12 



SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 94 



margin of the tympanal capsule just above the spiracle ; the long slender 

 dilator muscle ( 147) , together with the tensor of the tympanum ( 146) , 

 arises ventrally on an inflection of the membranous body wall (fig. 

 9 A) posterior and mesad of the hind coxa behind the small triangular 

 lateral sclerite of the metasternum (fig. i, t). The occlusor muscle 

 closes the inner aperture of the atrium into the spiracular trachea by 

 bringing the inner margin of the movable plate of the ventral atrial 

 wall against the inner margin of the immovable dorsal wall. The 

 antagonistic dilator muscle counteracts against the occlusor and o^^ens 

 the tracheal aperture. 



The other abdominal spiracles have essentially the same structure 

 as the first spiracle, though they are successively smaller to the eighth, 



f 



Fig. 5. — Structure of the abdominal spiracles. Dissosteira Carolina. 



A, right spiracle of first segment in rim of tympanum, inner view, showing 

 occlusor {148) and dilator (/-//) muscles. B, right spiracle of eighth segment 

 with end of trachea, inner view, showing occlusa {osp) and dilator {dlsp) 

 muscles. C, same, trachea removed, showing tracheal entrance (t) from atrium 

 and movable anterior valve {e) with manubrium (</) on which muscles are 

 attached. 



which again is of larger size (fig. i) ; also the obliquity of the aperture 

 is more pronounced in these spiracles (fig. 5 B, C), so that the movable 

 wall of the atrium {c) becomes anterior, with the manubrium {g) 

 directed downward, and the immovable wall (/) posterior. The short, 

 fan-shaped occlusor muscle of each spiracle {osp) arises on the tergal 

 wall behind the spiracle, and the long dilator muscle (dlsp) takes its 

 origin ventrally on the anterior part of the lateral margin of the cor- 

 responding segmental sternum. 



THE TYMPANAL ORGANS 



On the lower part of each lateral area of the first abdominal tergum 

 just behind the spiracle is located the large tympanal organ of Acridi- 

 dae (fig. I, Tm). In Mclauophis the tympanum is contained in a 



