11 



Main Street, Rockford, indicating the survival of this 

 brood, despite low numbers, over the past 136 or so 

 years of human occupancy of that city. 



In a woods at Castle Rock, Ogle County, the 

 eastern portion contained no cicadas, whereas the 

 western portion, having the same terrain but possibly 

 less sand in the soil, contained a good singing popula- 

 tion. There I found adults, skins, and holes in the 

 ground. This observation and others bear out the 

 contention that each woods is not necessarily uni- 

 formly inhabited. 



FUTURE EMERGENCES OF 

 PERIODICAL CICADAS IN ILLINOIS 



Future twentieth-century emergences of periodical 

 cicadas in Illinois are shown in Table 2. 



Table 2. — Future twentieth-century emergences of 

 piriodical cicadas in Illinois. 



Year of 

 Emergence 



Brood 



Cycle 

 in Years 



1976 Lower Mississippi River Valley Brood 



(Marlatfs XXIII i 

 1980 lowan Brood (Marlatfs III) 

 1985 Great Southern Brood ( Marlatfs XIX) 

 1987 Great Eastern Brood (Marlatfs X) 



1989 Lower Mississippi River Valley Brood 



(Marlatfs XXIIK 



1990 Northern Illinois Brood (Marlatfs XIII) 



1997 lowan Brood (Marlatfs III) 



1998 Great Southern Brood (Marlatfs XIX) 



DISCUSSION 



A composite map of the distribution of the Illinois 

 broods of periodical cicadas is shown in Fig. 5. The 

 borders of the range of each brood are only approxi- 

 mate except in a few regions, as critical areas and 

 woods which should have been put under close sur- 

 veillance for more precise information could not be 

 recognized until after two or three emergences had 

 been surveyed. In general, the distributions of these 

 broods fit nicely with the distributional patterns of 

 other organisms viewed in relation to geological 

 events. Most of the region occupied by the Northern 

 Illinois Brood lies within the Wisconsin glacial plain. 

 The lowan Brood, except for the region inhabited 

 by the disjunct population in De Witt, Piatt, and 

 Champaign counties, occupies a western spur on 

 eroded hills of the northern part of the Illinoian glacial 

 plain. The Great Southern Brood, except for the 

 east-central Illinois extension and the corridor in the 

 Shawnee Hills, is mostly on Illinoian glacial plains 

 onto which most of our southern biota also extends. 

 The Great Eastern Brood occupies an area which 

 supports many other relict or adventive eastern spe- 



Txm 



^ 



Fig. 5. — Distribution of periodical cicadas in Illinois. 

 The ranges of the five Illinois broods are shown on the map. 

 Each brood is identified by Marlatfs number. 



cies: beech, tulip poplar, and some fishes and sala- 

 manders. The Lower Mississippi River Valley Brood 

 occurs for a considerable part in the region where the 

 bald cypress also meets its northern limits. 



Occupation of a woods by two or more broods 

 is extremely rare, according to my data. Woods can 

 be almost adjacent, but each woods usually supports 

 one brood only. Sand areas are usually, if not always, 

 devoid of cicadas. 



The opportunity for 13-year and 17-year broods 

 to meet and hybridize may occur every 65 or 78 years 

 within the total range of the majority of the periodical 

 cicadas of Illinois. It is highly possible that no more 



