Bat, the popular name for any individual of the. order Chiroptera. Down to the end of the seventeenth century the zoological position of these animals was little understood; and so late as 1681 Grew, in the Catalogue of the Museum of the Royal Society, says that they stand "in the rear of beasts and in the front of birds," which added nothing to men's knowledge, for it was only a formal phrasing of the popular names "flittermouse," i.e. the flying or fluttering mouse, and "reremouse," from A.S. hreremus - the mouse that flaps (its wings). Two years after this Ray fully recognised their mammalian character; and Linnaeus (1707-78) placed them in his chief order Primates (q.v.), which also contained the lemurs, the apes, and man. Modern writers, however, do not admit the bats to such a high zoological rank, and they are now regarded as Insectivora (q.v.), modified for flight, one of the surviving intermediate forms between the two orders being Galeopithecus, the Flying Lemur (q.v.).
The fore limbs are much longer than the hinder ones, and the digits of the former, with the exception of the pollex or thumb, are extremely elongated. The volar membranes (or those employed for flight): are three: (1) The ante-brachial membrane, extending from the shoulder to the base of the thumb; (2) the wing membrane stretched over the digits, carried along the side, and reaching to the feet; and (3) the interfemoral membranes, between the hind limbs. Well-developed clavicles are always present, and the radius cannot be rotated on the ulna. The bones though slender are not pneumatic.
Bats appear first in the Upper Eocene, and the oldest known fossil form is "very similar to existing European bats," so that the period of divergence of the Chiroptera from the Insectivora must be very remote. The living forms are universally distributed over the tropical and temperate regions of both hemispheres, and fall into two natural groups or suborders.
I. Megachiroptera. Fruit-eating bats, generally of large size, limited to the tropical and subtropical parts of the Old World. The crowns of the molar teeth are marked with a longitudinal groove; index finger with three phalanges, the last phalanx generally armed with a claw; pyloric end of the stomach generally much elongated; tail, when present, inferior to, but not contained in, the interfemoral membrane. This sub-order consists of a single familv, Pteropidae.
II. Microchiropteea. Bats ranging over the tropical and temperate regions of both hemispheres, living for the most part on insects, though some are fruit-eating, and two species are known to suck the blood of higher animals. They are much smaller than the bats of the first sub-order, and have the crowns of the molars with tubercles or cusps; generally one rudimentary phalanx in the index finger, which is never terminated by a claw; stomach simple; tail, when present, contained in the anterfemoral membrane, or appearing on its upper surface. The sub-order is divided into two groups or alliances.
Bats are small nocturnal or crepuscular mammals, furnished with true wings, and having the power of flight. They generally fly abroad in the morning and evening twilight, and retire during the day to caves or crevices in the rocks, or to the inner parts of the roofs of barns or churches, where they suspend themselves by means of the hooked claws on their thumbs. Their senses are intensely acute, as was proved by some interesting but cruel experiments of Spallanzani on various species, towards the close of the eighteenth century. Their eyes are small and bead-like, and the proverb "as blind as a bat" must refer to the dazed condition of these animals when suddenly exposed to a glare of light, and not to their normal state in fitting environment. Their ears are generally large and directed well forward, and they seem to have a special power of directing their flight in places so dark as to render the keenest vision useless. This power Cuvier thought was due to an exceptional development of the sense of touch in the volar membrane. His conclusion is now generally accepted; and later research shows that the wings of bats are very freely supplied with blood-vessels, and that these vessels have contractile walls, so that the circulation must be so active as to induce a condition closely akin to inflammation, and everyone who has suffered from a "gathering" knows how keenly inflammation heightens the sensibility of a part. The curious membranous appendages attached to the nose of many species doubtless serve the same purpose. When not used for flight the wings of the bat are folded up by the long fingers being drawn together, and up towards the fore-arm, and the wing membrane then forms leathery folds at the sides of the body. In running or walking progress is effected by the action of the hind limbs and of the claws of the thumbs, which are placed on the ground. Doubtless it was from their appearance in this position that these animals derived their names of "flittermouse" and "reremouse." The teats, always two in number, are usually on the breast, sometimes on the sides. Some species are said to have them in the groin, but this is a mistake, for the nipple-like projections have been proved to be only warts. The reproductive organs in both sexes closely resemble those of the Primates (q.v.), a fact which influenced Linnaeus in his classification. The majority of the species hibernate.