288 



KELLOGG AND BELL 



the anterior margin of the pronotum. From an examination of 

 this series of spots in many individuals we found ourselves able 

 to describe the variation in these spots most conveniently by 

 assuming the normal condition of the series to be that in which 

 it would be composed of nine spots, one in the center and four 

 symmetrically disposed on each side (fig. 56). This arrange- 

 ment together with the relative size of the spots and the distin- 

 guishing numbers assigned them are all shown in fig. 56. As 

 a matter of fact this hypothetical normal was not found to exist 

 in a single individual ; instead of normal therefore we may call 



FIG. 56. The leaf hopper, Typhlocyba comes, and diagram showing hypo- 

 thetical, symmetrical arrangement of spots along the cephalic margin of the 

 prothorax. 



it the " condition of symmetry," an ideal condition not existent 

 in any of the specimens examined. 



These insects have an incomplete metamorphosis and this pro- 

 thoracic pattern is exposed to be tested for usefulness or to be 

 modified by direct reaction to environment throughout the im- 

 mature life of the leaf hoppers, which life is the same as re- 

 gards food and most other relations to the external world as 

 that of the adult and is spent in the same environment. The 

 various classes of variation of these prothoracic spot markings 

 and the frequencies of these classes are as follows : i individ- 

 ual has an added spot beyond 1 4 1 ; 2 individuals have an added 

 spot beyond 4*, and 4*; i individual has an added spot beyond 



1 Spots i 1 , 2 1 , 3 1- and 4' are spots i, 2, 3 and 4 of fig. 56, while spots i r , 2 r , 3 r 

 and 4 r are spots 9, 8, 7 and 6, respectively of fig. 56 : spot o is spot 5 of fig. 56. 



