To convert this framework into an organ of flight its various parts are, as already stated, united by a membrane of more or less leathery appearance, although often so thin and delicate as to be somewhat translucent. It is an expansion or wide fold of the skin of the body like those forming the parachutes of the flying Squirrels, &c., and often called by the same name—patagium. We shall employ the simpler, if rather longer term, “wing-membrane.” The bones of the arm, with their accompanying muscles, and those of the fingers, are enclosed between the two layers of skin of which the membrane is composed, and which they serve to extend and support. In front of the arm there is a small portion of membrane filling up the angle of the elbow, and called the antebrachial membrane. The thumb is left free. Behind the arm is the great expanse of the wing, which springs from the sides of the body, and is also attached to the hind legs, generally extending down to the ankle.
The wings are expanded by the spreading of the fingers, which radiate from the wrist something like the sticks of a fan. The second, or middle finger, which is the longest, runs to the extreme tip of the wing, but before reaching this it generally joins the extremity of the first, or index finger, which thus acts as a sort of stay to it, and the two fingers together form a tolerably stiff support for the outer margin of the wing. The other two fingers (the third and fourth) traverse the wing to its hinder border, where they carry out the membrane into small pointed projections; so that when the wing is expanded, this border shows two points besides that at the apex of the wing, and three more or less rounded notches, the last of which is between the tip of the fourth finger and the attachment of the membrane to the hinder limb.
In most Bats the membrane does not stop short at the legs, but encloses them after the same fashion as the arms, leaving only the foot and sometimes a part of the shank free. The portion of membrane that passes within the legs, sometimes filling up the whole space between them and enclosing the whole or a part of the tail, sometimes forming only a narrow border to these limbs, is called the interfemoral or intercrural membrane, and the characters furnished by it and its relations to other parts are of great importance in the classification of Bats.
The rest of the structure of these animals may be dismissed in a few words. The skull, and all the other parts of the skeleton, are generally light and delicate in their construction, as might be expected in animals destined to support themselves in the air; but there is no trace of those pneumatic cavities which, in birds, enable the air to penetrate all parts of the skeleton. The jaws are well armed with teeth, which differ in their character in accordance with the food consumed by the animals. The ribs are well developed, and enclose a large chest cavity. The pelvis (p) is long, slender, and somewhat bird-like in some respects; the legs are short, generally slender, and articulated in such a manner that when used in walking the knees are directed backwards, like our elbows; the fibula (the second bone in the shank) is usually imperfectly developed, in the same way as the ulna in the fore-arm; and the foot consists of five distinct toes, armed with small but sharp claws, by which the animals suspend themselves from the surface of rocks, walls, and other objects, in the dark retreats to which they retire for their repose. From the heel-bone (calcaneum) in most Bats there springs a cartilaginous or bony rod or spur, which is regarded by some zoologists as forming part of the bone itself. This spur, which is often of considerable length, runs along the margin of the interfemoral membrane, which it no doubt helps materially to stretch. When long, and more or less curved, it often causes a projection of the side of the interfemoral membrane, as shown in the [figure of the Marsh Bat] (p. 259). The tail is very variable in length.
In repose, or rather when not flying, the wings of the Bat are folded up by a reversal of the process by which they were extended for flight; the long fingers are drawn together, and up towards the fore-arm, and the membrane forms leathery folds at the sides of the body. This is also their position when the animal is walking or running on the ground (see the [engraving on the next page]), which it does in a somewhat awkward fashion, by the action of its hind feet and the claws of its thumbs. When seen thus engaged there can be little doubt as to the quadruped nature of the Bat. Our little European species have a Mouse-like appearance, which fully justifies their old popular name.
The teats are usually situated on the breast; but sometimes they are placed quite on the sides, immediately beneath what we must call the armpits. They are two in number. In addition to these chest or pectoral teats, some species have been described as possessing a second pair of such organs situated on the groin, but recent investigations prove clearly that these are merely nipple-like warts.
The organs of the senses are well developed. The ears are almost always of considerable size, sometimes very large and membranous, and in most cases there is in front of the cavity a sort of lobe of variable form, called the earlet, or tragus, representing the little rounded lobe which, in the human ear, projects from behind the cheek over the opening (see the [woodcut of the Head of the Long-eared Bat]). The nostrils are either simple slits or apertures at the end of the muzzle, or surrounded by leaf-like organs, often of the most extraordinary forms (see the [Head of the Spectacled Vampire, p. 264], and other illustrations later on), in fact, this tendency of the skin in Bats to run out into membranous expansions is one of their most remarkable characteristics, and, from their mode of life, this great development of the skin system would seem to be almost essential to their existence.
The old proverbial expression, “As blind as a Bat,” is certainly not founded on a due appreciation of facts, for Bats are by no means blind; on the contrary, they are furnished with very efficient eyes, although, in most cases, these are little bead-like organs, very unlike the eyes usually seen in animals whose activity is nocturnal or crepuscular. But it would appear that the office of the eyes in guiding these animals is, at all events, supplemented by some other means. Towards the end of the last century, the Abbé Spallanzani made some exceedingly interesting, although certainly cruel experiments on various species of Bats. He blinded these animals, sometimes by burning the eyes with a red-hot wire, sometimes by removing the organs altogether, and even filling up the orbits with wax, and then allowed them to fly. In spite of the mutilation, the unfortunate little creatures continued quite lively, and flew about as well as those which still retained their eyes; they did not strike against the walls of the room, or the objects in it, avoided a stick held up before them, and showed a greater desire to keep out of the way of a Cat or the hand of a man than to escape contact with inanimate objects. One of these blinded Bats was set free in a long underground passage, which turned at right angles about its middle. It flew through the two branches of this passage, and turned, without approaching the side walls. During its flight it detected a small cavity in the roof at a distance of eighteen inches, and immediately changed its course in order to conceal itself in this retreat. In a garden a sort of cage was prepared, with nets, and from its top sixteen strings were allowed to hang down. Two Bats were introduced into this enclosure, one blinded, the other with its eyes perfect. Both flew about freely, never touching the strings with more than the tips of the wings. Finally, the blind Bat discovered that the meshes of the enclosing net were large enough for it to get through, and made its escape; and, after flying about for a time, made its way rapidly and directly to the only roof in the neighbourhood, in which it disappeared. In a room containing numerous branches of trees, or in which silk threads, stretched by small weights, were suspended from the ceiling, the Bats, though blinded, avoided all these obstacles; and when, after tiring themselves with their aerial evolutions, they settled on some object for the sake of rest, they would immediately rise again on an attempt being made to seize them with the hand.
BARBASTELLE WALKING.