ABSTRACT
The Arachnida, a class of arthropod animals that includes prominent examples such as spiders, ticks and scorpions (Figure 10.1), comprises some of the most successful biological radiations on Earth. The lineage is extremely ancient and has a fossil record that dates back to the Palaeozoic (Dunlop 2010), but it is also highly diverse, with some 114,000 named species (Zhang 2013). The vast majority of arachnids are terrestrial, but some, such as water mites and marine mites, have independently evolved an aquatic lifestyle (Walter and Proctor 2013).
