Morphologically, arthropods have segmented bodies, including jointed appendages, and are covered with a chitinous cuticle that serves as an exoskeleton. Arthropods must undergo periodic molts as growth and development proceeds. Sexes are separate. Morphological differences between sexes are few in some species, marked in others.

Parasitic arthropods are usually small, ranging from microscopic to 1-2 cm in length. Body segments may be grouped into discrete regions, such as the head, thorax and abdomen of insects, or show various degrees of fusion. In some cases, segmentation has been reduced to the extent that the organism is barely recognizable as an arthropod. The body surface is covered in a series of plates. Each of these plates, and the separate segments of each jointed appendage (including the mouthparts), have their own names. Fortunately, the terminology is relatively consistent within each of the arthropod classes.

The digestive system is complete. The mouth is surrounded with jointed mouthparts that gather (possibly through piercing or biting) and manipulate food. The gut is usually divided into a foregut, midgut, and hindgut. The circulatory system is open; blood is pumped by a heart and circulates through the body cavity. The nervous system is well developed, usually including eyes, antennae and other sensory structures.

The male reproductive system consists of paired testes, each connected by vas deferens to a seminal vesicle, and then uniting to form an ejaculatory duct that enters a penis, which may have complex supporting structures associated with it. The female system consists of paired ovaries, each of which is connected by an oviduct to a vagina. A spermatheca is present. Accessory glands are present in both male and female systems.