STUDIES OF VARIATION IN INSECTS 



309 



tions of 3-3, or 4-4 spines (both modal); 3 individuals with 

 combinations of 3-2 spines (mode and below) ; 3 individuals 

 with combination of 4-5 spines (mode and above) ; i individual 

 with combination of 4-2 spines (extreme difference). 



In the outer rows of right and left tibiae of the same individual 

 there are, among the males (same 50): 31 individuals with 



FIG. 75. Frequency polygon of 

 the variation in number of spines in 

 outer row of right tibiae of 49 female 

 seventeen-year cicadas, Cicada septen- 

 decim; mean, 2; index of variability, 

 .04; coefficient of variation 2. 



FIG. 76. Frequency polygon 

 of the variation in number of 

 spines in outer row of left tibiae 

 of 47 female seventeen-year 

 cicadas, Cicada septendecim j 

 mean, 2. ; index of variability, 

 .04 ; coefficient of variation 2. 



combinations of 2-2 spines (both modal) ; 15 individuals with 

 combinations of 1-2 spines (mode and below) ; 4 individuals 

 with combinations of 2-3 spines (mode and above). 



Among the females there are : 44 individuals with combina- 

 tions of 2-2 spines (both modal) ; 8 individuals with combina- 

 tions of 1-2 spines (mode and below) ; 6 individuals with com- 

 binations of 2-3 spines (mode and above). 



