262 WM. REES B. ROBERTSON 



shows that probably twelve of the twenty-three chromosomes 

 have fused to form the six V's which are always present. If 

 we admit that this may have occurred, the number continues 

 to be twenty- three for this as for other genera. The case de- 

 scribed by McClung seems to be an exception I cannot explain. 

 A third exception is that described by Montgomery in 1906 in 

 Syrbula acuticornis, w^here the number was reported as varying 

 from twenty to twenty-four. This, however, may have been a 

 mistake, since I have shown in the present paper that this species 

 may have twenty-three. For these three subfamilies of the 

 Acrididae, the Truxalinae, Oedipodinae and Acridiinae, the 

 fundamental number therefore seems to be twenty- three. 



The phenomenon of linking as shown in the compound V- 

 chromosomes of Chorthippus leads us to suspect that funda- 

 mental numbers will probably be found for the subfamilies of 

 the Gryllidae and the Locustidae also, where V's are known 

 frequently to be present. A start in this direction has been 

 made for the Locustidae, in the genus Jamaicana, as described 

 by Woolsey ('15) and in this paper. Similar conditions have 

 been found by Metz ('14) for the Diptera in the species of Droso- 

 phila. It is to be expected, I believe, that a great deal of the 

 variation in chromosome numbers among nearly related species 

 may be ascribed to this cause. 



As the number of the chromosomes is, within certain limits, 

 constant, so too are their sizes. It seems that no group of 

 species or of genera so far studied presents such unmistakable 

 evidence in constancy of size gradations as do the genera of 

 the Tettigidae. There may readily be seen in all cells of the 

 body, both somatic and germinal, a series of relative sizes con- 

 stant for the six pairs of autosomes and for the single (cf) or 

 paired ( 9 ) sex chromosomes. These size gradations are con- 

 stant for the individuals of a species and, with slight variation, 

 for the species of a genus; likewise, with somewhat greater varia- 

 tions, for the genera of a family. Of the autosomes there may 

 be recognized two very small pairs, two intermediate ones and 

 two extremely large ones. The two pairs within each of these 

 three groups may in turn be distinguished by slight difference 



