234 N - M - STEVENS. 



that it was certain that it was in a position such that it might 

 divide transversely, but I was not able to find any cases of actual 

 transverse division of X. Later at Mountain View I did find 

 two anaphases where X appeared to have divided transversely 

 and unequally (Figs. 3 and 5) and one in which X was caught in 

 the cell plate between the daughter cells (Fig. 4). Now the 

 supernumeraries are usually very uniform in size and certainly 

 less than one half the sixe of X. I have one individual noted as 

 having an unusually large supernumerary, about one-half as 

 large as X, and a few cases where an unusually small one occurs. 

 One of the latter cases is shown in '08, PI. III., Figs. 76 to 78. 

 From the evidence now at hand I should infer that the probable 

 origin of the supernumeraries in the Diabroticas has been an 

 occasional transverse division of X followed by a longitudinal 

 division of the two parts. Evidently the transverse division has 

 usually been an equal one, but that it may be unequal is show r n by 

 Figs. 3 and 5, and the rather rare occurrence of unusually large 

 and unusually small supernumeraries. Figs. 6 to 10 are from a 

 male captured at Mountain View, July 29, 1910. Here we 

 have a large and a small supernumerary in the same individual. 

 In the metaphase (6a and 6b) X and the two supernumeraries 

 were all near one pole of the spindle, while in Figs. 8 and 9 the 

 supernumeraries are at opposite poles and in Fig. 9, X is near 

 the equatorial plate. In Fig. 10, X and both supernumeraries 

 have gone undivided to one second spermatocyte. No cases of 

 the division of either supernumerary in the first maturation 

 mitosis were found in this individual. 



In Metapodius Wilson found no somatic variations correspond- 

 ing to the variation in the number of supernumeraries. In fact 

 the insects with X alone, X and Y, or X, Y and I to 6 super- 

 numeraries are described as indistinguishable. These speciec of 

 Diabrotica are very variable in size, and in regard to size and 

 fusion of the 12 black spots on the elytra, but as I showed in 1908 

 there is no significant correlation between these somatic varia- 

 tions and the presence or absence or number of supernumeraries 

 ('08, Tables I. and II., and p. 465, text). In Metapodius the 

 indications are that the chromosome Y is of no hereditary value, 

 and the supernumeraries, as duplicates of Y would not be ex- 



