The common characteristics of arthropods are they are invertebrates, which means they lack backbones. In fact, they lack bones of any kind but possess a protective exoskeleton, which they molt in order to grow and repair themselves. They also have segmented bodies.
Some arthropods undergo metamorphosis, which means they need to pass through several stages from larva to adult. Often, the larva does not resemble its adult form. This can be seen most clearly in insects such as butterflies.
Arthropods also have segmented legs, and in some of the more primitive arthropods, each segment of the body has a pair of legs. Arthropod legs are most often used for swimming or walking, but some arthropods have pairs of legs that have been modified. Some are used for defense or hunting, and some have been modified into sucking or sense organs. Insects have only three pairs of legs, but many of them have one or two pairs of wings.
The arthropod exoskeleton is made up of a tough material called chitin. Some arthropods, such as houseflies, have very thin and weak exoskeletons, while the exoskeletons of other arthropods are very tough. This can be seen in animals such as crustaceans and beetles. Almost all arthropods have fairly well developed nervous and circulatory systems.