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The claim is the Daddy Long Legs (Pholcus phalangioides) spider is the most venomous spider in the world, but that its fangs are too small to be able to penetrate human skin.

The wikipedia article calls it an urban myth and links through to some supposed "research", but its just some guy claiming:

There is no scientific basis for the supposition that they are deadly poisonous and there is no reason to assume that it is true.

Which I don't feel a lack of evidence should stand as claimed scientific research.

There is also a link to a myth busters episode where they apparently get a Daddy Long Legs spider to bite someone.

Is it possible for the Pholcidae spiders fangs to penetrate the skin and has there been any official measurement into the common dose size and toxicity of the venom?

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+1 - Ha! I was always told this as a child. Nice question. –  user2466 Jul 22 '11 at 9:50
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Maybe in 100,000 years when it will be evolution's response to children constantly pulling their legs off. –  LarsTech Jul 22 '11 at 14:18
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I had always heard this as a child as well. –  Monkey Tuesday Jul 22 '11 at 22:36
    
What we in the UK call a daddy long-legs isn't a spider at all, but an insect. And it's not poisonous. –  Daniel Roseman Jul 28 '11 at 10:17
    
@LarsTech - yes, but then the response of evolution might be to add poisonous spines to the legs. –  user3344 Sep 17 '13 at 13:57

3 Answers 3

up vote 36 down vote accepted

It's a myth.

From the University of California, Riverside:

... There is no scientific basis for the supposition that they are deadly poisonous and there is no reason to assume that it is true.

There is no reference to any pholcid spider biting a human and causing any detrimental reaction.

If these spiders were indeed deadly poisonous but couldn't bite humans, then the only way we would know that they are poisonous is by milking them and injecting the venom into humans.
For a variety of reasons including Amnesty International and a humanitarian code of ethics, this research has never been done.

Furthermore, there are no toxicological studies testing the lethality of pholcid venom on any mammalian system (this is usually done with mice). Therefore, no information is available on the likely toxic effects of their venom in humans, so the part of the myth about their being especially poisonous is just that: a myth.

What about their fangs being too short to penetrate human skin?

Pholcids do indeed have short fangs, which in arachnological terms is called "uncate" because they have a secondary tooth which meets the fang like the way the two grabbing parts of a pair of tongs come together.

Brown recluse spiders similarly have uncate fang structure and they obviously are able to bite humans.

There may be a difference in the musculature that houses the fang such that recluses have stronger muscles for penetration because they are hunting spiders needing to subdue prey whereas pholcid spiders are able to wrap their prey and don't need as strong a musculature.

So, again, the myth states as fact something about which there is no scientific basis.


A video of the MythBusters can be watched here:

Supposedly, daddy longlegs possess extremely powerful poison, but their fangs are too short to penetrate human skin.

To find out, Jamie and Adam hunted down a host of daddy longlegs and took them to a spider specialist who could milk out their venom.

Next, the spider specialist compared the toxicity of daddy longlegs venom to black widow venom (on mice). The red-bellied widow won out, busting the myth.

A microscopic measurement of the long-legged spider's fangs proved their miniscule quarter-millimeter length could puncture human skin, taking a double bite out of the daddy longlegs myth. [Source]

As seen in the video Adam did let himself get bitten and only felt "a tiny little burning ".

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one of the better (scientifically speaking) Mythbusters episodes :) –  jwenting Jul 22 '11 at 13:15
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@jwenting: Indeed, the didn't even blew up the spider afterwards! –  Martin Scharrer Jul 23 '11 at 10:37

This is from the Straight Dope:

Herman Vanuytven came forth from the Arachnology Home page (http://www.ufsia.ac.be/Arachnology/Arachnology.html). Herman's response:

"The problem with the 'Daddy Longlegs' is that the name is used for several kinds of animals, 2 of them arachnids:

1) The family Phalangiidae (a part of the opilionids [harvestmen], a different order than spiders)

2) a spider: Pholcus phalangioides

Number one doesn't have poison glands. Number two has poison glands but as far as is known in the scientific world, nobody has ever been bitten by one of them. It's not sure if the poison has ever been investigated since there was no need for it."

So, in short, this has never been investigated, and it hasn't been determined whether or not Daddy Long-legs are the most poisonous spiders. It is possible that they are the most poisonous spiders, but it hasn't been found out.

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Of course, if it has never been investigated, the chances of it being the most poisonous, out of the at least thousands of types of spiders, is rather small. –  thursdaysgeek Jul 25 '11 at 23:05

If the Pholcus phalangioides spider is not venomous to humans and their fangs cannot penetrate human flesh then how come a woman has just been bitten by one of these spiders and hospitalised in Hull, Great Britain due to the bite of one?

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Eileen: Welcome to Skeptics!. You didn't provide any reference to support your claim. @ChrisW kindly added one but it demonstrates two things: the genus of the spider is not clearly known, and even if it is known, it is just the genus not the species. –  Oddthinking Sep 17 '13 at 12:50
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The fact that she was prescribed antibiotics indicates that the severity of the injury wasn’t due to spider venom but rather a secondary bacterial infection: antibiotics only work against bacteria, nothing else. –  Konrad Rudolph Sep 17 '13 at 12:57
    
The delayed reaction (of at least one day according to the article), as well as the blisters, are consistent with a bacterial skin infection. –  ChrisW Sep 17 '13 at 13:04
    
The mother-of-four was hospitalised for two days after the bite turned bright red and continued to grow in size. The spider was a pale-whitish colour and had a really small body, but really long legs. A woman at pest control has since told her it could be a Genus Pholcus spider. This was reported in September 2013 in Hull, Great Britain. –  Eileen Hobkirk Sep 17 '13 at 13:06
    
Could be is no evidence at all. It's hearsay, based on the observation of somebody with no expertise in this field. As for the redness, this is more likely to be a symptom of infection or mild allergic reaction than any evidence of toxicity. –  itsbruce Sep 18 '13 at 21:51

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