Every year in late summer / early autumn we receive a shedload of questions about these creatures. "What sort of spider is this?" "Is it a tropical spider?" "Is it a poisonous spider?" "Will it eat my children / dog / cat / grandmother etc?"
These creatures certainly look like spiders, and they are a kind of arachnid. They have eight legs (long ones), and they even move in a spider-like way, but they're not actually spiders.
They're known as 'Harvestmen' because of the time of year they appear - harvest time. They're also sometimes called 'daddy long legs' or 'opiliones'.
Although they're normally active at night you'll probably find some right now if you look outside your house, under the window ledges.
They differ from spiders in that they have no fangs, and they have a one part body. Spiders have two part bodies (and insects as you know have three part bodies). Fossil harvestmen found in the Aberdeenshire area (Rhynie) show that the basic structure of these creatures has hardly changed over the last 400 million years.
Harvestmen don't produce silk, they have just two eyes and no venom glands. In fact they're relatively harmless. Their main defence mechanism is a pair of glands which produce a smelly odor. The stink glands are positioned at the sides of the eyes. If you pick one up you might see a drop of fluid forming in this area. Sniff your fingers if you want to get the full effect!
They do have one other defensive action which involves losing a limb. Harvestmen are able to shed a leg. If you try to handle one you might find that a leg falls off in your hand. The leg will continue to twitch about for a while, which is often long enough to distract a would-be-predator (or inquisitive naturalist) while the harvestman makes its escape.
Although this trick can help the creature escape from its predators it seriously reduces its survival potential. The legs, especially the second pair, are the harvestman's sensory organs. They act as nose, tongue, ears and eyes. They're literally loaded with nerves, each containing thousands of tiny sense organs, all housed inside minute slits in the legs.