Spiders have two noticeable body sections separated by a waist, while opiliones do not have a noticeable waist, so they appear to only have a single body segment. The bodies of opiliones are usually about 1/4 inch long, but their legs can be as long as 2 inches. If an opilione had a body the size of an adult male human, its legs would be over 40 feet long. Spider bodies vary greatly in size, from about 1/10 inch, to about 3.5 inches, with a leg span up to 10 inches. All spiders have six or eight eyes, while opiliones only have two.
Opiliones, unlike spiders, do not have silk glands, or spinnerets, so they do not spin webs. Not all spiders, such as tarantulas and wolf spiders, spin webs, but many do. Spiders use webs to catch prey, swing from place to place, enclose egg sacs and line their burrows. The thread is spun from one of seven pairs of spinnerets located in the middle or at the end of their abdomens. Each silk-producing spider can produce different types of silk, depending on the set of spinnerets used, as well as different types of webs, depending on the purpose of the web. Spiders eat old webs to recycle the silk. Spider web silk is about 10 times stronger than steel.
Both spiders and opiliones have fangs, but opiliones do not have venom glands. Almost all types of spiders have venom glands and are poisonous, but not usually deadly to humans because their fangs are too weak or too short to puncture our skin. Also, a spider's venom is usually only deadly to its prey, and not humans, because of the size of the prey. Some spider bites, like the black widow's, are painful and can cause cramps, nausea and vomiting, and rarely, death. To protect themselves from predators, opiliones emit a repulsive scent from odiferous glands.
Male opiliones, unlike male spiders, have a penis, so there is direct copulation. Male spiders, on the other hand, produce a sperm web, release sperm into the web and then dip one of its pedipalps -- which looks like an antenna with a bulb at the end -- into the sperm. He then puts the pedipalp into a duct on the female that leads to a seminal receptacle. Female opiliones lay several batches of eggs in different locations in the ground and never return. Spiders spin a protective web around the eggs and then some watch over them in a nest, while others carry them around with them. Opiliones can lay up to several hundred eggs, while some spider species can lay up to 7,000 eggs.