[BigV]

From here.

In the animal class Arachnida, there are several lower level divisions called Orders. Scorpions are in the Order Scorpiones, spiders are in the Order Araneae, ticks and mites are in the Order Acari.

The creatures most correctly called daddy-longlegs are in their own separate Order which is Opiliones. Common names for this Order are 1) daddy-longlegs, 2) harvestmen and 3) opilionids. They are characterized by having one basic body segment which shows segmentation on the posterior portion, at most 2 eyes and all 8 legs attach to the pill-like body segment.

Another creature often called daddy-longlegs are actually spiders. These long-legged spiders are in the family Pholcidae. Previously the common name of this family was the cellar spiders but arachnologists have also given them the moniker of "daddy-longlegs spiders" because of the confusion generated by the general public. Because these arachnids are spiders, they have 2 body basic body parts (cephalothorax and abdomen), have 8 eyes most often clumped together in the front of the body, the abdomen shows no evidence of segmentation, have 8 legs all attached to the front most body part (the cephalothorax) and make webs out of silk. This is most probably the animal to which people refer when they tell the tale because these spiders are plentiful especially in cellars (hence their common name) and are commonly seen by the general public.

Possible envenomation

Is there any truth to this oft-repeated tale?

Daddy-longlegs (Opiliones) - these arachnids make their living by eating decomposing vegetative and animal matter although are opportunist predators if they can get away with it. They do not have venom glands, fangs or any other mechanism for chemically subduing their food. 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-long-legs, the tale is clearly false.

Daddy-longlegs spiders (Pholcidae) - Here, the myth is incorrect at least in making claims that have no basis in known facts. 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. There is no scientific basis for the supposition that they are deadly poisonous and there is no reason to assume that it is true.