Fallacious yet widespread and documented beliefs courtesy of Wikipedia.
According to urban myth, the daddy longlegs spider (Pholcus phalangioides) is the most venomous spider in the world, but the shape of their mandibles leaves them unable to bite humans, rendering them harmless to our species. In reality, they can indeed pierce human skin, though the tiny amount of venom they carry causes only a mild burning sensation for a few seconds. In addition, there is also confusion regarding the use of the name daddy longlegs, because harvestmen (order Opiliones, which are not spiders) and crane flies (which are insects) are also known as daddy longlegs, and share (also incorrectly) the myth of being venomous.