46 Annals Entomological Society of America [Vol. XIII, 



V. THE MUSCLES OF THE SPIRACLES. 



The Mesothoracic Spiracle (Fig. 37) lies transversely 

 in the pleural membrane behind the prothoracic leg. The 

 anterior lip is composed of a broad ventral (a. l.i) and a narrow- 

 dorsal (a. 1.2) sclerite. Two tracheal chambers open into the 

 vestibule and between the two chambers there is a thickened 

 chitinous septum (c. t.) 



The First Occlusor of the Mesothoracic Spiracle (Fig. 37, 

 oc.i sp.) originates near the anterior margin of the episternum 

 of the mesothorax and is inserted into the ventral anterior 

 valve. It closes the orifice of the spiracle by pulling the anterior 

 lip against the posterior. 



The Second Occlusor of the Mesothoracic Spiracle (Fig. 37, 

 0C.2 sp.) originates from the ventral sclerite of the anterior lip 

 and is inserted into the anterior side of the chitinous septum 

 between the two tracheal chambers. It closes the dorsal cham- 

 ber by pulling its anterior and posterior walls together. There 

 seems to be no mechanism for closing the ventral chamber apart 

 from the first occlusor. 



The Metathoracic Spiracle (Figs. 38 and 39) has a 

 crescent shaped anterior lip (a. 1.) and a somewhat sickle-shaped 

 posterior lip (p. 1.) with an expanded ventral end. There is but 

 one tracheal chamber, the ventral wall of which has a chitinous 

 thickening which serves for the insertion of an occlusor muscle. 



The First Occlusor of the Metathoracic Spiracle (Fig. 38, 

 OC] sp.) originates from the outer border of the posterior lip 

 and is inserted into the anterior lip. It closes the spiracle by 

 pulling the two valves together. 



The Second Occlusor of the Metathoracic Spiracle (Fig. 39, 

 0C.2 sp.) has the same origin as the first, but is inserted into 

 the chitinous thickening of the wall of the tracheal chamber. 

 It closes the chamber by pulling its walls together. 



The Abdominal Spiracles (Figs. 40 and 41) have a- 

 triangular posterior valve and an arcuate anterior valve. Each 

 valve bears a narrow ridge beset with minute chitinous pro- 

 jections. The ridge of the anterior segment is the "bow" of 

 Landois, and bears near its ventral end an inward projecting 

 process, the closing lever or peg. 



