According to answers.com, Pholcus Phalangioides, commonly known as daddy long legs, cellar spider, or skull spider, is a species of spiders belonging to the Pholcidae family. It is a synantropic species, which can be often found in human houses. It feeds on different insects, from flies to other spiders. It can attack and devour pray that is much larger than itself, even woodlice. When they run out of food, they resort to eating their own kind! The habitat of the species was initially limited to the Palearctic warmer regions. It is currently spread throughout the continents, except Antarctica. It cannot survive cold weather, so it is restricted to human houses during the winter, where it often finds shelter in the cellars or lofts. They breed throughout the year, as they are not influenced by climate changes.
The daddy long legs spider is considered useful in many parts of the world because it can kill and devour other dangerous species like the redback spider or the hobo spider. Daddy long legs has become famous because it is known to attack the Black Widow spider, an extremely venomous species. This means that the daddy long legs venom is extremely toxic. However, after a thorough analysis, researchers have found that the daddy long legs venom is not nearly as powerful as that of the Black Widow spider. Although it can kill other insects, its venom is not potent enough to harm humans. Daddy long legs spiders do have venom glands.
Although its venom is strong enough to kill other species of spiders, including the Black Widow,i t can not bite you because the fangs are to small and to weak to bit human skin. There is no scientific evidence that they can harm humans and there is no document about a person being bit by a daddy long legs and having any adverse reaction. There's a myth about this species saying that their fangs are too short to bite humans. But in fact they are not any shorter than those of other species known to bite humans. Its fangs are very similar to those of the brown recluse spider, an extremely venomous species which does bite. They never attack humans and usually hide when they feel their presence.