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KELLOGG AND BELL 



ber (i) below the mode (2) is more abundantly represented than 

 the numbers (3 and 4) above the mode. In the inner rows the 

 curve of frequency is nearly perfectly symmetrical, with a very 

 high modal point, and nearly equal in extent and character on 

 each side of this point. 



To indicate the degree of correlation of the variation in num- 

 ber of spines in the various individuals of this series studied, 

 the following compilation has been made : 



Taking the number of spines in the inner rows of the right 

 and left tibiae of the same individual there are, among the 



FIG. 73. Frequency polygon of the varia- 

 tion in number of spines in inner row of 

 right tibiae of 98 male seventeen-year cicadas, 

 Cicada septendecim ; mean, 3.4; index of 

 variability, .66 ; coefficient of variation, 19.4. 



FIG. 74. Frequency polygon of 

 the variation in number of spines in 

 inner row of left tibiae of 97 male seven- 

 teen-year cicadas, Cicada septendecim ; 

 mean, 3.25; index of variability, .7; 

 coefficient of variation, 21.5. 



males (only 50 individuals taken at random from the series of 

 100, in order to make the comparison with the 50 females more 

 obvious) : 39 individuals with combinations of 3-3, 3-4, or 4-4 

 spines (both modes) ; 7 individuals with combinations of 3-2 

 spines (mode and below) ; 2 individuals with combinations of 

 2-2 spines (both below mode) ; I individual with combination 

 of 5-3 spines (above mode and below) ; i individual with com- 

 bination of 1-4 spines (extreme difference). 



Among the females there are : 42 individuals with combina- 



