Every animal needs to maintain a steady internal environment in order to carry out the various physiological processes that together allow life to continue. This maintenance is known as homeostasis. One of the key organs in homeostasis is the animal's surface covering, the skin, cuticle or other tegumentary substance. In mammals and birds the surface covering is extended to incorporate an outer insulating layer of hair or feathers. In many such animals this layer has proved to be an excellent home for permanent and periodic ectoparasitic insects, providing many different species with a relatively constant environment in which to live.

Vertebrate skin is formed of an inner dermis and an outer epidermis (Fig. 7.1). The thickness of these two layers, and the ratio between them, varies considerably between different parts of the body. The dermis is a connective tissue layer containing blood and lymphatic vessels, and nerves. Embedded in the dermis are the acini of the gland systems that open onto the skin's surface. There are two basic types of skin-associated gland in mammals. The sebaceous glands produce an oily secretion called sebum, which helps prevent the skin and hair from drying out and possesses anti-microbial components. The sweat glands (which are not present in carnivores or rodents) produce a watery secretion used in temperature regulation and in maintaining water and salt balance in the body.