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In the fantasy culture it's not uncommon to find Giant spiders with the upper body of a human.

There are many variants as the spider body having only 4 legs and the upper human body have 2 additional arms or scorpion-like giant stings attached on the shoulders while some other variants even include ant-mimic spiders.

For my question I will use what to me seems the most classic option:

spider girl

On a planet with conditions similar to Earth's, why and how would this thing evolve? Would it be capable of producing and using its own silk as in the image above, and if so, how much silk to be more precise? If this thing is anyway realistic,how to make it even able to walk?

Bonus if you can describe its anatomy with precision, like if it's more convenient to give birth or lay eggs, or breath with the human part or the book lungs, and things like that.

This question is part of the Anatomically Correct Series.

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    $\begingroup$ A.k.a. Drider in some media (chiefly D&D and Drowtales). BTW I really like this question :) $\endgroup$ Aug 16 '16 at 18:08
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    $\begingroup$ @MichaelKjörling Web stuff comes out of their butt. On a more serious note, I imagine the sensory capabilities and approximate strength implications should be attempted to be carried over. $\endgroup$
    – Ranger
    Aug 16 '16 at 18:10
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    $\begingroup$ @MichaelKjörling these guys can crawl up on walls and ceilings, jump insanely high and their feet can work like hands as well. And then there's the fact that I've never seen anyone who is afraid of horses, but if I saw a bull-sized spider in front of me, human or no human parts, it'd be fire and chemical warfare time. $\endgroup$ Aug 16 '16 at 21:24
  • $\begingroup$ the strange thing is the biggest problem is the size of the spider not the rest of the anatomy. $\endgroup$
    – John
    May 19 '20 at 13:10
  • $\begingroup$ Spider body does not scale up very well, but that is not the limitation. Spider LUNGS do not scale up at all well, and over body mass of about 20kg would be impossible to keep the spider oxygenated, even if 100% of its mass was just lung. $\endgroup$
    – user79911
    Nov 20 '20 at 8:33

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You guys are approaching this as if the only way for an Arachne to exist would be for it to be a giant spider thing without any modifications to its spider anatomy. This is silly.

We all evolved from very small animals that lived underwater, yet here we are in human form. A much larger stretch than having arthropods growing into big sizes.

Specially because in prehistoric times, we did have arthropods that were quite big. The largest arachnid ever was a ~1 meter long scorpion (who was also kind enough to let me know about him in the comments :)). That's a little over three feet. It had aquatic cousins that could grow up to 2.5 meters long (that's over eight feet), though. Granted, these beasts had their weight supported by water, but look at that... An arthropod larger than a man! Not only that, but we know that there's less oxygen available in water than in air, so how could the square-cube law allow for that?

Well, besides evidences that our atmosphere was more rich in oxygen millions of years ago, there is also the fact that those arthropods had evolved their internal anatomy to allow for those sizes.

Let's go back to our drider, then. Yes, drider. It's easier to spell and more catchy than "arachne".

Imagine if you will a prehistoric tarantula. It lives in a world where there is more oxygen available than Earth right now, because that's how prehistoric Earth was like. Our prehistoric tarantula is under many different evolutionary pressures, and a larger size is conferring it increased rates of survivability and reproduction.

To allow for that, the tarantula starts developing, throughout generations, some unusual characteristics for a spider. For example, a mutation introduces a third body segment to it. This new segment will go in front of the cephalotorax, much like the head of an insect. It will contain the mouth, eyes, chelicera and pedipalps. It will also be filled with extra book lungs. This will allow for more oxygenation.

Over time, as this species of tarantula grows, it will evolve a circulatory system much like a worm's at first, then that of a vertebrate. A fourth, smaller body segment develops in front of the third segment, containing the eyes and chelicerae, but not the pedipalps.

As for the abdomen, it resembles that of a reptile or amphibian more and more, with different organs going through convergent evolution to take the roles of a liver, a spleen, a pancreas and so on. Spiders have excretory organs in their legs, but this species's excretory organs move to the abdomen over time.

Now, spiders are known for being very agile with their legs, and weaving webs with them. Our specific tarantula, though, is growing larger by the millenia, so its six frontmost feet are only used for supporting weight now. Eventually they will be completely flat and fingerless (yes, spiders have fingers - actually microscopic claws, which is how they weave and cling to stuff). Only the last pair of legs will have fingers for weaving.

To make up for that, the pedipalps start evolving into manipulators. They evolve more and more segments until they end up looking like vertebrate arms with hands, again due to convergent evolution.

Two more things are needed for this spider to reach a "drider template":

  • It will evolve a "neck" between the two frontmost segments, to allow it to rotate its "head". It may have eyes in all directions but the two eyes with the best sight are still fixed, and rotating a head is cheaper than rotating a whole body to better look at stuff;

  • The legs... Azuaron suggested that a drider would have to have its legs below the body and not splayed. I propose a midterm solution which is close to the anatomy of a real spider, so the legs should be fixed below the cephalothorax, as in the picture below. A drider is supported by eight feet, and they don't have to be incredibly heavy for their size, so I think this setup works.

A drider hanging from a thread of silk Source: this page at Deviant Art.

I estimate that, once the two first segments (head and "upper torso") reach the size of an adult human, the drider should be 6 feet tall and six feet long in a rest position, giving it an "L" shape somewhat. But it can put itself into much taller or shorter stances easily. It should weight 120 kilograms (approximately 265 pounds), with its weight divided almost evenly among its eight feet (the hindermost feet would support a smaller fraction of their weight), so on average each foot would be supporting 15 kg / 33 pounds.

More mythical creature greatness (More mythical creature greatness. I found this picture in an image board. It had no source, but I'd bet my soul it comes from Drowtales. Go read it. Great webcomic with an awful lot of spiders in it.)

Such a drider would probably be warm-blooded, with all the adaptations required for that. It should eat at least as much as a human with the same weight.

Everything else is just cosmetic details. Here is how I would have it:

  • The head looks like a human's due to convergent evolution (evolution usually places most sensory organs in a head) and blind luck/coincidence. They probably have no external years and their noses will be kinda different from ours. The freakiest part may be them having six to eight eyes.

  • The upper torso looking human would be pure blind luck. They would probably be furry all over, even females, less than a spider but slightly more than a human. Females won't have breasts like mammals do.

  • Remember these beasts have external, not internal skeletons. Even if they develop a hide or skin in the upper torso that looks like ours, it will still have plates of chitin underneath. Their upper torso belly will be hard as a rock.

  • They could easily evolve hair out of the hair that spiders already have. Why they would evolve hair would be a mystery, just as it it a mystery to me why we have hair like we do.

  • Spiders have no actual brains... like insects they have ganglia. But the arachnid ganglia are all fused together and the bulk of it is located around the esophagus in the cephalotorax. A drider would have a very interesting neural anatomy, with a very long, diffuse "brain" surrounding the esophagus, going from the upper torso neck to the part where the original cephalotorax connects the abdomen.

  • Speaking of which, they don't have a backbone, so no medula as well. Also no ribs, if it wasn't clear enough when I mentioned chitin plates under the skin.

  • Last but not least, they are, as James pointed out, separated from humans by a long distance in the tree of life. Their physiology would be alien compared to ours. Any medicines and drugs will potentially have completely different effects on them compared to what they do to us. If you research enough into that you'll see that many invertebrates species have synapses and neurotransmitters different from those of vertebrates. In the very least, a doctor would have to learn medicine again practically from scratch to be able to help a drider.

A last, anecdotal commentary on that: spiders are so unlike humans, that coffee makes them less able to concentrate, but LSD makes them get into the flow somehow. I suppose that's because the arachnid brain already sees the world as a web of interconnected things or something like to that effect.

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    $\begingroup$ spiders have hair over their body to sense sound vibration ... I suspect with ears, sensitive hair will not be so needed... $\endgroup$
    – Charon
    Aug 19 '16 at 10:08
  • $\begingroup$ More things you have to do: endoskeleton instead of exoskeleton, different joint types, fewer joints in the legs, and even after all that, I'm still 98% sure the biomechanics don't work out. $\endgroup$
    – Azuaron
    Aug 19 '16 at 12:16
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    $\begingroup$ This is the type of answer that I love to see on this site. You did a really good work, sir! $\endgroup$
    – Mermaker
    Aug 23 '16 at 13:36
  • $\begingroup$ Not bad, not bad, but you avoided digging into problems with mass and muscle strength. Mass is roughly proportional to cube of length, muscle size to cross-section. With this large "butt", legs so thin would have hard time to support it when spread to sides. Most of weight would need to be on muscles, not bones like horses do. The fact that legs are connected under his belly hardly matters - you can't put pillar nearly horizontally and expect it to work. $\endgroup$
    – Mołot
    Aug 23 '16 at 14:55
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    $\begingroup$ Leg width is not a problem, Have you have tried doing push ups on your fingers ? it's feasible, I can do weighted push ups on my fingers and they won't break, I can even do pull ups and fingers and they still won't break. Why wouldn't this apply to giant spiders too? $\endgroup$
    – Ekaen
    Mar 20 '18 at 18:22
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Sorry, but you've once again run into the dreaded square-cube law (it's the LAW). The largest spider is the goliath birdeater, which weighs a spectacular... 6.2 ounces. There are crabs with splayed legs like that, of course, and they get much bigger. The coconut crab weighs in at an amazing... 9 pounds.

The simple fact is, you can't have legs like that and weigh that much. To support that much weight, you must have legs directly beneath your body.

The seminal paper on this is On Being the Right Size by J. B. S. Haldane. What determines an animal's strength is the cross-section of their muscles and bones, which is an area (unit: meters squared). What determines an animal's mass is (roughly) volume (unit: meters cubed). So, if I double in height and become 4x as strong (2 squared), I'll have the problem of being 8x as heavy (2 cubed). 10x the height, 100x as strong, 1000x as heavy. Very quickly, I will not be able to breathe, let alone stand.

Splayed legs are great for small creatures (allows quicker darting movements, greater stability and surface area for climbing), but impossible for large creatures (can't support the cubed increase in weight).

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    $\begingroup$ How about approaching this like a giant spider crab? They can get fairly big: i.ytimg.com/vi/f78d72KypOY/maxresdefault.jpg I imagine you'll be able to get at least somewhat bigger than the crab, and forced to put a somewhat smaller-than-normal torso ontop. $\endgroup$
    – Ranger
    Aug 16 '16 at 18:32
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    $\begingroup$ Spider crabs are non-terrestrial (live at 500 feet below sea level), and still only weigh 40 pounds. $\endgroup$
    – Azuaron
    Aug 16 '16 at 18:38
  • $\begingroup$ Well the post doesn't impose a minimum body weight. You don't have to run with the idea, but it seems feasible enough to try to make it work. Younger children weigh not much more than 40 lb, and if you ignore the weight for the legs, which I imagine is close to half... $\endgroup$
    – Ranger
    Aug 16 '16 at 18:41
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    $\begingroup$ Assuming an average sized female torso on the picture, that spider woman probably weighs 400 or 500 pounds (look at that abdomen). Beyond that, the "40 pound" limit is for sea dwelling creatures not spiders. The best land-based limit I've seen is still the 9 pound coconut crab. And I don't think a 9 pound spider person is what 渡し守シャロン had in mind... $\endgroup$
    – Azuaron
    Aug 16 '16 at 18:44
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    $\begingroup$ I've updated my answer to more fully explain the square-cube law. In short, weight increases at an exponentially greater rate than strength. "Size" can mean many things (weight, height, width, depth, volume, surface area, cross-section, etc.) so I try to avoid using it. $\endgroup$
    – Azuaron
    Aug 16 '16 at 19:20
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To create a creature which looks half-spider and half-human, you must reject the concept of it being primarily based on the anatomy of either - trying to graft a human torso onto a spider isn't going to work, but something similar could be produced from novel origins.

Exoskeleton don't work for a human-like portion, nor does it permit legs capable of supporting the larger body size, so somewhere along the line an arthropod (before spiders fused head and torso) made a switch to a partial endoskeleton (with the exception of the thorax and abdomen which maintains a chitinous shell).

In earlier evolution, it had a protohead (with palps), thorax (with legs), and abdomen. As it started to go for food above it (easier than trying to shift its weight onto just back legs as it got heavier), it developed a stiffened ridge on the inside of upper protohead to provide better structural support - this eventually became retained during molting (such a large creature probably has a very long delay before the new exoskeleton hardens into sufficient structural support) and eventually became the spine. Likewise, in order to support the ever-larger body, the legs developed similar structural reinforcement which was no longer discarded with molting.

Eventually these extra structural supports became more and more predominant as the creature grew in size. One divergence with a spider development is that these creatures developed leg function more like insects, so they use muscles instead of hydraulics, which those attached to the permanent support would be more effective than the weakened exoskeleton, making this functionally more of an endoskeleton musculature structure. This permits legs capable of supporting great weight without needing excessively thick exoskeleton (only a vestigial thickness on that).

It began to develop the ability to push out its mouthpards and sensory organs a little while searching for food, until eventually this became a kind of stalk-like appendage. Greater flexibility in this led to its exoskeleton not fully hardening around this part, and eventually further segmentation of the protohead appendage into what we will now call the head (containing the mouth and sensory organs) and the bulk of the protohead becoming the torso (with the palps).

The torso used great flexibility (easier than moving around the massive bulk of the body), which favored ever-thinner exoskeletons, but more developed structural support ridges - eventually something roughly akin to bones. The palps obviously developed greater dexterity, and even more developed digits (better manipulation of stuff), while dispensing with the molting exoskeleton. Evolution is imprecise - those mutations got the head and whole torso to eventually end up with an endoskeleton and thin non-hardening skin instead of an exoskeleton. Most of the head retained a patch of exoskeleton - more of a helmet-like protection than body structure, upon which hairs grew considerably longer than before (weird mutations sometimes persist and perhaps this helped protect the exposed skin of the head and torso a little). With the transition may also have been a transition to an exothermic metabolism (to better cope with potential loss of insulation) if so desired.

Now we have a roughly human looking upper half. You can even throw in some similar internals like a heart in the central torso to help pump hemolymph all the way up into the head (the big heart in the abdomen might not be enough), and maybe lungs as well (secondary just for ease of oxygenating the upper parts - perhaps a tracheae type path to a lung cavity which eventually merged with the esophagus), and even a brain in the head (close to the eyes and mouth so that bit of neurology specialized there leading to higher brain functions - though the parts of the brain most responsible for locomotion would be in the thorax or abdomen).

The thorax would likely be an extremely tough mass of heavy exoskeleton to support the weight of all this plus be a central point for all the legs. I would probably think of it as a bowl like pelvis for the 8 legs, plus neural tissues sitting on top of it to coordinate the spider half. The middle of this bowl-shaped structure could sit digestive chambers (like a spider) to hold nutrients for the legs and leg-brain before passing the rest back to the abdomen for wider circulation.

The abdomen would probably be the most spider-like. A hindgut, a heart to pump hemolymph throughout the body, book lungs, etc. etc. I see no reason why it couldn't have silk glands and spinnerets for silk production.

The reproductive strategy of larger organisms tend to favor small numbers of live births than lots of tiny young. Gestating a large well-developed child for a year before giving live birth could even give reason for human-like breasts - mammary glands could evolve to suckle such live young, whereas egg-laying makes this implausible. Alternately instead of specialized mammary glands, a more spider-like solution is a pair of foregut ceacum (part of the digestive system into which the female diverts easier to digest or already digested foods) which hold material to then be discharged out a couple of ducts for feeding young (like birds pre-chewing food, just not coming out the mouth).

A more complex and interesting breeding cycle would have significant gender dimorphism. Female offspring are gestated individually before live birth, then raised with significant maternal investment, while developing on similar timelines to humans. Males are laid in clutches of eggs and not especially cared for - the comparatively tiny males lack significant intelligence, grow to breeding age quickly and are not long-lived (only a few years?), perhaps even lack the more human components entirely. Environmental factors could significantly determine which strategy is pursued in that female offspring are gestated when there are ample resources which could be devoted to expensive offspring, while males are often produced during lean times when the mother has little investment beyond producing the egg sack. Females develop civilization, while males are more like short-lived small dogs kept as pets.

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  • $\begingroup$ It makes me wander, how big could an arachne get with both bones and exoskeleton/armor, also their weight would be distributed very oddly. $\endgroup$
    – TobyB
    Jun 25 '19 at 13:07
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The top answer on here uses convergent evolution as an explanation for why a giant spider would develop a semi-humanoid appearance, but I'd like to suggest an alternative model: mimicry. Spiders are among the most prolific mimics in the animal kingdom, and often adopt the appearance of other species in order to deceive predators or prey. Take a look at the following image, which shows both an ant (on top) and a jumping spider (below). enter image description here

Something else we know about fantasy worlds is that they tend to have a superabundance of large, predatory creatures that love to prey on humans; manticores, giants, ogres, dragons etc etc. Therefore, it seems we have an evolutionary niche available for a species that exploits this habit in order to prey on them in turn.

How it works is as follows. The human 'torso' on an Arachne is in fact nothing of the sort; it's an elaborate lure made of chitin, which contains no essential organs but can be made to twitch in a rough approximation of human motion. This is similar to living lures exhibit by many real-world ambush predators (the Alligator Snapping Turtle, for instance, has a fleshy extension to its tongue which it flicks about in imitation of a worm, luring hungry fish within reach of the turtle's jaws). The rest of the Arachne's anatomy and behaviour partially resemble a scaled-up funnel-web spider. It hunts via deception, not speed, so it doesn't mind so much about the square-cube law; rather than supporting it, its legs drag it slowly along the ground while it weaves a web of sticky strands along the ground in a circle around it.

Next it uses its thick back legs to dig itself into the soil at the centre of the web (note that many of the largest real-world spiders burrow), burying its hind-quarters and leaving only the humanoid appendage visible. Then it waits patiently until a wyvern or some other large predator sees what appears to be a delicious human snack and comes over to investigate. As soon as the predator lands or comes near, it will become ensnared in the tripwire-style web, at which point the Arachne drags itself over and injects the target with paralysing venom. It can then drain the unfortunate monster of vital fluids at its leisure. An added bonus of this hunting system is that supplies a supplementary diet of unwary human adventurers, who come either to slay the 'sleeping' wyvern or to save what appears at first glance to be a damsel in distress.

As for the evolutionary history of this spider, I imagine it to be one of a group of mimic-spiders who evolved from some large prehistoric tarantula. The earliest breeds found that mutant head-growths which resembled worms (or something similar) were useful at luring over small mammals and birds, and from there they slowly evolved ever-more sophisticated lures. The Arachne's ancestors steadily grew in size in order to be able to tackle larger targets, and during the last few million years have developed the humanoid lure to attract those species which preyed predominantly on hominids, and then on modern humans.

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  • $\begingroup$ I'll note that while this would be great to explain the intermediate stage to the first post eventually ends up. this entity will need a lot of food to create that much silk. I suspect the outcome is that some will like like halflings/children and have more web and the larger ones will operate more like hunting spiders. $\endgroup$
    – zoboso
    Apr 19 at 13:50
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The square-cubed law is a big problem, and makes this creature impossible. But another problem exists and that is the fact that this is a hybrid.

Not only is this creature a hybrid, but it is a hybrid of two creatures drastically removed from each other on the evolutionary tree.

Take a look at this.

enter image description here

The further you have to go back to find a common ancestor the more difficult it becomes to have a plausible hybrid...and in the case of the dryder you have to go way way way back.

Arachnids are invertebrates and their systems are completely foreign to what makes up a human. Finding a plausible way for the two systems to work together is simply not possible. Thus the answer is magic on this one.

  • Creating a centaur: Difficult
  • Creating a bird person: Very Difficult
  • Creating a kangaroo person: Super Very Difficult
  • Creating a platypus person: Awesome
  • Creating a Spider Person: Super Duper Very Difficult (also known as impossible)
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    $\begingroup$ On a planet similar to Earth, that family tree is irrelevant. Their equivalent of protists will be entirely different, and any possible human-arachnid analogue will have evolved that way because of environmental pressure. $\endgroup$
    – Rory Alsop
    Aug 18 '16 at 17:23
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    $\begingroup$ IMO, this is a much lesser problem, given that we're, "On a planet with conditions similar to Earth's," not Earth, so we're already talking about a completely different evolutionary tree. Although, that does raise the question, "Why does it look half-human/half-spider?" Strangest case of convergent evolution I ever did see... $\endgroup$
    – Azuaron
    Aug 18 '16 at 17:27
  • $\begingroup$ @RoryAlsop Similar to Earth or Earth itself, arachnid and mammalian biology don't mix. You can't use two Earth based creatures and then pretend that a different planet makes a difference. $\endgroup$
    – James
    Aug 18 '16 at 18:07
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    $\begingroup$ @James We are not really trying to make a creature that has a mix of human and spider biology - just one that, externally, looks half-spider and half-human. Their internal workings could be something way different than both humans and spiders! $\endgroup$
    – Mermaker
    Aug 18 '16 at 19:55
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    $\begingroup$ @James, that "Kangaroo" is a dinosaur. Kangaroos are mammals. $\endgroup$ Aug 23 '16 at 21:13
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Not sure how to help with the size of the spider. The max you could get (and even this is probably pushing it, due to differing anatomy) is the size of a coconut crab.

enter image description here

This is a pretty big creature (I believe some specimens have leg spans of 3 feet), but it's nowhere near the size of a human.

However, in regards to the torso, getting it to look fully human is an issue, but might I suggest something somewhat similar.

enter image description here

This is a Pelican Spider. They're quite small (only 2 to 8 millimeters long), but their head shape could work (I don't see why it couldn't evolve in a larger spider). Their long fangs could take the place of arms as they are used to grab prey (probably no way to have fingers though). I understand if this isn't what you're looking for, but I found it fairly interesting.

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If it evolved on a planet similar to Earth then all issues regarding respiration and taxonomy are irrelevant.

So, millions of years ago a large decapodal silk-producing organism existed on a planet similar to earth. It has an endoskeleton as well as a chitinous exoskeleton. Its foremost limbs developed into humanlike arms so that it can better manipulate objects in its environment. The front portion of the body became a torso to increase the reach of the arms in any direction. Then a defined head, neck and shoulders develop. The exoskeleton on the torso disappears for greater flexibility. The hindmost portion of the body grows into a distinct abdomen to counterbalance the torso and so there is more space for internal organs. They then become social for protection and food and their intelligence slowly increases over the next hundred thousand or so years. The similarities between their facial features and that of a human's are simply convergent evolution. Hair is optional.

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  • $\begingroup$ 'So, millions of years ago a large decapodal silk-producing organism existed on a planet similar to earth. It has an endoskeleton as well as a chitinous exoskeleton' This quite accurately describes a spider $\endgroup$ Jun 1 '20 at 15:09
  • $\begingroup$ @IchthysKing but it isn't a spider, because it didn't evolve on Earth. Spiders belong to the order araneae, which only exists on Earth. $\endgroup$ Jun 2 '20 at 2:45
  • $\begingroup$ It could be Earth, as you only stated it was a planet similar to earth, and Earth fits that description $\endgroup$ Jun 2 '20 at 7:26
  • $\begingroup$ Earth isn't similar to Earth, it is Earth. $\endgroup$ Jun 3 '20 at 19:35
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An arachne could evolve from a spider that evolved to have looping digestive tubules. They may later evolve to actively pump fluids through this system, which may become more separated from the intestine. Eventually, the digestive tubules will be completely separate, and full of a distinct fluid. This may take on the role of blood, entering the prosoma and replacing the hemolymph. In order to make the blood more efficient, some vessels may expand and transform into a gland that produces an oxygen-carrying pigment, such as hemocyanin. There may still be digestive tubules that do not loop. These tubules, not needed to spread food through the body, might become glandular and fuse into a hepatopancreas. They may increase in size, due to predation. In order to support themselves, they may harden and expand their endoskeleton. Their endoskeletom may become their primary skeletal system, as is is better protected against predators. They may also become able to pump their trachea, eventually turning them into true lungs. They would also need to get rid of more waste, and so the malpighian tubules and coxal glands might expand and become more efficient, until they become similar to the kidneys of vertebrates. They may gain irises in the central eyes, which would allow these eyes to become more powerful, but would lead to the lateral eyes being lost and reduced. Due to the loss of the lateral eyes, they may gain a joint in the carapace around the eyes, mouth, pedipalps, and chelicerae. This regoin may extend outwards, to allow a greater range of articulation, allowing them to see in more directions without needing to move their prosoma. The pedipalps may split at the ends, in order to grab food and bring it closer. This may lead to the pedipalps bending backwards, to allow food to be brought to the body, so that the spider's young can more easily be fed. The pedipalp's motion might cause the maxillae to turn up, and oppose the chelicerae, which would adapt accordingly. The maxillae might become fixed together, as a jaw. Due to the lack of attacks to the head, the exoskeletom may be reduced in that region, which might turn the maxillae and chelicerae into fleshly jaws which can hold liquid. This would allow their diet to expand, allowing them to gain more energy. They might use this energy to become endothermic, allowing them to become more active without the need to stay in the sun to warm up. The fleshy jaw would also be a good guard against solids entering the mouth, which could lead to many groups losing their oral filters. They may begin to live together, in familial groups. Another adaptation may be gaining sound-sensitive setae on their head. These setae might lead to pits in the head to focus sound, which may later turn into deep cochleae, with an eardrum to help pick up sound, and a pinna surrounding it to make sounds easier to pick up. They may evolve to squeal using their stomach, as a way to alert members of their family to approaching threats. This might make the frontal gut-loop take on the role of stomach, with the stomach becoming soley for sound production. Another effect may be that olfactory receptors might appear in the throat and mouth, so that the creature could inhale air to smell it. In order to look around more easily, the root of their maxillary bone might become cartilagenous, to allow some motion of the head. This trend would increase until the maxillary bone is completely disjoint from the scapula, and the head is attached to a neck. The changes of the gut and jaw might allow them to consume solid food. This would lead to a more solid jaw, with the chelicerae fixed under the skull, the maxillae fused as a lower jaw, and both appendages bearing bony spikes similar to teeth. However, there may still be channels above the chelicerae, which allow air to flow into the throat while the mouth is closed. They might also add plants to their diet. This would lead to their teeth-spikes changing, so that the rear teeth might become square, peg-like molars, in order to chew through fibrous plants. They would also need cheeks to prevent the food from being lost from the sides of the mouth. At some point, humans may come into their habitat, and hunt them due to their strange appearance. They might evolve to become more intelligent, to try and avoid and outsmart the humans. They might evolve to use language, which will require several changes, such as the labrum being freed from the lower jaw as a tongue, and the region between the cheliceral fangs broadening into an alveolar ridge. These changes would make them into an arachne

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A quick short answer: Things that evolve can't de-evolve to gain new abilities, be them any better. We can't get worse than we already are. If they evolved from humans, it wouldn't be feasible outside of artificial genetic modification. They would need to de-evolve their good legs for worse spider legs then best spider legs, which is impossible. If they evolved from spiders, it wouldn't make sense, because it would have to change pretty much every thing about their respiratory systems, size, prey, and things like that. Smaller things breath VERY differently compared to us. So much so that smaller creatures would just die if they get too big without evolving different respiratory systems. They square-cube law would also make it harder for the air to get to them, as they breath through tubes as of now. They just wouldn't be able to evolve in that way without being worse for a period of time, which would likely make them unable to reproduce, killing them off.

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It's possible but unlikely. The ratio between the surface area and volume of a creature increases exponentialy as it get's bigger, thus a large arachnid body would be largely unable to support it's own weight.If an animal were isometrically scaled up by a considerable amount, its relative muscular strength would be severely reduced since its mass would increase by the cube of the scaling factor. As a result of this, cardiovascular and respiratory functions would be severely burdened.

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They have already repeated it a lot, the biggest problem is weight support due to having to follow the cube-square law. So maybe this will not be the most important aportation, but I read some "escapes" for this.

  1. Could molt in sections instead of all at once, like some crabs.
  2. "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". So keep in mind though that if you fix the oxigen circulation in insects, their metabolism will most likely increase as well, especially in warm habitats.
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Okay, we have an humanoid upper body, which we’ll assume has a mass of 46.7 kg, and an arachnid lower body, which we’ll put at 3 feet, or .914 m, long. The largest spiders have an body length of .09 m, or less than a tenth of that, and have a mass of .175 kg. Scale that up, and we have a mass of 184 kg, and 8 legs=8 support structures. The total mass is about 230 kg, and spiders can lift 170 times their mass. As size increases, proportional strength decreases in an linear and inverse relationship, so scaled up, you’re looking at a being who can lift up to 2.84 tonnes. I figured that out as mass increases cubically, and strength increases quadratically with size, and thus, n^2/n^3=n^-1.

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Diverge just before the rise of Tetrapods

You're creature resembles both an arachnid and a mammal, but these two groups of animals diverged a VERY long time ago. Arachnida are already too far down the evolutionary rabbit hole to just switch to an endoskeleton and centralized pulmonary system, and mammals are already too far committed to the tetrapod body plan to just spring for an extra 6 appendages. So instead of going back to some where an ancestor might look like something between a spider and a person, you should go all the way back to this planet's origin of animals by looking out how our Phyla diverged in the very early epochs of macroscopic life.

There are many answers already stating why giant arachnids would likely not get you the desired result, so instead I will focus on how one might arrive at this body plan following an evolutionary path more similar to the human part.

Back when human and spider ancestors all still looked like worms, two groups of worms emerged called Ecdysozoa and Deuterostomes. The Ecdysozoa favored less centralized body mechanics which would eventually lead to spiders where Deuterostomes favored more centralized systems. The proclivity of Deuterostomes to specialize in centralization led to the Chordate which in turn further specialized into the Vertebrata. Basically, it was a huge evolutionary push towards centralizing things that eventually lead to fish having internal skeletons, central pulmonary systems, and all the things that form the foundation human like life forms. Basically you just keep following this general evolutionary path until you hit something like Sarcopterygii (lobe-finned fish).

It was at this particular evolutionary junction that Tetrapods evolved to eventually become humans, and this is where you need to make your split. While Tetrapods gave us our 4 appendages, back when lobe finned fish were still fish, they could have had any number of appendages and still be evolutionarily viable. So instead of your planet's "reptiles" "birds", "mammals", etc. all evolving from a primordial 4 finned fish, they would have come from a 10 finned fish instead. Then, just like arthropods specialized into classes with fewer and fewer limbs; so too could your world see some groups that lose limbs as evolution marches on while others retain them.

At this point though, your ancestor is already very mammal or reptile like complete with teeth, eyes, lungs, heart, brain, etc. with the only key difference being 10 limbs instead of 4. From here it is just a matter of convergent evolution. Your 10 legged creature will take on mammal like traits including fur and hands. It could even evolve armor plates similar to an exoskeleton the way that armadillos have. Eventually one group that prefers tree-dwelling would have enough evolutionary pressure to develop webbing and/or opposable hands.

As the creature evolves more towards intelligence and tool usage, the forward part of its body will begin to curve up and become more flexible freeing up the hands for better tool manipulation while leaving the other appendages as feet. This adaption would happen for the same reason humans evolved towards bipedalism.

Bonus if you can describe its anatomy with precision, like if it's more convenient to give birth or lay eggs, or breath with the human part or the book lungs, and things like that.

Following the pattern of centralization you see in this animal's ancestry, it will have a heart and lungs similar to a human's that fill up the human like part putting them closer to the nose for more efficient respiration, then the spider like abdomen would contain the digestive and reproductive organs.

Since you are describing a large and intelligent species, you are best off with a K-selected reproductive strategy. While this could technically be done by egg-laying or live birth, the more important question to answer will be how many children your Arachne has at a time. Instead of making hundreds of offspring like spiders do, these creatures will likely lean towards 1 child at a time like humans do to make sure they can commit the resources they need toward education.

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While evolution can make an arachne-like creature, you'd need to involve some design for a true, lone spiderhuman

  • Firstly, the human and spider parts will need to be connected together. The most common way to do this in most depictions is for the human part above the legs to be set into the spider's face above the chelicerae. This seems like a good basis for an arachne

  • Secondly, we'll need to make sure it can support itself at its size. They'd need some sort of internal skeleton for support. The walking legs and chelicerae could contain a tetrapod-like structure with many repeated stylopods to form the correct amount of segments for the limb. However, this system may be insufficient for the pedipalps; these structures would benefit from having a jaw-like system in the maxillae, that could be used as a shoulder for a tetrapodal limb in the rest of the pedipalp. The skeleton of the torso would need to surround the organs. One solution, that seems good enough, would be to have a single pelvis structure on each side, with 4 joints to which all of the legs connect. The maxillae would have hinge joints, and could connect to the front of the multipelvis by a 5th joint. The organs could be supported by a ribcage-like structure set below the sternum, which would provide support for the organs. The carapace can remain exoskeletal. The pedicel would be wider, and have strong bones and muscles. The abdomen would have to be much smaller, and could have something similar to the sternum-bone for support

  • The organs will similarly have to be adapted. A human's mouth seems far too small for an arachne, and spider-like digestion is inefficient. One solution could be giving the spider part an enlarged humanoid mouth and stomach, which would be the main way they eat. A similar substitution could be done with the spider's heart and lungs, with the addition of a closed circulatory system. The intestines could be kept roughly as they are (with many tubules from a straight intestine), especially if there's some way to filter out indigestible matter at the base of the tubules. The leg-based section of the intestines will probably have to be removed, though. The reproductive system may also need to be reworked for a live birth. This could be achieved by adding a humanoid uterus under the spermathecae

  • The two bodies (alongside a few other parts) will also need to be connected. The chelicerae might be attached to the human pelvis. The spider part's humanoid jaws will also need to be connected. One way would be to add in a pair of bones which would fuse to the maxillae and provide a joint for the mandible. The whole system could be opened and closed by masseter and pterygoid muscles. Both of these systems might be supported by a pair of rib-like structures in the front of the carapace. On the organs, the human part could have a large stomach filling the abdomen with a short duodenum that connects to the front of the spider part's intestine loop. The spinal cord will also need to extend beyond the spine and go into the spider's brain

  • Finally, we need arachne to be able to develop from an embryo. One plausible sounding method is this: First, the embryo develops as monoamniotic twins in the human. One of the twins will develop as a human, and the other will develop more like a spider, receiving nutrients from a yolk that is fused to the uterine wall. The humanoid/tetrapodal parts will develop in a more tetrapodal way. The legs specifically could grow as complete replacements for the spider-type limbs. The spider maxillae could be derived from pharyngeal arches, which would be required for the humanoid mouthparts. The human twin would also develop chelicerae in place of legs. At some point, these twins would have to fuse together. This should happen early on. At this point, the neural tube should extend to combine with the spider's nerve cord, and the end of the human gut should be extended to connect with the spider's intestine

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