Arthropods (from the Greek arthron “joint” and podos “foot”) represent the vast majority of animal species on earth (Fig. 2.1): About 62% of known species belong to this phylum! (Fig. 2.2). There are typically five major subdivisions (Fig. 2.3): (1) Malacostraca, including higher crustaceans such as crabs and shrimp; (2) Hexapoda, primarily insects, plus springtails, diploures, and protoures; (3) Myriapoda, including centipedes (e.g., Scolopendra subspinipes); (4) scorpions; and (5) spiders. This book focuses on terrestrial organisms, which are more exposed to sunlight, with insects being the majority, hence the title. Properties of other classes, such as arachnids (e.g., spiders, scorpions), are discussed when they present some remarkable particularities. A simplified arthropod phylogeny is presented in Fig. 2.4.