People often mistake the daddy longlegs, also called the harvestman, for a spider. Admittedly, it does have some spider-like qualities. Daddy longlegs do have eight legs, and tend to skitter about the way spiders do. We often see them in the same places where we see spiders. They are arachnids, just like spiders. But that's where the similarities end.
First, the daddy longlegs belongs to the order Opiliones. In opilionids, the head, thorax, and abdomen are fused widely into one. Spiders, of the order Araneae, have a distinct waist between the cephalothorax and the abdomen. Opilionids have just two eyes, compared to the usual eight in spiders.
Second, daddy longlegs do not produce silk. They don’t spin webs, and they don't use silk to capture prey. If you find one in a web, it doesn't live there. It's about to be devoured by the spider that built the web. Daddy longlegs stink when threatened, thanks to defensive stink glands.
Anyone who's tried to catch a daddy longlegs knows they have a tendency to shed their legs. Grab one by the foot, and it promptly lets go of the entire leg and runs off. My clumsy childhood efforts to capture harvestmen often resulted in one-legged arachnids walking in circles.
As an adult with some knowledge of arachnids, I feel a bit guilty about my early years tormenting daddy longlegs. Their legs are not just vital to locomotion, they're also nerve centers. Through its legs, the daddy longlegs may sense vibrations, smells, and tastes. Pull the legs off a harvestman, and you are limiting its ability to make sense of its world.
Finally, daddy longlegs have penises, and spiders do not. Spiders use an indirect method of transferring their sperm to females. The daddy longlegs, on the other hand, has a specialized organ capable of doing the job directly.