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So I am building giant spider species in my world, but I want to make semi-realistic changes to their physiology in order for them to be possible.

Given these requirements, what changes would have to be made? How big would they have to be? I know for instance, that small spiders pump fluid into their legs to move, but this doesn't really work on a larger scale. I was thinking as well, that given current earth-like conditions, they'd also need to process oxygen better than regular-sized spiders generally do.

Can creatures with an exoskeleton get this big? What adjustments would have to be made in order to meet my requirements?

I know I can hand-wave a bunch because it's a mythical world, but I would like for them to work without magic if at all possible.

EDIT: THIS HAS BEEN FLAGGED AS A DUPLICATE of a question about exoskeletons and size. Not only had I already seen this question and the answers there, it's the reason why I mention oxygen processing. THAT question was about crustaceans, this one is about SPIDERS. MY question has specific requirements for the creature I am building, including what they should be able to drag, and asking about what other changes need to be made (for example the hydraulic leg system scaling up). The biology of a spider and the biology of a crustacean ARE, in fact, different, from how they move, to how they breathe, what they eat, and how they hunt.

@Will for example, answered the question by saying that they don't have to be large, just push the cooperative nature of them, so that they drag things together.

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  • $\begingroup$ good stuff here. worldbuilding.stackexchange.com/questions/57348/… $\endgroup$ – Willk Apr 18 '17 at 3:16
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    $\begingroup$ Possible duplicate of Maximum size of an exoskeletal creature $\endgroup$ – Aify Apr 18 '17 at 7:11
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    $\begingroup$ Add lungs to their anatomy & physiology and give the world's atmosphere a higher percentage of oxygen. Possibly, an internal skeleton too. The exoskeleton could be simply plates of chitin for protective armour & not as a skeleton proper. $\endgroup$ – a4android Apr 18 '17 at 7:32
  • $\begingroup$ Why do you think that hydraulics doesn't scale? We use hydraulics to move huge machine members, for example the huge arms and cups of enourmous backhoes. $\endgroup$ – AlexP Apr 18 '17 at 12:52
  • $\begingroup$ @a4android: Spiders have lungs. $\endgroup$ – AlexP Apr 18 '17 at 12:54
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I was interested to see that your linked spiders have 2 castes of females. Different castes are well known for social insects.

from http://www.alexanderwild.com/ enter image description here

My proposition takes a page from Komodo dragons. They are no good for sustained speed. The Komodo bites and then stalks the prey for days. I watched a tiny spider do this to a wasp caught in its web: wait, bite a leg when given the opportunity, and then wait again. All day and all night. The next day the wasp was being wrapped.

Spiders are not good with sustained speed either. Spiders are good with patience and poison.

You will have a biting caste of spiders. These are little and quick: regular spiders. They bite the prey and then run or get smashed.

Once bitten the prey starts to get sick. It takes a while. The stalking spiders come. These are larger. They are not fast. They see very well. They wait in the shadows. When the prey gets sick enough, they come in as a team and wrap it up.

The stalking spiders are not huge either: maybe the size of guinea pigs. Once the prey is wrapped it is slippery, and the wrap is parallel to the long axis and the eventual direction of travel. The silk acts like runners. The stalking spiders drag it away like a dogsled team, each attaching its own silk and pulling. More weight to pull = more spiders to pull it.

The period between getting bit and the stalking spiders taking you away offers narrative possibility. The stalking spiders are like death. They are very, very difficult to escape. Sentient prey will know they are coming.

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  • $\begingroup$ Even better, they don't have to be large, just highly cooperative. $\endgroup$ – Erin Thursby Apr 18 '17 at 16:02
  • $\begingroup$ Eusocial insects tend to communicate with pheremones - I'd imagine that the venom of the biter spiders would cause injuries that emit a foul odor, which the stalker spiders follow. $\endgroup$ – Tacroy Apr 13 '18 at 16:42
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    $\begingroup$ This deserves more upvotes than it has. $\endgroup$ – FoxElemental Sep 9 '18 at 21:19
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Simple answer: your giant spiders aren't really all that much like regular arachnids.

In detail: your basic need is to figure out which features or regular spiders will scale and which ones won't, while preserving the (at least perceived) sportiness.

Exoskeleton: this should scale okay. You might have some features such as legs and joints a little overscaled, as they may need space for actuation. Animals of large scale already possessing exoskeletons are lobster (and other creations), turtles (perhaps this is a stretch, bit the mechanisms could be carried over), and armadillo (again, not strictly an exoskeleton, but an option).

Number of limbs: this should be able to stay. More limbs means each one doesn't need to be as powerful, although your spiders probably won't be breaking land speed records, as the increase in limbs around a comparable space for animals of a similar size means less space for limb operation, and thus a higher number of limb direction reversals over a given distance than a creature with fewer legs.

Cardiopulmonary: give it lungs and a heart like all the other big animals and it'll be just fine. The lungs could work off a relatively fast moving pumping muscle that forces air around the gas exchanges in its abdomen and out through backflow-blocking cartilage somewhere. Or they could be more material, perhaps with a shared wall diaphragm, where one lung contracts while the other expands to maintain constant volume.

Actuation and digestion: this may be more tricky. You could use the fluid pumping method of smaller arachnids, but this would require significant musculature and hydrolics in what will be an already crowded body. Here's the note in digestion: a spider typically emulsified and drinks it's prey. In a large scale, this may not be practical. They'll need a more mamillian digestive tract to extract the amount of energy they need from the amount of prey then y can catch. Big predators like wolves (in packs) or cougar have to for days between meals sometimes. If megarachnid gets all her food from just the available juices, even with acid spit up for some predigestion of solids, it simply won't last long enough between meals to be competitive with other predators. That said, moving back to actuation , the legs may actually be pretty easy. An exoskeleton gives you a whole lot of room for muscle. And, when you have eight legs, you don't need as much power in each leg. It would probably be just fine by that reasoning to replace the hydraulics with some muscle tissue.

Webs: not specified in the original question, but worth dealing with. Spider webs are made almost entirely of special proteins. So, your spider needs a lot of high energy density food (definitely a predator then). And, while it may seem like that would be a problem on large scale, it may not. Spider silk is close to steel for tensile strength and much tougher on a deformation v. strength curve. It would probably not be inconceivable then for a spider of this magnitude to spin enough silk for low quantity regular use or the occasional net trap or web nest.

Spider appearance: let's see, eight legs, exoskeleton, more than 2 eyes, general scariness—yep, I think you're good!

Edit: to address the pulling capacity: a 30 to 40 pound dog can drag its own weight if the distance is not to great. If this spider has similar musculature spread around eight legs, it could do the same. If it had double the musculature (that is, similar per-leg musculature to the dog), it could be half the size (of said dog, an example of which would be a large Spaniel or a small Labrador or Bulldog).

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  • $\begingroup$ How large would they have to be to drag 30-40 pound prey? $\endgroup$ – Erin Thursby Apr 18 '17 at 4:11
  • $\begingroup$ Pulling capacity addressed in edit. $\endgroup$ – user29032 Apr 18 '17 at 4:28
  • $\begingroup$ Spiders already have lungs. They are not insects... $\endgroup$ – AlexP Apr 18 '17 at 12:55
  • $\begingroup$ @AlexP they're not the common manifestation of lungs though, they don't really breathe $\endgroup$ – Callum Bradbury Apr 18 '17 at 14:51
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You should take a look at Coconut Crabs:
enter image description here which are amazing/frightening depending on one's perspective and distance from them. They can spend long periods out of the water and can do fairly dexterous tasks, as shown!

More info in these critters at: https://en.wikipedia.org/wiki/Coconut_crab

Among other things, they can climb coconut trees, cut the nuts free and then break the nuts open for food. Imagine if they could move faster, e.g. with greater O2 concentration or some other bit of handwavium....

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  • $\begingroup$ Yes, I have seen these. I need to know how big my spiders would have to be in order to drag 30 or 40 pounds. $\endgroup$ – Erin Thursby Apr 18 '17 at 15:39
  • $\begingroup$ "They can spend long periods out of the water" actually adults spend their entire lives out of the water, they'll drown if submerged. They drop their eggs in the surf, and spend their larvae stage at sea, but must return to land when mature. $\endgroup$ – apaul Apr 18 '17 at 15:52

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