THE BATS 83 



Britain, depend but little for guidance on their sense of vision, 

 the eyes being very small, and often partially concealed under 

 the large ears and the fur. They are, however, extremely 

 sensitive in their hearing, and the considerable ear surface pre- 

 sented by some of them to the air undoubtedly guides them 

 in their avoidance of obstacles by receiving and recording the air 

 currents which may indicate by their vibrations the proximity of 

 any solid body. The extraordinary development of cartilage 

 round the nose in several families of bats is, no doubt, an additional 

 organ of sensation. 



In most parts of the temperate regions and in Britain bats 

 hibernate ; that is to say, they retire in companies to a sheltered, 

 dark place of security, where they suspend themselves by their 

 hind claws, often holding on as well to their neighbours, so that 

 they are sometimes crowded together in large numbers. The 

 period of hibernation varies according to species. Some bats " go 

 into retreat " in August, others in October or November, while 

 the commonest of British bats, the pipistrelle, may only sleep for 

 two or three weeks at a time during the winter, emerging from its 

 hiding-place on a mild day, and pursuing such insects as may remain 

 to haunt the air. Most bats are out again at the end of April, and 

 breeding goes on amongst them at the end of the hibernation. 



Although the food of this group is mainly insectivorous, most 

 of them will eat meat greedily when they can get it, and even 

 fish ; while some species of microcheiropterous bats in America 

 live wholly on fruit or on fish, or, in the true vampire, on the 

 blood of live beasts. 



Family: VESPERTILIONID.^. THE TYPICAL BATS 



This is the largest family in the whole order. It extends 

 to something like 190 species, though the number of genera 

 is considerably less. This group in some respects is the least 

 specialised of the bats. Its members have, as a rule, three 

 pairs of incisors in the lower and two in the upper jaw. In 

 the upper jaw there is a remarkable open space between the inner 

 pair of incisors. The ears are furnished with a tragus, and 



