188 WM. REES B. ROBERTSON 



gradations. The differences depend upon degree of remote- 

 ness in relationship. 



First of all, our short-horn grasshoppers may be divided into 

 two large groups on the basis of number of chromosomes: the 

 family Tettigidae having 13 (cf) and 14 (9), and the family 

 Acrididae with its three subfamilies having 23 (cf) and 24 ( 9 ). 



Within each of these groups the chromosomes present certain 

 general size relations, which are fairly constant, but there are 

 minor variations. The larger variations are found between 

 different genera. More distantly related genera are less likely 

 to be similar. The smaller variations between species of a genus, 

 though more difficult to recognize, follow the same principle. 

 The greater are likely to be found between more distantly re- 

 lated species, the lesser between those more closely related. 

 This does not mean that more distantly related species may not 

 occasionally be found with chromosomes more nearly alike than 

 those of less distantly related species. Observations have not 

 been extended far enough to determine that point. 



1. Tettigidae 



1. Acridium granulaius Scudd. a. Chromosmnes of the male 

 germ cells. Spermatogonial and somatic chromosomes of this, 

 and of all other species of the family thus far studied, are of the 

 rod-shaped type, and the spindle fibers are attached at the 

 proximal end; i.e., the end which points toward the center of 

 the cell plate at the time of cell division (figs. 23-28). Great 

 extremes in the relative sizes of different chromosome pairs 

 are characteristic of all the Tettigidae, but are most marked 

 in this genus. Compare chromosomes 1, 1 or 2, 2 with 6, 6 

 (figs. 23-28). The chromosomes may be readily arranged into 

 a series of six pairs besides the accessory chromosome. The 

 latter is so nearly the size of the smallest pair that it would be 

 difficult to distinguish between them, were it not that frequently 

 the sex chromosomes show the 'woolly' surface texture (no. 2x 

 in figs. 23, 25, 26, 27) often seen in other acrididaean spermato- 

 gonial stages (Sutton, '02; McClung, '05; Pinney, '08). Of 



