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 ".
