Order XIV. Cheiroptera.* - This order is undoubtedly "the most distinctly circumscribed and natural group" in the whole class of the Mammalia. The most obvious peculiarity of the Bats is the modification of the hand for the purpose of supporting a flying-membrane; but with this are correlated other structural characters of importance.
The Cheiroptera are essentially characterised by the fact that the anterior limbs are longer than the posterior, the digits of the fore-limb, with the exception of the pollex, being enormously elongated (fig. 442). These elongated fingers are united by an expanded membrane or "patagium," which is also extended between the fore and hind limbs and the sides of the body, and in many cases passes also between the hind-limbs and the tail. The patagium thus formed is naked, or nearly so, on both sides, and it serves for flight. Of the fingers of the hand, the pollex, and sometimes the next finger as well, is unguiculate, or furnished with a claw ; but the other digits are destitute of nails. In the hind-limbs all the toes are unguiculate, and the hallux is not in any respect different from the other digits. Well-developed clavicles are always present, and the radius has no power of rotation upon the ulna. The mammary glands are two in number, and are placed upon the chest. There are teeth of three kinds, and the canines are always well developed. The molars are tuberculate or grooved in the frugivorous forms, and cuspidate in the insectivorous species. The ulna is rudimentary. The bones are not pneumatic. The testes are abdominal except during the breeding season. The stomach is complex and the intestine long in the fruit-eating Bats; but the reverse of this obtains amongst the insectivorous forms. The Cheiroptera are cosmopolitan in their distribution, and the oldest known species is from the Eocene rocks.
* The Cheiroptera were, placed by Linnaeus in his order Primates, which contained also the Lemurs, the Apes, and Man.
The most striking features in the structure of the Cheiroptera are those connected with the conformation of the limbs (fig. 442). The fore-limb is larger than the hind-limb, the strong and moderately long humerus articulating with a very large scapula. The clavicles are complete, and the sternum is keeled. The radius is long and well developed, but the ulna is reduced to a mere splint-bone, which is anchylosed with the proximal end of the radius, all power of rotation of the fore-arm being thus lost. The thumb is short, and its last phalanx carries a claw. The index is long, but is shorter than the other digits, and often consists of its metacarpal only, in other cases with two short phalanges in addition. It is usually clawless, but may (as in most of the Pteropidae) be unguiculate. Of the remaining digits the medius is the longest; and all are clawless, and possess two or three phalanges.
Fig. 442. - Skeleton of the Mouse-coloured Bat (Vespertilio murinus). a Humerus; b Scapula; d Radius, with the rudimentary ulna at its proximal end; e Carpus; f Thumb; gg Metacarpal bones; s s Sternum; p Pelvis; i Supplementary bone attached to the calcaneum.
In the hind-limb, the fibula is mostly incomplete, and the foot is furnished with five clawed toes. To the os calcis is attached, in most Bats, a cartilaginous or bony process or spur, which is directed inwards along the lower margin of the inter-femoral membrane, and serves to put this upon the stretch during flight.
The Bats are all crepuscular and nocturnal in their habits, and are sometimes carnivorous, sometimes frugivorous. The eyes are small, but the ears are very large, and their sense of touch is most acute. During the day they retire to caves or crevices amongst the rocks, where they suspend themselves by means of the hind - feet, which are provided with curved claws. In their flight, though they can fly in the genuine and proper sense of the term, and can turn with great ease, they are by no means as rapid and as active as are the true birds. The tail is sometimes short, sometimes moderately long, and is usually included in a continuation of the leathery patagium, which stretches between the hind-legs, and is termed the "inter-femoral membrane." The body is covered with hair, but the patagium is usually hairless, or nearly so. Most of the Bats hibernate.
The Cheiroptera are conveniently divided into the two sections of the Insectivora and Frugivora, according as the diet consists of insects or of fruits.