174 ILLINOIS STATE ACADEMY OF SCIENCE 



This phenomenon will be discussed more fully in a later 

 paper. 



The most striking phenomenon of the entire period 

 covered by the collections was the autumn hibernating 

 reaction evidenced by migration inward from the forest 

 margin and downward to the forest floor. In most of 

 the species observed the stimulus initiating migration 

 seems to be gradual increase in daily range of tempera- 

 ture and lower night temperature. Some species evident- 

 ly require the additional stimulus of frost of greater or 

 less intensity, while others which appear in the forest 

 very early seem not to require this. The disappearance or 

 maturation of food-plants may be a factor in some cases. 

 In the forest the insects appear first in large numbers 

 in the stratum corresponding to that in which the sum- 

 mer portion of their life history is spent. If the princi- 

 pal summer food plant is found in the high forest margin, 

 the first appearance is in the shrub stratum. If the sum- 

 mer habitat is the low forest margin or meadow the in- 

 sects first appear in the herb stratum. A downward mi- 

 gration follows under the stimulus of an additional fall in 

 temperature or additional frosts. Insects from the shrub 

 stratum seem in most cases to spend a short period of 

 time in the intermediate herb stratum before seeking 

 the final place of hibernation in the leaf or soil strata. 

 Warm days may reverse the course of migration at any 

 point. The fact that a large number of species react alike 

 and at the same time to the same stimulus or combina- 

 tion of stimuli shows a large degree of similar adjust- 

 ment of the climatic rhythm of the temperate savanna 

 region on the part of the characteristic insects of the local 



area. 



TABLE 1. 



Animal Population of the Lower Strata of University Woods, 

 Urbana, 111. 



July, 1921, to June, 1922. 



Per 

 Per Hec- 

 Week Week Soil Str. Leaf Str. Herb Str. Shrub Str. Acre tare 



Ending No. Coll. Ave. Coll. Ave. Coll. Ave. Coll. Ave. Total (Thousands) 



July 4 1 1 11 2 24.5 3 9 3 .6 45.1 496 1217 



July 11 2 1 16 1 20 6 13.5 .... 2 51.5 566 1390 



July 18 3 1 10 1 14 3 15 3 4.8 43.8 481 1182 



July 25 4 1 15 2 5 3 8.4 .... 4.2 32.6 359 980 



Aug. 1 5 3 14.7 1 11 .... 11.5 .... 3.9 41.1 452 1109 



Aug. 8 6 2 14.5 1 12 3 14 3 3.6 44.1 485 1190 



Aug. 15 7 



Aug. 22 8 1 18 1 15 3 8 2 4.5 44.5 490 1101 



