Order XII. Chiroptera

We may thus define the Bats:—Flying mammals, with the phalanges of the four digits of the hand following the pollex greatly elongated, and supporting between themselves and the hind-limbs and tail a thin integumental membrane, which forms the wing. The radius is long and curved; the ulna rudimentary. The knee is directed backwards, owing to the rotation of the limb outward by the wing membrane. From the inner side of the ankle-joint arises a cartilaginous process, the calcar, which supports the interfemoral part of the wing membrane. The mammae are thoracic; the placenta discoidal and deciduate. The cerebral hemispheres, which are smooth, do not extend over the cerebellum.

Fig. 254.—Barbastelle. Synotus barbastellus. × ½. (After Vogt and Specht.)

This large order of mammals was once placed with the Primates. There is no doubt, however, that they form a perfectly distinct order; no knowledge of fossil forms in any way bridges over the gap which distinguishes them from the highest mammals. The most salient feature in their organisation is clearly the wings. These consist of membrane, an expansion of the integument, provided with nerves, blood-vessels, etc., which mainly lie stretched between the digits 2 to 5. These digits themselves, which are enormously elongated, act like the ribs of an umbrella, and when the wing is folded they come into contact. Besides this part of the flying apparatus there is a tract of membrane lying in front of the arm, which corresponds to the wing membrane of the bird, but which in the Bats takes quite a subordinate place. In the bird, on the other hand, there is a metapatagium, which is the main part of the wing of the Bat. It seems just possible that in Archaeopteryx the metapatagium was more Bat-like. Furthermore, a steering membrane, like that which fringes the tail in some Pterosaurians, lies interfemorally in Bats, and includes the whole or a part of the tail. The pollex takes no share in the wing, but projects, strongly armed with a claw, from the upper margin.

The bones of this order of mammals are slender and marrowy; they are thus light, and subserve the function of flight. A most remarkable feature among the external characters of the Bat tribe is the extraordinary and often highly complicated membranes which surround the nostrils. These are at least often more strongly developed in males than in females, and may perhaps be partly relegated to the category of secondary sexual characters. But it seems that they have also an important tactile function, and enable the creatures to fly without touching bodies which intrude themselves upon their way. The ears, too, are frequently very large, and it may be supposed that the sense of hearing is correspondingly acute. In the common Long-eared Bat of this country, the ears are not greatly inferior in length to the head and body of the animal combined. The ears are of every variety of shape, and offer characters which are valuable in the systematic arrangement of the members of the order.

Fig. 255.—Skeleton of Flying Fox. Pteropus jubatus. × ⅛. (After de Blainville.)

In the skull of Bats there is very rarely a complete separation between the orbital and temporal fossae; the lachrymal duct is outside the orbit. The tympanics are annular, and in a rudimentary condition. The centra of the vertebrae tend to become ankylosed in old individuals; the caudals have no processes, but are like those quite at the end of the series in long-tailed animals. The sternum is keeled for the better attachment of the pectoral muscles, the chief muscles of flight. The ribs, which are much flattened, are occasionally ankylosed together by their margins. There is a well-developed clavicle. In the carpus the scaphoid, lunar, and cuneiform are all fused together. In the hind-limb the fibula is rarely fully developed.

The Bats are divisible into two primary groups, which are those of the Megachiroptera and the Microchiroptera.