NO. 2 THORACIC MECHANISM OF A GRASSHOPPER — SNODGRASS IO5 



lobe (n) of the spiracle. There is no special device for opening this 

 spiracle ; the lips diverge by their own elasticity, as is shown by the 

 fact that the spiracle is always open in a dead insect. The occlusor 

 muscle brings the edges of both lips together. 



The abdominal spiracles. — The eight spiracles of the abdomen in 

 Dissosteira are quite different from either of the thoracic spiracles. 

 They are not provided with projecting external lips (fig. 53 B), 

 the body wall being directly inflected in each spiracle to form an open 

 atrial chamber (Atr). The atrium leads by a narrowed aperture at 



Fig. 52. — Second thoracic spiracle of Dissosteira. 



A, outer view of left spiracle. B, inner view of right spiracle, c, anterior lip 

 of spiracle ; d, posterior lip of spiracle ; Epvi2, epimeron of mesothorax ; EpS3, 

 episternum of metathorax ; Isg, intersegmental fold ; wb, membrane surrounding 

 spiracle; n, ventral lobe of spiracle uniting the lips and giving insertion to 

 spiracular muscle (///) ; 0, internal lobe on rim of coxal cavity on which 

 spiracular muscle arises; r, internal intersegmental fold; Tra, trachea; iii, 

 closing muscle of spiracle. 



its inner end into the spiracular trachea (Tra), and the occlusor 

 mechanism regulates this opening. 



The longer axis of the first abdominal spiracle is obliquely hori- 

 zontal (fig. 54 A) with the anterior end a little higher than the 

 posterior. The other spiracles (C, D) are placed more nearly vertical, 

 so that the dorsal end of each corresponds with the anterior end of 

 the first spiracle. In each spiracle one wall of the atrium is rigid 

 (fig. 53 B, t), and the other (s) is movable. The rigid wall is dorsal 

 in the first spiracle (fig. 54 A, /) and posterior in the other spiracles 

 (C, D). It is strengthened by a thickening in the external body wall 

 (figs. 53 B, 54 A, It) from which it is inflected. The movable wall 

 of the atrium (s), which is ventral in the first spiracle (fig. 54 A) 



