Sept., 'o6] ENTOMOLOGICAL NEWS. 25 1 



Now if we can find some factors which these regions, from 

 which the species tabulated above are derived, have in common, 

 it may be possible to reach some conclusions in regard to the 

 influences which have made the fauna of northern Illinois 

 what it is. 



Mr. Edgar N. Transeau in a paper on "Forest Centres of 

 Eastern America," published in the American Naturalist, vol. 

 xxxix, p. 885, fig. 6, 1905, gives a map which is most sugges- 

 tive in this connection. This map shows graphically the ratio 

 of rainfall to evaporation in the eastern United States and 

 Canada, expressed in percentages. 



A very interesting fact is disclosed on comparing the distri- 

 bution of the species of insects as enumerated in the table, 

 with the distribution of rainfall and evaporation, given by 

 Transeau in the map quoted above. For we find the species 

 readily divide into two series, whose food plants pertain to 

 regions of very different humidity, and consequently of dif- 

 ferent vegetal growth. 



The first series embraces the species from Canadian sources, 

 and all fall into zones in which the percentage of rainfall to 

 evaporation is no per cent, or more. The second series em- 

 braces all the species described from regions east of the great 

 western plains, and south of the Canadian sources, and fall 

 into zones whose percentage of rainfall to evaporation ranges 

 from 80 to 1 10 per cent. 



In the first series from Canada and the north east are 201 

 species, which is 54 per cent, of the species given by Cresson 

 and Ashmead, and 33 per cent, of all the Algonquin species. 

 In the second series from regions east of the great plains and 

 south of the Canadian region there are 153 species, or 41 per 

 cent, of the species given by Cresson and Ashmead, and 25 per 

 cent, of all in the Algonquin list. This leaves only 17 species, 

 or less than 5 per cent, of the species given by Cresson and 

 Ashmead from other areas. 



Taken altogether, this gives 95 per cent, of the species 

 assigned by Cresson and Ashmead to definite habitats, as orig- 

 inating in or inhabiting the two zones or areas described. This 

 is a sufficiently large percentage to warrant conclusions of 



