Spiders and such (Arachnida)

It seems to be a simple rule: all small animals with 6 legs are insects and all small animals with 8 legs are spider-like arthropods. Unfortunately the mere feature of having 8 legs does not mean that their owners are closely related. However, counting 6 or 8 legs is a useful rule-of-thumb to distinguish between the numerous species belonging to the insects. Small animals walking on 8 legs are usually well known: spiders, harvestmen, scorpions and ticks. Yet the animals differ considerably from one to another. Spiders have a body consisting of three parts, just like insects have: head, thorax and abdomen. The head and thorax however, have fused into one visible structure called the cephalothorax. Ticks have only two bodyparts: head and body. Harvestmen even consist of one part, the body, only. This feature shows obviously that these animals are not closely related at all. And to make the matter even more complicated: these 8 legs are not always visibly present (yet). A newly born mite or tick has 6 legs only. Worldwide there is an impressive number of different spiders. The number of mites is considerable as well, even though many are rarely ever seen. They are small and most are parasites to mammals, birds or insects. There are far less harvestmen species. Of the pseudoscorpions only some 15 species are present in the Benelux. Many spiders and harvestmen are relatively big while pseudoscorpions and mites are much smaller. Many of these species can hardly be seen by the naked eye.