20 



inois Natural History Survey Circular 49 



as "north woods" prevailed as far south as southern Illinois. Evidence 

 for these climates comes mainly from small southern populations of 

 primarily northern species of insects and other animals that, because 

 of restricted flight or sedentary habits, cannot disperse across con- 

 siderable distances having conditions unsuitable for the existence of 

 the organisms. In spots such as cool ravines or spring-fed rocky 

 recesses protected from the extremes of summer heat prevailing in 



/>•.%. 



Fig. 12. — Clump of bearberry on sand ridge near Zion, Illinois. The insect at the left is the 

 bearberry leafhopper, Texananus cumulatus (actual length about three-eighths inch). 



the surrounding terrain, colonies of these northern species have per- 

 sisted far south of their present continuous range to the north or 

 northeast. A large number of these species are stoneflies and caddis- 

 flies whose young live in the spring-fed streams occurring in these 

 habitats. A northern caddisfly and a northeastern stonefly of this 

 type occur at Lusk Creek, near Eddyville, Pope County, in extreme 

 southern Illinois. In the small stream at Rocky Branch, Clark County, 

 in east-central Illinois, a peculiar relict stonefly persists. In the cold, 

 spring-fed brooks in the Elgin Botanical Garden, in the city of Elgin, 

 two kinds of stoneflies and four kinds of caddisflies persist 300 to 

 500 miles south or southwest of the main range of each species. These 

 typically northern kinds of stoneflies and caddisflies are quite dif- 



