I have seen quite a number of harvestmen - also known as daddy long legs - in the last couple of weeks, and realised I hadn't posted on them yet. At a distance harvestmen look like skinny, long-legged spiders, but a close-up look reveals they are not spiders (see above a female possibly of Opilio canestrini). Harvestmen belong to the order Opiliones, within the class Arachnida, and they are related to mites, spiders and scorpions and so they have eight legs. They can be distinguished by their fused body, as opposed to the cephalothorax and abdomen of spiders, and more similar in looks to that of scorpions -but without the tail. They have two eyes close together on top of their body, on a tubercle, unlike spiders which have eight eyes. Their legs are very long, and the first pair is even thinner and longer and acts like antennae, constantly sensing the ground like a blindman's stick. They also have a pair of pedipalps and chelicers. Unlike spiders, harvestmen produce no silk, have no venom and are generalist feeders. They have two forms of defence: if trapped they emit a foul-smelling substance which acts as a deterrent to predators. They also shed their legs easily and the severed leg carries on moving for a while, which allows them to scape while the potential predator is distracted with the moving leg. One of the harvestmen I saw yesterday (below, possibly female Phalangium opilio) had only one leg on one side of the body: it wasn't the most elegant walker, but it managed to move quickly enough.
Another important difference from spiders is that harvestmen actually copulate, as males have an intromitent organ. This drawing, from Hillyard and Sankeys' monograph, shows the larger female and the male facing each other 'in copula'.
More information Harvestmen: keys and notes for the identification of the species.1974 By P. D. Hillyard, John H. P. Sankey.Here.