It turns out that the notion is false on both counts. But a little clarification is needed.
According to entomologists at the University of California, Riverside, the term "daddy longlegs" is commonly used to refer to two distinct types of creatures: opilionids arachnids with pill-shape bodies and eight long legs that are actually not spiders, and pholcids, which have long legs and small bodies, and thus resemble opilionids, but which are true spiders.
Opilionids true daddy longlegs live in moist, dark places and eat mostly decomposing vegetable and animal matter. "They do not have venom glands, fangs or any other mechanism for chemically subduing their food," the UC entomologists write on their website. "Therefore, they do not have poison and, by the powers of logic, cannot be poisonous from venom. Some have defensive secretions that might be poisonous to small animals if ingested. So, for these daddy longlegs, the tale is clearly false."
Pholcids, or daddy long-legs spiders, are venomous predators, and although they never naturally bite people, their fangs are similar in structure to those of brown recluse spiders, and therefore can theoretically penetrate skin. For these reasons, "This is most probably the animal to which people refer when they tell the tale," the entomologists assert.
But is pholcids' venom extremely poisonous? Surprisingly, because they almost never bite, scientists have never bothered to conduct research to determine their venom's toxicity to humans . In 2004, the Discovery Channel show "Mythbusters" stepped in to fill this knowledge void. The team set out to coax a daddy longlegs spider into biting the arm of the show's co-host, Adam Savage.
Their official conclusion? Myth busted. The spider was able to penetrate Savage's skin, and he reported nothing more than a very mild burning sensation from the venom that lasted just a few seconds.