GENERAL STRUCTURE. 249 



the usual manner; and this peculiar arrangement renders a bat's movement on 

 the ground an awkward kind of shuffle. 



O 



In order to afford space for the attachment of the powerful muscles necessary 

 to move the wings, the chest of bats, like that of birds, is remarkably large. But 

 as these animals are poor walkers, the haunch-bones are relatively small and weak. 

 The great majority of bats feed solely on insects, and have their 

 cheek-teeth furnished with a number of sharp cusps, admirably 

 adapted for holding and piercing the tough integuments of beetles and many other 

 insects. A few bats, however, are blood-suckers, and these have the front teeth 

 specially modified for piercing the skin of the animals they select as their victims. 

 Others, and among them the largest representatives of the order, are fruit-eaters : 

 and these accordingly have a quite different kind of cheek-teeth, in which the 

 crowns are nearly smooth, and without cusps. 



The number of the different teeth in different bats is variable, and is of great 

 importance in distinguishing the different genera ; but as some of these teeth may 

 be exceedingly minute, their enumeration requires great care. No bat, it may be 

 observed, has more than two pairs of incisor teeth in the upper jaw ; neither are 

 there ever more than three premolars on each side of the upper and lower jaws, so 

 that the number of teeth behind the tusks, or canines, never exceeds six. 



So thoroughly are bats adapted for a life in the air, that most of them but 

 seldom resort to the ground, and even when they do so they generally endeavour 

 to leave it as soon as possible by ascending a tree, rock, or wall, whence they either 

 again take flight, or settle themselves into their favourite position of repose, 

 suspended head downwards by the feet. Not only do most bats reed and drink 

 while on the wing, but the females even carry their young tightly clinging to their 

 bodies. 



In their active life bats being mostly crepuscular or nocturnal, 

 Sense of touch. 



while their eyes are relatively small, it is obvious that they must 



be provided with some special means of avoiding contact with objects during 

 flight. This appears to be effected by the extreme development of a sense more or 

 less akin to our sense of touch, by which the neighbourhood of objects is per- 

 ceived without actual contact; and it was demonstrated as long ago as 1793, by 

 the cruel experiment of depriving bats of sight and then allowing them to fly 

 in a room across which silken threads were stretched in such a manner as to leave 

 just sufficient space for them to pass between with outstretched wings. The unfor- 

 tunate bats not only succeeded in passing between these threads without contact, 

 arid likewise avoided the walls and ceiling of the room, but, when the threads were 

 placed still nearer together, they contracted their wings in order to be able to 

 pass without contact. In the same manner they flew between branches and 

 twigs of trees placed in their course, and suspended themselves when tired of 

 flight on the walls of the room, just as easily as when they enjoyed the use of 

 their eyes. In the great majority of bats it appears that this sense of touch is 

 situated in the wing-membranes, and in the delicate and frequently enormously 

 elongated ears, which are often provided with a kind of secondary inner ear, known 

 as the tragus. There are, moreover, certain bats provided with an additional organ 

 of perception, which takes the form of expansions of skin from the nose and 



