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Daddy Longlegs Spiders, officially "Pholcidae" are known to be the worlds most venomous spiders, sort of.

We are supposedly unaffected by them because their fangs are too small, the shape of their fangs, or their venom doesn't affect us due to size.

According to Rick Vetter of the University of California at Riverside, the daddy long-legs spider has never harmed a human and there is no evidence that they are dangerous to humans. - https://en.wikipedia.org/wiki/Pholcidae

I would like to know how large one of these friendly but venomous spiders would need to be to cause significant harm/damage or even death to an average sized human. I'm creating something along the lines of the movie Eight Legged Freaks (2002)

Any information or speculation is appreciated!

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    $\begingroup$ "to cause significant harm/damage or even death". Significant damages such as? Squishing a human? Bus sized, I'd say. Spearing one through the torso with a leg? Car sized. Maim by biting, big dog should be good (I'm eyeballing this one, don't take my word for it). Venom? Probably hard to accurately guess, since you seems unsure about the reason why it doesn't affect us. I think you should rephrase to indicate by which means you want the "significant damage" to be done ;) $\endgroup$ – Nyakouai Aug 19 at 15:01
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    $\begingroup$ Read the rest of the wiki entry that you linked. They did a myth busters, and it looks like your premise that they are "known to be the worlds most venomous spiders" is quite incorrect. They seem to have relatively weak venom. $\endgroup$ – Mathaddict Aug 19 at 15:09
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    $\begingroup$ As usual, just scaling up the current design results in a non-functional organism. Respiration is often the limiting factor in scaling up insects, I suspect spiders suffer a similar limitation (though I don't know literature for this). $\endgroup$ – Gary Walker Aug 19 at 15:54
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    $\begingroup$ Spiders or insects large enough to kill Humans (without being extremely venomous) are fortunately impossible, though that doesn't stop filmmakers from sometimes making hit movies about it. And there are examples in literature, such as Shelob in Lord of the Rings. If you decide that some of your spiders get dissected, you may be able to put in a description of what enables them to function at their size to make it seem more plausible. $\endgroup$ – M. A. Golding Aug 19 at 19:36
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    $\begingroup$ @Sean "Daddy longlegs" includes different groups. Opiliones is not a spider. Pholcidae are spiders and are venomous. And sometimes tipulidae, which is not a spider either, sometimes get called daddy longlegs. These all have different common names as well, but they do share the common name "daddy longlegs". $\endgroup$ – forest Aug 20 at 4:20
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As noted in the comments, the Wikipedia article you cited contains a description of a Mythbusters episode in which Adam Savage allowed himself to be bitten by a daddy-long-legs. He experienced a "mild, short-lived burning sensation." From this we can conclude that pholcidae can bite humans, but their venom is not toxic to us.

While the venom of pholcidae has never been specifically studied, we can extrapolate some things from the study of other spider venom. Venom is characterized into two categories; necrotic and neurotoxic. Because Mr. Savage did not experience any ill effects to his nervous system, like the paralysis that often results from the bite of the black widow, we can assume that pholcidae venom is not neurotoxic, at least to humans. So, it's necrotic. This means that the main active components are peptides, and the main purpose of the venom is to aid the external digestion of the spider. This lines up with what we know about the hunting behavior of pholcidae; they immobilize their prey with silk-like threads and then envenom and consume them. It also lines up with Mr. Savage's experience; a mild burning sensation is consistent with being partially, mildly digested. It was a very small amount, so his body was able to very quickly recover from the damage.

This has lots of potential in a horror scenario. Victims do not die instantly of a poisonous bite; they are entombed in silk and then injected with acid that burns them alive very, very slowly, while a giant spider takes small bites out of the burned bits. That's terrifying. You just need to make the pholcids large enough that their webs could catch and hold a person. Pholcids are known to hunt huntsman spiders, which are roughly the same size as they are. So, your pholcids should be roughly person-sized.

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    $\begingroup$ A remarkably logical answer. Also horrifying. But logical! +1 $\endgroup$ – Joe Bloggs Aug 19 at 16:02
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    $\begingroup$ Nah, they'd die soon enough; people aren't as robust as arthropods and have stupid things like needing functioning muscles to breathe, and to have actual blood pressure to perfuse their bodies. Can you imagine that? Enough injected digestive enzymes to melt a decent mouthful of human would probably finish em off in a few hours, tops. $\endgroup$ – Starfish Prime Aug 19 at 16:40
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    $\begingroup$ @StarfishPrime I would argue that "a few hours" is a very long time to be alive while being digested, both externally and internally. $\endgroup$ – IAntoniazzi Aug 19 at 16:43
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    $\begingroup$ @JohnDvorak it won't have to be in the neck... the effect of a circulatory system is to ensure that large quantities of the venom will be well distributed. I suspect that any bite near a large vein will cause death from cardiac or respiratory failure in relatively short order. $\endgroup$ – Starfish Prime Aug 19 at 18:41
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    $\begingroup$ When you say they would need to be roughly the same size as a person, do you mean their body, or including their legs. I ask only to prepare for how scary my dreams will be tonight. $\endgroup$ – fixer1234 Aug 20 at 4:02
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While the question may suggest that it's looking for an answer involving the spider's venom, there is no such specification directly asked for. As such I challenge that a spider need only be large enough to choke on to cause death.

Perhaps these spiders of yours are prone to hunting slugs that like to dwell in shallow caves and confuse the human tongue for their prey and in the process of trying to hunt it wind up suffocating the unfortunate persons.

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As their venom would be useless against humans, the only ways such a creature could harm us would be if they were large enough that they could either crush a human being or their mandibles were large enough they can kill through the sheer force of their bite.

Each would require the spider to have a body at least the size of a human's, probably much larger.

At that size, its legs however would be unable to support the weight of the body unless you change the physiology of the spider to have legs that are far thicker and more muscular, which would potentially give it another way to inflict harm: striking with one of those legs.

With a total size for the creature maybe 4-5 times that of the body, you'd have a spider with a body 2 meters long and half a meter wide, with legs up to 5 meters long each.

At that size, it wouldn't need to bite or crush most people, they'd die of a heart attack just seeing one coming at them through fear :)

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  • $\begingroup$ Their venom is harmless only because at "normal" size, their fangs are too small and there isn't enough venom. That wouldn't be the case if they were a lot bigger. $\endgroup$ – fixer1234 Aug 20 at 3:57
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    $\begingroup$ @fixer1234 nope, it's also not potent enough. Though a spider the size I suggest would have enough venom that someone could drown in the stuff :) $\endgroup$ – jwenting Aug 20 at 4:05
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    $\begingroup$ @fixer1234 Fang size has nothing to do with it, that's just a common myth. "Not enough venom" is closer to mark - the venom doesn't really have any systemic effect, so you need enough venom to do large enough damage to the tissue to kill the human. Remember, most spider "venom" is really a digestive fluid - instead of eating their prey whole and digesting it in an internal cavity, they use the body of their prey as a "stomach". I suspect this would make a giant spider die of starvation soon, as the digested fluid would leak out of the necrotic tissue, rather than conveniently remaining inside. $\endgroup$ – Luaan Aug 20 at 7:13
  • $\begingroup$ Though granted, large spiders have little trouble consuming e.g. mice, so it's possible they'd get enough of a meal before the digestion reaches the skin. Most of the animal remains undigested and essentially worthless, but spiders don't exactly need a lot of food anyway, even if you scale them up (assuming that doesn't kill them, of course :)). $\endgroup$ – Luaan Aug 20 at 7:16
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    $\begingroup$ @Luaan Interesting follow-up question: are any spiders' web casings watertight enough that the fluid would remain inside even after digestion? I'm just musing here, but I suppose someone could ask on Biology if they were curious enough. $\endgroup$ – called2voyage Aug 20 at 15:33
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Daddy Longlegs Spiders, officially "Pholcidae" are known to be the worlds most venomous spiders, sort of.

We are supposedly unaffected by them because their fangs are too small, the shape of their fangs, or their venom doesn't affect us due to size.

According to the very same WP page you linked in the question, in fact the two paragraphs around the one quoted in the question, that is a myth:

There is a legend that daddy long-legs spiders have the most potent venom of any spider, but that their fangs are either too small or too weak to puncture human skin; the same legend is also repeated of the harvestman and crane fly, also known as "daddy long-legs" in some regions. Indeed, pholcid spiders do have a short fang structure (called uncate due to its "hooked" shape). Brown recluse spiders also have uncate fang structure, but are able to deliver medically significant bites.

(paragraph quoted in question removed)

The legend may result from the fact that the daddy long-legs spider preys upon deadly venomous spiders, such as the redback, a member of the black widow genus Latrodectus. To the extent that such entomological information was known to the general public, it was perhaps thought that if the daddy long-legs spider could kill a spider capable of delivering fatal bites to humans, then it must be more venomous, and the uncate fangs were regarded as prohibiting it from killing people. In reality, it is able to cast lengths of silk onto its prey, incapacitating them from a safe distance.

As for how big such a spider would have to get, assuming it keeps to its same predation behavior, it appears it would need to get big enough to kill the human with a normal (non-envenomed) bite:

The web of pholcids has no adhesive properties and instead relies on its irregular structure to trap prey. When pholcid spiders detect prey within their webs the spiders quickly envelop prey with silk-like material before inflicting a fatal bite. The prey may be eaten immediately or stored for later.

Wolves have been known to kill humans with neck bites. One would imagine a significantly smaller animal could pull off the same trick with an incapacitated human, (which is where the projected silk comes in). So the limiting factor is likely to be some combination of ability to produce enough silk to disable a human-sized creature, and mandible size.

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This is a bit of a weird question.

In North America we have two spiders that have medically significant venom - the Black Widow and the Brown Recluse. They are both small spiders, but rarely lethal. In fact, for most health youth and adults a bit from a Black Widow is only incredibly painful. The mechanism by which it causes death is actually because it causes paralysis, typically the diaphragm, causing suffocation.

I'm not aware of a brown recluse bite ever being responsible for a death, and they are also the most misdiagnosed spider bite.

All of that is background information - either your killer pholcid spider (not to be confused with a harvestman, which is an arachnid but not a spider) is going to require a different venom, or it's going to need to go through some pretty radical changes.

We know that their venom is not particularly potent. Unless you have some sort of severe allergy, you might get a mild burning and some redness in the extremely unlikely case of actually getting bitten.

So if we rule out any changes to venom (aside from perhaps the amount produced, due to size changes), we've basically just got to go with sheer size.

If you've hung around worldbuilding for long you'll know that the square cube law comes into play a lot when we're scaling stuff up. I don't know if the numbers could even pan out, but basically what we're looking for is going to be fang size. According to that Wikipedia article, their fangs are 0.25mm in length. Let's say we need their fangs to be 100 (about 4") long, based on the typical legal knife-length limit.

That's 400x the size. That means that their 10mm body will be 4,000 - or about 160 inches, or 13 feet. Their legs will be 20,000mm long, or 65 feet. That's just ridiculous. At that point they're probably able to cut you with the claws on their feet more effectively than using their fangs.

This says nothing else about the physiological changes they're going to need - their book lungs will no longer be effective, their hydraulic blood system will be problematic... in general, they're going to have a lot of problems, even if you can handwave that way.

Although on the plus side, they will definitely be large enough to kill an average human.

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    $\begingroup$ That would be sooo embarrassing, being eaten alive by a 65' rickety spider with circulatory and breathing problems. $\endgroup$ – fixer1234 Aug 21 at 21:40
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The size of a Brazilian Wandering Spider, if a mutation occurs in a species of Pholcidae so that it creates a cytotoxin venom instead of a necrotic venom.

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