10 SMITHSONIAN MISCELLANEOUS COLLECTIONS \0L. 94 



with the metasternum in the notch between the sternellar lobes (SI 3). 

 There are no ventral muscles that extend from the thorax into the 

 abdomen in Acrididae, and the first ventral muscles of the abdomen 

 take their origin on a transverse ridge of the first abdominal sternum 

 at the base of the acrosternite (fig. 8, 143). This ridge, therefore, is 

 evidently the antecosta of the first sternum, and corresponds with the 

 phragma of the first tergum, that is, it marks the true intersegmental 

 line of the venter between the thorax and the abdomen. In the Tetrigi- 

 dae the sternum of the first abdominal segment (fig. 2D, IS) has a 

 rounded anterior edge inserted into a wide emargination of the meta- 

 sternum, but it is attached to the latter by a narrow, flexible mem- 

 branous suture, and, therefore, does not give the abdomen a firm 

 ventral connection with the thorax as in Acrididae. There is no evi- 

 dence, therefore, that the small median area between the bases of the 

 metasternal apophyses (sas) in the Tetrigidae represents the acroster- 

 nite of the first abdominal sternum ; it appears rather to be the ster- 

 nellum of the metathorax, which is suppressed medially in the 

 Acrididae. 



When we turn to the Tridactylidae by way of comparison it is to 

 be seen that there is little similarity, either in the thoracic sclerotiza- 

 tion or in the basal structure of the abdomen, between this family and 

 the Acrididae or Tetrigidae. The pleural sclerites of the pterothorax 

 in the tridactylids are reduced and widely separated by membranous 

 areas (fig. 2 E). The sterna are simple segmental plates (F, 6^2, ^3) 

 entirely separated from each other. In the mesosternum the bases of 

 the apophyses (sa) are far apart at opposite ends of a transverse 

 sternacostal suture (k). The metathoracic apophyses are somewhat 

 more approximated, and from each a suture extends forward in the 

 basisternal region. These sutures in Rhipiptcryx (fig. 2 F) are con- 

 tinuous anteriorly in a transverse arc, but in Tridactylus they remain 

 separate, as shown by Ander (1934). The sternellum of each ptero- 

 thoracic sternum is a narrow margined area behind the sternacostal 

 suture {k), and is not produced into lateral lobes as in Acrididae. The 

 first abdominal sternum {IS) is entirely distinct from the metasternum. 



In the relations of the base of the abdomen to the thorax the tri- 

 dactylids present some very unusual features. The tergum of the 

 first abdominal segment is much reduced and does not contain the first 

 spiracles (fig. 2 E, IT) ; the posterior dorsal and lateral parts of the 

 segment are membranous. The acrotergite (PN3) is a strongly de- 

 veloped though narrow sclerite on the anterior margin of the first 

 abdominal tergum, but it is widely separated dorsally from the wing- 

 bearing plate of the metathorax (AA^s) by a large membranous area 



