The answer is yes and no. It turns out that someone wasn’t very creative when naming little critters with long legs, and it can lead to a lot of confusion!
The not-spider
The critters most commonly referred to as daddy longlegs are not spiders. They are Opiliones and also known as harvestmen. They are also arachnids, but are more closely related to scorpions than to spiders. A few characteristics differentiate Opiliones from spiders. For starters, Opiliones just have two eyes. They cannot produce silk and therefore cannot make webs. They also lack venom glands. Spiders usually have two distinct body segments, but Opiliones have a compact oval-shaped body. It looks like one segment but the cephalothorax and abdomen are fused together so that the joint is hard to see.
The spider
Where it gets confusing is that there is also a spider called the daddy longlegs spider. They are also called cellar spiders. They are pale grey or tan and can have markings such as banding or chevron markings. Cellar spiders have typical spider features, like six or eight eyes and two distinct body segments. They produce silk and build a messy irregular web in corners. The threads are not sticky but used to alert the spider when prey has hit the web.
Are they the most venomous in the world?
There is a myth floating around that daddy longlegs are the most venomous spiders, but their fangs are too weak to bite a human. I have to admit that someone told me this years ago at summer camp (there were always dozens of them in the showers). However this statement is wrong for both daddy longlegs. The Opilione doesn’t even have any venom glands. The show Mythbusters took on this myth and found that black widow spider venom is much more toxic than the venom of the daddy longlegs spider. There aren’t any cases of this spider causing any harm to humans and no evidence that they are dangerous.
On a side note
Crane flies are also colloquially referred to as daddy longlegs because, you guessed it, they have long legs. Even though they look like giant mosquitos, most of the adults live for such a short time (only to reproduce) that they don’t eat anything.
All three of these daddy longlegs are found all over the world and are common in houses. Now you know what they really are!