306 KELLOGG AND BELL 



numbers differed by two in 6 cases, numbers differed by three 

 in i case ; in females (50) numbers were the same in 15 cases, 

 numbers differed by one in 24 cases, numbers differed by two 

 in 9 cases, numbers differed by three in i case. 



In corresponding rows in right and left tibiae. In outer rows 

 of males (39) numbers were the same in 17 cases, numbers dif- 

 fered by one in 15 cases, numbers differed by two in 5 cases, 

 numbers differed by three in 2 cases ; of females (50) numbers 

 were the same in 25 cases, numbers differed by one in 16 cases, 

 numbers differed by two in 7 cases, numbers differed by three 

 in i case. In inner rows : of males (39) numbers were the same 

 in 20 cases, numbers differed by one in 18 cases, numbers differed 

 by two in i case, numbers differed by three in no case ; of fe- 

 males (50) numbers were the same in 16 cases, numbers differed 

 by one in 24 cases, numbers differed by two in 8 cases, numbers 

 differed by three in i case. 



This case of variation, as well as numerous others in our list, 

 notably that of the antennal segments of Phenacoccus (p. 311), 

 which have to do with characters that happen to be ones used 

 by systematists in classification, illustrates what an informing 

 light statistical studies of variation have to throw upon syste- 

 matic practise. The modern sys- 

 tematist will have to take into 

 account the actual status of the 

 existent variation in the charac- 

 ters of which diagnostic use is 

 made. 



Variation in Tibial Spines of 

 Cicada septendecim (the Seven- 

 FlG 70 teen-year Locust or Periodical 



Cicada). The periodical cicada 



(family Cicadidas, order Hemiptera) has an incomplete meta- 

 morphosis, the young or larval stage being passed under- 

 ground in active burrowing about and feeding. The fore legs 

 are fitted for digging but the middle and hind legs are purely 

 locomotory. On the ventral aspect of each hind tibia there are 

 certain small but distinct spurs or spines arranged in two longi- 

 tudinal series, one along the outer margin of this aspect, the 

 other along the inner (fig. 70). 



