Arthropods are a phylum within the animal kingdom. They include four classes: Chelicerates (such as spiders, mites, ticks, scorpions, and horseshoe crabs), the extinct Trilobites, Crustaceans (such as lobsters, crabs, and shrimp), and Uniramians (millipedes, centipedes, and the most numerous group of all, the insects). The defining features of arthropods are their exoskeletons (hard outer coverings), segmented bodies, and jointed appendages , from which they derive their name ( arthro means "joint," pod means "foot").
The exoskeleton, secreted by the outer tissue layer, is composed of protein and a nitrogenous carbohydrate called chitin , which in crustaceans is fortified with calcium carbonate crystals. To grow, most arthropods either shed (molt) the exoskeleton periodically or grow as soft-bodied larvae before undergoing metamorphosis into the adult, hard-bodied form. Some arthropods (such as millipedes) have legs on nearly every segment. However, most arthropods have evolved reduced numbers of legs, with many other appendages taking on highly specialized roles. Examples include the antennae and hardened mouth parts on head segments, and egg-clasping ovipositors on rear segments.
Arthropods are the most numerous of all animal phyla, both in numbers of species and numbers of individuals, primarily due to insect diversity and numbers. There are at least one million recorded species of arthropods, with the actual number probably ten or even twenty times that amount.
Daly, H. V., J. T. Doyen, and A. H. Purcell. Introduction to Insect Biology and Diversity. New York: Oxford University Press, 1998.