from only the inside will work. You need wide strong independent joint surfaces and thick walled tubes to support the weight. Keeping the muscles inside is possible it just makes the legs very thick. Your best bet is a skeleton grown internally but only covered by a skin of living cells similar to how wood or scales grow, but still hollow inside. Chitin could still work for this but it needs to grow like wood or bone and not be shed, because it needs to be thick. Engineering wise, we don't normally build this way but it is possible, there are hollow load bearing joints in nature, it is just tricky and rare. What you loose is joint mobility, the joints end up far less flexible than what you can can achieve with an fully internal skeleton. Thankfully there is not that much mobility in spider leg movement anyway, so you could still make legs that work. The skin of of external tissue also will not work at the joints, you still need more space for something like ligaments to support wide independent joint surfaces. You could make something that superficially looks like a thick legged spider with swollen joints but you basically need to throw out everything spider except the general layout and start from scratch to work out the biomechanics.

Another thing you can't get is a wide spider like splay, the legs need to be held close to the body to reduce the stress on them, more like your bottom picture. But note in your image the upper segments of the legs are way to thin so you can't just use it as is.

John
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Actually... Chitin can do the trick.

"Traditional" chitin (the chitin currently present in the spiders we are familiar with) may not be able to carry that weigth if you just scale up the animal (I won´t enter in the details of the organism specifications and square cube law because it has already been answered in the "Anatomically correct Arachne" question you mentioned, and has already been reviewed in the comments and other answers. So let´s focus on the leg material.

Chitin comes in many flavours. Actually an animal can combine chitin with other substances to make it very strong or very flexible:

https://en.wikipedia.org/wiki/Arthropod_exoskeleton

"Generally the exoskeleton will have thickened areas in which the chitin is reinforced or stiffened by materials such as minerals or hardened proteins."

Typically the mineral crystals, mainly calcium carbonate, are deposited among the chitin and protein molecules in a process called biomineralization.

"Typically" the biomineralization is with calcium carbonate. But what if your spider uses another component? for example iron.
And it is actually not fiction. Sea snails have teeth that reinforce the chitin with iron oxide and create "the strongest material on earth" (Organically generated) according to this article:

https://www.livescience.com/49844-limpet-teeth-strongest-natural-material.html

The teeth fibers withstood a pulling force that was equivalent to a spaghetti strand hoisting 3,000 bags of sugar, equivalent to about 3,300 pounds (1,500 kilograms)

And then, the articulations:

A standard spider leg articulation is as shown in the picture:
https://www.researchgate.net/figure/Fig1-ASketch-of-a-spider-leg-showing-all-segments-joints-without-extensor-muscles_fig1_221779105
Sketch of a spider leg
The hinges represent a weak point structurally speaking. So the creature may need to grow an aditional reinforcement in the articulations (made with the same iron-based chitin):

enter image description here

The chitin reinforcement in the tubes must be very good sealed to avoid fluid leakage, and may resemble the articulations in the crabs exoskeletons.

Carlos Zamora
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    except that force is transferred at only a few millimeters of joint surface, remember an arthropods leg is essentially a tube with complex bends in it. And it cant be grown very thick due to the problem of molting and the geometry of a surface growing from only one side. its not about the strength of the chitin as much as the geometry of the chitin. – John Jul 13 '21 at 19:08
  • I have edited the answer to include that. – Carlos Zamora Jul 13 '21 at 21:04
  • that doesn't help, its a nice addition but doesn't solve the cross sectional area problem. your are trying to support the weight of a large human on something with less material than the tube at the center of a paper towel roll. to be clear even steel would have a hard time supporting it. – John Jul 13 '21 at 21:35
  • It is good information but I still have the problem of the strength of the muscles and the space they have to function – Drakio-X Jul 16 '21 at 07:35