NO. 2 THORACIC MECHANISM OF A GRASSHOPPER SNODGRASS 



45 



suture (/) which is continuous through the anterior part of the meso- 

 sternum {S2) and sets off from the sternum and the two episterna a 

 narrow anterior marginal piece, or prepcctus (Ppct), which is analo- 

 gous to the similar sclerite of the Ichneumonidae and some other 

 Hymenoptera. To the posterior margin of the epimeron of the meta- 

 thorax {Eprn^) is attached the large lateral extension of the meta- 

 thoracic postnotum (PN3). 



The two pairs of spiracles of the thorax are presumably the meso- 

 thoracic spiracles and the metathoracic spiracles, each pair being dis- 

 placed anteriorly. The first spiracle on each side (fig. 26, Sp2^ is 



PIS 



.D PlR. 



Fig. 2/. — Pterothoracic pleura of orthopteran nymphs. 



A, pterothoracic pleura of an acridid nymph, showing laterosternal arms of 

 pleura (Ls) separated from sterna (S2, Ss) by the pleuro-sternal sutures (r, r). 



B, mesopleuron and coxa of young nymph of Grylliis assiiiiilis, showing a dis- 

 tinct precoxal sclerite (Acx) between episternum (Eps) and sternum (S). 

 C, inner view of B, showing the basalar and subalar muscles of the nymph 

 (M', M") attached dorsally on edges of episternum and epimeron, respectively. 



situated laterally in the intersegmental membrane between the pro- 

 thorax and the mesothorax, where it is covered by the posterior fold 

 of the protergum. The second spiracle (fig. 26, Spz) appears in the 

 adult to lie in the lower posterior angle of the mesepimeron {Epuu) 

 just above the base of the middle leg, and anterior to the fold between 

 the mesothorax and the metathorax (fig. 28), but in the nymph 

 (fig. 27 A) it occurs in the intersegmental fold. 



The structural pattern of the internal surface of the pleural wall of 

 the pterothorax (fig. 28) is a replica of that of the outer surface, 

 except that the impressed lines of the latter are represented by ridges. 

 Each pleural ridge (PlRo, PlRs), however, gives off from its lower 

 end a large pleural arm, or pleural apophysis {PIA2, PlAs), that 

 projects inward across the coxal cavity, where it is closely associated 

 with the lateral arm of the corresponding sternal apophysis (fig. 31, 



