222 WM. REES B. ROBERTSON 



Ascaris, Tomopteris (Schreiner, '06a), in amphibia and, possibly, 

 among the chromosomes of an unidentified acridid described 

 by McClung ('14, figs. 59-62). McClung suggests that "V's 

 may have been derived from straight rods through an altered 

 transverse segmentation of a continuous spireme thread, which 

 would result in chromosomes with fiber attached at a subter- 

 minal or median position, instead of a terminal." Such V's 

 might be grouped with the bent-rod type. 



In the second group the V's bear at the apex unmistakable 

 achromatic bridges of clear non-staining material. At this 

 point there is also a constriction. Such chromosomes I be- 

 lieve to have been derived in many cases from the linkage proxi- 

 mally of two non-homologous rods (nos. 8 and 10 in Chor- 

 thippus, nos. 14 and 16 in Jamaicana unicolor). In this type 

 may also be placed those chromosomes which are loosely linked 

 together, possibly in the stage of forming V's (Browne ['10, 

 '13], in Notonecta insulata; Woolsey ['15, figs. 5-8, A], in Jamai- 

 cana flava). 



As to the validity of the former group, I am unable to say. 

 I merely suggest that such may exist (Agar, '12, fig. 9). By far 

 the larger number of the V's and U's which have been figured 

 have been represented as such, exhibiting neither constrictions 

 nor transverse clear regions and would therefore be grouped 

 with this type. But certainly not all such described V's may be 

 so grouped. 



I am certain of the occurrence of the second type of V's. 

 I also believe that this type will be found to be much more fre- 

 quent than one might suppose, for in the case of the genus Chor- 

 thippus (Stenobothrus) alone, de Sinety ('01), Davis ('08), 

 Gerard ('09), and Meek ('11, '12a) have overlooked the trans- 

 verse segmentation which I describe here. I should expect 

 to find this linkage type of V in those genera which show a smaller 

 number of chromosomes than is characteristic of the family 

 to which they belong, and at the same time a relatively larger 

 number of V-shaped chromosomes — conditions which are found 

 in comparing Chorthippus with other genera of the Truxalinae. 



