Insects, not having lungs, receive air into their bodies through holes in the skin. These are called spiracles. They are so protected by hairs within the holes that water will not enter them. This is why it is so difficult to drown an insect. But if you touch the abdomen of one of these skin-breathing creatures, for instance the yellow part of a wasp, with a drop of oil, the minute openings become almost immediately clogged and the insect falls dead as if choked completely.
The skin consists of two layers, both of which are exceedingly interesting. The outer or scarf skin is called the cuticle on the outside of the body, while wherever the skin dips into the body it is modified into what is called mucous membrane. This outer skin is not what is rubbed off the surface in a Turkish bath manipulation or what is brought off by the rubbing one gives the body with a rough towel. These rubbings bring off merely the dead outer surface of the cuticle which should be out of the way because no longer useful. In man it continually wears off, in serpents it is shed annually in one slough.
The cuticle is the portion of the covering of the body which may best be noticed when a blister has been raised in the skin. The blister is an accumulation of fluid between the cuticle and the true skin.
The cuticle, or epidermis, is modified in many other ways than the one in which it becomes mucous membrane. Where the habits of the animal make warmth desirable the epidermis dips into the skin and without any break in its connection rises in the form of wool, which covers the body of the sheep so effectually. Where the animal is designed for flight there is the same characteristic dip into the material of the body, and out of the little sac so formed rises the feather which gives the bird its beauty and powers of flight. The feather is a modification of the scarf skin.
Where protection is needed for the body beneath the surface of the water this changeable substance covers the true skin with hard scales that make the friction of the water as slight as possible, while giving a firm and light resisting surface to prevent wounds. Horns and hoofs are modifications of the scarf skin. Where claws or talons are needed in the business of fighting or tearing food in bits or digging holes in the ground or elsewhere, the scarf skin changes itself at the extremities of paws and feet and produces nails, talons, and claws, whose powers are both marvelous and varied. For the protection of most mammals the whole of the body is favored by this power of the scarf skin to produce whatever seems necessary for the comfort of the individual, and the body is indented with innumerable minute holes called hair follicles into which the scarf skin dips and rises again to the surface transformed into hairs of varying degrees of fineness and color, beautifully arranged in order, and all pointing in such directions as will add to the beauty or comfort or terrifying aspect of the animal.
Not only are our hairs numbered, but each particular hair is furnished with a little individual muscle of its own running from the base of the follicle to the inner surface of the true skin, so that when the proper occasion arises for erection of that individual hair the muscle contracts apparently of its own accord, and up stands the hair along with its fellows, ready to frighten the animal that dares to approach in hostile attitude the owner of the precious coat. Similar muscles erect the feathers of the owl, and the gorgeous tail of the peacock dazzles us in the sunlight moved in like manner, while to those more powerful dermal appendages, the claws, talons, and nails, are attached more powerful muscles still, with proper nerve connections for the most effective use of the weapons nature has formed out of the soft outer skin, which is usually so mild and yielding as to have earned the name of scarf skin.
This outer skin is formed of cells, flat on the surface, but near the true skin where they originate, rounded and in many cases even tall and apparently reaching out towards the surface. It gives the color to the person by means of pigment cells which lie in its midst.
The black man is dark because of the abundance of pigment cells in his scarf skin. The albino is light because of their absence. The colors of hair and feathers are due to these cells in their receptacles, but white and iridescent feathers are doubtless so partly because of their absence and partly because of hollow spaces which catch and reflect or refract the light.
This arrangement of cells into scarf skin has much to do with the healing of wounds. In cases of old sores that refuse to heal, or where the skin has been extensively destroyed, the doctors have found that good, healthy skin may be grafted upon the sores in such a manner as to invigorate and perfect the process of healing. Small particles of fresh skin taken from a healthy subject or from some other part of the patient's body are placed upon the sore, the portions used being about the size of a small pinhead, and new life seems transplanted in the deadened part. The skin of a black man grafted upon that of a white man shows afterwards no trace of its origin, but becomes the same shade as that which it adjoins.
Several animals change their tints to correspond with their surroundings. This subject has been exaggerated by observers of an imaginative turn of mind, but the fact remains that there is a decided change in the coloring of certain crabs and shrimps as well as in soles, chameleons, tree-frogs, and two kinds of horned toads wherever they are found against any well-defined shade or color. Some have maintained that man takes on a tint somewhat resembling the soil of the territory where he abides in an uncivilized condition, but Beddard considers Schweinfurth's statement that the Bongos have a reddish-brown skin similar to the soil of their country, and the Dinhas, their neighbors, are as black as their alluvial ground, merely as an account of what is purely accidental in the instances given.