In this chapter, we examine the nature of those animals that collectively form the biggest group in the animal kingdom – Arthropoda – and explore their evolutionary relationships, both with each other and with other animals. The Arthropoda is the most species-rich phylum by a very long way. There are well over a million species of arthropods (most of them insects); this means that they make up more than three-quarters of the known animal kingdom. Arthropod species inhabit almost all environments on Earth, from the deepest oceans to the driest deserts.

The key feature underlying the success of the arthropods is the exoskeleton. In fact, a hard structure acting as, among other things, an anchor-point for muscles, is a key feature in the second- and third-largest groups of animals too. Most molluscs have a shell, or a pair or series of them, as we noted in Chapter 12; and species of our own group, the vertebrates, are characterized by possession of endoskeletons. Not only do all these hard structures provide a key part of a crucial biomechanical system that allows movement, but they also allow more frequent fossilization than do soft tissues.