The largest number of species in the animal kingdom belongs to the phylum Arthropoda. Members of this phylum, called arthropods, include such animals as spiders, ticks, centipedes, lobsters, and insects. All animals in the phylum have an external skeleton, a segmented body, and jointed appendages.

The external body skeleton of arthropods is called the exoskeleton. It surrounds the animal and provides support. The bodies of arthropods are often divided into distinct regions called the head, thorax, and abdomen. Mouthparts exist in the head region. Antennae, the sense organs, are located on the head also, if they are present. Respiration in the arthropods occurs through microscopic holes in the exoskeleton and body wall. Branching networks called tracheae extend from these holes to all parts of an arthropod’s body.

Five major classes divide the phylum Arthropoda. The first two classes include the centipedes and millipedes. These animals have multiple appendages and are able to move efficiently on land. Another class of arthropods is the crustaceans, including crabs, shrimp, lobster, and crayfish. In these animals, the exoskeleton, hardened with calcium salts, serves as a protective shell. The arachnids comprise the fourth class of arthropods, including spiders, ticks, mites, and scorpions. Arachnids have a fused head and thorax and have four pairs of legs.

The largest group of arthropods is the insects, including grasshoppers, butterflies, beetles, and cockroaches. Insects live in every conceivable environment on Earth and are among the most highly adapted of all animal species, with well-developed organs for various senses, including smell, touch, taste, and hearing. Insects have three pairs of jointed legs, and many have one or two pairs of wings. The body is divided into three parts: head, thorax, and abdomen. In many cases, the embryo develops into an immature form called the larva. The larva transforms into a pupa and then becomes an adult. This change is called metamorphosis.