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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 :) $\endgroup$ – Renan 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. $\endgroup$ – Renan Aug 16 '16 at 21:24
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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 nope 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 70 cm long scorpion. That's a little over two feet, or 28 inches. 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 nope 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 nope 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 nope 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 nope 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. Nopes spiders have excretory organs in their legs, but this species's excretory organs move to the abdomen over time.

Now, nopes spiders are known for being very agile with their legs, and weaving webs with them. Our specific nope 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, nopes 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 nope 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 nope 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.

The stuff of nightmares, really 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 nopes 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 nopes spiders already have. Why they would evolve hair would be a mistery, just as it it a mistery to me why we have hair like we do.

  • Nopes spiders have no actual brains... like the other nopes 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 synapsis 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, annedoctal commentary on that: nopes 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! $\endgroup$ – T. Sar 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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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! $\endgroup$ – T. Sar Aug 18 '16 at 19:55
  • $\begingroup$ @James, that "Kangaroo" is a dinosaur. Kangaroos are mammals. $\endgroup$ – Salmoncrusher Aug 23 '16 at 21:13
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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 at 13:07

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