STUDIES OF VARIATION IN INSECTS 



307 



The variation in number of the setibial spines was determined 

 in a lot of 150 individuals (100 males, 50 females), collected by 

 B. T. Riley at Indianapolis, Indiana, in the summer of 1898. 

 The following frequency polygons show the character of this 

 meristic variation. 



As will be seen from the frequency polygons the range in the 







; a 



ft! 



m 



FIG. 71. Frequency polygon of the 

 variation in number of spines in outer 

 row of right tibiae of 98 male seven- 

 teen-year cicadas, Cicada septendecim ; 

 mean, 1.9; index of variability, .42; 

 coefficient of varaition, 2.21. 





a 



FIG. 72. Frequencj' poly- 

 gon of the variation in number 

 of spines in outer row of left 

 tibiae of 98 male seventeen-year 

 cicadas, Cicada septendecim ; 

 mean, 1.9; index of variability, 

 .47; coefficient of variation, 

 2.47. 



outer row of right tibiae in males (100 specimens), is i to 4, left 

 tibiae i to 3 ; right tibiae of females (50 specimens), i to 3, left 

 tibiae i to 3 ; in inner row of right tibiae in males, i to 5, left 

 tibiae 2 to 5 ; right tibiae of females, 2 to 5, left tibiae, 2 to 5. 

 The mode in outer rows is 2, the inner rows 3 and 4 are nearly 

 equal modes. In the outer rows it will be noted that the num- 



