NO. 6 GRASSHOPPER ABDOMEN SNODGRASS II 



(Mb). Laterally, however, it is connected on each side with the pos- 

 terior angle of the metanotum (ANs), and by a strong postalar arm 

 (Pa) with the lower end of the narrow metapleuron (Ph)- The third 

 phragma (sPh) consists of a pair of long lobes projecting posteriorly 

 from the antecostal suture of the first abdominal tergum through the 

 first and second abdominal segments. The extraordinarily long dorsal 

 muscles of the metathorax extending back to the third phragmatal 

 lobes are plainly visible through the membrane separating the post- 

 notum from the metathoracic alinotum. 



THE ABDOMINAL SPIRACLES 



The spiracles of insects, the writer assumes, belong to the dorsum. 

 In a generalized arthropod the limb bases lie between the dorsum and 

 the venter, and there is no evidence that the insect spiracles are de- 

 veloped on the bases of the limbs. The spiracles may be included in 

 the tergal sclerotization of the dorsum, or they may lie free in a 

 laterodorsal membrane, or again, they may be situated in small latero- 

 dorsal sclerites. The abdominal spiracles of Acrididae are all con- 

 tained in the lower parts of the tergal plates (figs. i. 2 A) ; in the 

 Tetrigidae all but the first lie in the laterodorsal membranes below the 

 terga (fig. 2 C) ; in the Tridactylidae the first two spiracles on each 

 side are in the laterodorsal membranes of their segments (E), the 

 others are contained in small laterotergites (Itg), except the last, which 

 lies in the lateral part of the eighth tergum (fig. 19 A). 



The abdominal spiracles of Acrididae are of the type of structure 

 in which the closing apparatus is at the inner end of the atrium where 

 the latter is joined by the spiracular trachea. They thus diflfer, as 

 abdominal spiracles usually do, from the thoracic spiracles, which are 

 closed by an approximation of the outer lips of the atrium. 



The large first abdominal spiracles of Dissosteira, as already ob- 

 served, lie in the anterior walls of the tympanal capsules (figs, i, 6 A, 

 9 A, ISp). Each of these spiracles presents externally an oval aper- 

 ture, the long axis of which is somewhat oblique. The walls of the 

 atrium are direct inflections of the body wall. The dorsal atrial wall is 

 immovable and is firmly supported by a dense sclerotization of the body 

 wall above it ; the ventral atrial wall, on the other hand, is a freely 

 movable plate, and a small area of the body wall below it is mem- 

 branous. Viewed internally (fig. 5 A), it is seen that the movable 

 ventral wall of the atrium (c) is produced posteriorly in a handle- 

 like process, or manubrium (g), on which the spiracular muscles are 

 inserted. The short occlusor muscle (i4(S) takes its origin on the 



