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This is a factoid I heard as a kid at some point, probably on a playground somewhere, and just accepted without question in the way that children do:

All spiders are venomous, but most of them aren't venomous enough to harm humans.

(I'm pretty sure the wording used was actually "poisonous", because the distinction between venom and poison is a little too technical for a nine-year-old, but I understood it at the time to mean what I now know is called "venomous".)

Is this true? Are all spiders venomous? Or is this just one of those made-up "facts" that circulates playgrounds and campsites?

Hearth
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  • If threre are herbivorous spiders, then no, not all spiders have venom. – anongoodnurse Dec 15 '23 at 19:43
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    @anongoodnurse I have never heard of herbivorous spiders. Are there some? That also doesn't mean they don't have venom for self-defense or just vestigially, either. – Hearth Dec 15 '23 at 19:44
  • There is an herbivorous spider, yes, Bagheera kiplingi. It may have been too subtle, but my point was, you should have researched this a bit before posting. We get a lot of "I heard..." questions here, which is rarely a reliable source. – anongoodnurse Dec 15 '23 at 19:47
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    @anongoodnurse I did look it up, but the state of search engines right now means it's hard to find anything, especially anything debunking common misconceptions and playground rumors (you're more likely to get some LLM arguing that they're true). – Hearth Dec 15 '23 at 19:55
  • Um... I don't believe you're correct. I'll leave it at that. – anongoodnurse Dec 15 '23 at 19:59
  • Try (not for herbivorous spiders): Uloboridae. – Jiminy Cricket. Dec 15 '23 at 20:00
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    @anongoodnurse Bear in mind that I'm an electrical engineer, not a biologist or zoologist or anything, and I almost certainly don't know the right terms to make a search engine give me actual scientific articles and not "10 cool facts about spiders you won't believe" repeated a million times. Yes, I'm exaggerating, but it's hard to find information that's actually reliable these days. – Hearth Dec 15 '23 at 20:02
  • https://s3.amazonaws.com/scschoolfiles/552/non_venomous_spiders.pdf – Nilay Ghosh Dec 16 '23 at 05:19
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    @NilayGhosh Beware of the information in your link, it states they are non-venomous, but that is incorrect in absolutely every case listed there. They may have venom insignificant to most humans, but that's a different thing. – Jiminy Cricket. Dec 16 '23 at 07:52
  • @anongoodnurse: B. kiplingi aren't 100% herbivorous, though, and will sometimes hunt e.g. ant larvae. I couldn't confirm with a quick Google search whether they have venom, but it seems that most jumping spiders (Salticidae) do and use it as a key part of their hunting method, so there's a decent chance that B. kiplingi have also retained the trait. – Ilmari Karonen Dec 16 '23 at 17:45
  • I would add that the venom of many (most?) spiders is technically dangerous indeed to humans, but because it is administered in such a small dose, it is not efficient. Also, while all spiders can bite, most of them won’t bite humans (except in rare cases of extreme self-defence), as they “know” their fangs are not long enough to pierce the skin. (In Québec, no spiders are harmful to humans, although we are starting to see some Latrodectus mactans—the (in)famous black widow spider.) – Pierre Paquette Dec 16 '23 at 17:51
  • @IlmariKaronen - I've read that the way they 'eat' ant larvae has never been explained. However, that wasn't really my point. I guess subtlety is not my forte. I had (erroneously) assumed all spiders were venomous as they prey on animals. – anongoodnurse Dec 16 '23 at 17:54
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    @PierrePaquette Is it true that many spiders' fangs are too short to pierce human skin? That's another thing I heard on playground rumors (usually in the form of "(whatever spider species they just saw) is the most poisonous spider but it's harmless to humans because its fangs are too short"; I expect the "most poisonous spider" part is wrong, but I'd never realized the "fangs are too short" part was actually true. (The spider in question in that rumor was usually the daddy longlegs in my experience, which I know isn't even a spider.) – Hearth Dec 16 '23 at 17:55
  • @Hearth: I would almost say that most spider’s fangs are too short, as there are multiple species of very small spiders—those I find in my apartment rarely exceed 2–3 mm in body length (when measuring a spider, only its body length is considered). Those definitely have fangs that are too short to pierce human skin! Even a black widow’s fangs are only about 1 mm in length: long enough to pierce skin, but it would rather run away than bite someone! (This does NOT mean you should try to manipulate them!) Only larger spiders have fangs that are long enough to pierce human skin. – Pierre Paquette Dec 16 '23 at 20:16
  • @Hearth There are three different kinds of creatures that are called "daddy longlegs". One of the three is a spider. – Dawood ibn Kareem Dec 16 '23 at 21:54
  • Note that "factoid" means something that looks like a fact but isn't, which in this case is indeed accurate. – OrangeDog Dec 18 '23 at 13:59
  • @OrangeDog https://www.merriam-webster.com/dictionary/factoid both "an invented fact believed to be true because it appears in print" and "a briefly stated and usually trivial fact" are definitions. – Bryan Krause Dec 18 '23 at 19:46

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I searched Google for "non-venomous spiders".

The first result that came up was Uloboridae (Wikipedia):

Uloboridae is a family of non-venomous spiders, known as cribellate orb weavers or hackled orb weavers. Their lack of venom glands is a secondarily evolved trait. Instead, they wrap their prey thoroughly in silk, cover it in regurgitated digestive enzymes, and then ingest the liquified body.

However, non-venomous spiders are atypical, and it is true that most spiders that humans don't consider harmful are still venomous to their prey; the playground statement is a reasonable approximation.

Laurel
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Bryan Krause
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    I don't know why this got a downvote; it definitely answers the question. Have an upvote. It didn't occur to me to use that specific search term; I just tried "all spiders poisonous", "is it true that all spiders are poisonous", and so on--which didn't turn up any useful results. – Hearth Dec 16 '23 at 02:11
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    @Hearth Which makes sense, because being poisonous is not the same as being venomous. Venom is injected, poison is ingested or touched, so for example Pitohui dichrous is poisonous but not venomous, while Acharia stimulea is venomous but not poisonous, despite physical contact being the hazard for in both cases. – Austin Hemmelgarn Dec 16 '23 at 13:43
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    @AustinHemmelgarn I didn't really realize that "venomous" would be the correct term until I was typing up the question here, yeah. It's not a distinction that comes up often when working as an engineer! – Hearth Dec 16 '23 at 13:45
  • You know, after this description I have decided that I actually prefer the venomous spiders ;-). – Peter - Reinstate Monica Dec 19 '23 at 01:26
  • Considering that all spiders inject digestive acids into a prey, the method and speed of the venom is debatable. Here's an interesting spitting spider: https://www.youtube.com/watch?v=vUUm2OCMCzY they catch their prey by spitting a fluid that congeals on contact into a venomous and sticky mass. The fluid contains both venom and spider silk in liquid form, though it is produced in venom glands in the chelicerae. The venom-laced silk both immobilizes and envenoms prey such as silverfish. https://en.wikipedia.org/wiki/Spitting_spider found in America. – bandybabboon Dec 21 '23 at 07:37