Family VESPERTILIONIDAE 77 



Toward spring many pipistrels desert the winter caves and 

 take up abodes in attics of buildings, under porches, and in other 

 caves. Apparently they return to the same "summer" roosts year 

 after year. If removed some distance from such a roost, they 

 exhibit a definite homing instinct to return to it. During the 

 "summer" months, eastern pipistrels are entirely social and 

 usually hang in clusters. 



In early summer, the sexes tend to separate while the females 

 bring forth their young. Usually an adult female has two off- 

 spring. 



Pipistrels are early evening fllyers and frequently hunt insects 

 before sundown. Their flight appears to be very erratic. 



Distribution. — The eastern pipistrel is common in much of 

 Illinois except in the Chicago area. It seems to be absent from 

 the Urbana region. It is more common in the southern than the 

 northern half of the state. Presumably, two subspecies occur in 

 this state, Pipistrellns siibflavus ohscurus Miller in the north- 

 western part and P. s. subflavus (Cuvier) in the remainder of 

 Illinois. The range of the species extends from New England 

 to Minnesota (except for Michigan and northern Indiana) and 

 southward as far as eastern Mexico and central Florida. 



EPTESICUS FUSCUS (Beauvois) 

 Big Brown Bat 



Description. — The big brown bat, fig. 55, as the name im- 

 plies, is both big and brown. It has a wingspread of about 

 a foot but it weighs only half an ounce. It is dark or bronzy 

 brown, except for blackish ears, wings, and tail membrane. Its 

 ears are short and its tragi blunt. 



Length measurements: head and body 2y^-2y% inches (60-72 

 mm.) ; tail \y~\]/s inches (35-48 mm.) ; over-all 3^-4^ 

 inches (95-120 mm.); hind foot about Yz inch (10-14 mm.); 

 ear from notch about ^ inch (16-20 mm.). Weight: about 

 ]/^ ounce (13-16 gm.). 



The skull is comparatively large and broad; it is 18.0-20.5 

 mm. (about ^ inch) in length. Dental formula: I 2/3, C 1/1, 

 Pm 1/2, M 3/3. 



Life History. — The big brown bat, common in Illinois in 

 summer, spends the daylight hours in a variety of places: in 

 attics of dwellings; in hollows of trees; beneath boards, shutters, 



