l8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 94 



first sternum (fig. 9B), and extends anteriorly and dorsally to its 

 insertion on the under surface of the anterior apodeme of the second 

 sternum. Its contraction separates the two sterna. 



146. Tensor of the tympanum (figs. 8, 9 A). — This muscle is the 

 only representative of the lateral muscles in the first abdominal seg- 

 ment of Dissosteira. It is a slender muscle arising laterad of the first 

 sternum in the membrane behind the base of the hind coxal cavity, 

 and extends dorsally to its insertion on the ventral process of the 

 tympanal capsule. Its contraction evidently serves to stretch the 

 tympanum. 



14/'. Dilator muscle of the spiracle (figs. 5 A, 8, 9 A). — A very 

 slender muscle arising with 146 in the membrane laterad of the first 

 abdominal sternum, extending dorsally to its insertion on the manu- 

 brium of the ventral atrial plate of the first spiracle. 



148. Occlusor muscle of the spiracle (figs. 5 A, 9 A). — A very 

 short muscle arising on the anterior part of the tympanal capsule above 

 the spiracle, inserted on the manubrium of the ventral atrial plate 

 opposite the dilator. 



MUSCLES OF THE SECOND SEGMENT 



The musculature of the second abdominal segment conforms closely 

 with that of the following segments except in the arrangement of the 

 lateral muscles. 



I4p. Median internal dorsal muscles (fig. 8) . — A flat band of four 

 more or less distinct groups of longitudinal fibers against the upper 

 part of the tergum within the pericardial cavity. Origins anteriorly 

 on the submarginal ridge of the second tergum, insertions posteriorly 

 on the anterior margin of the third tergum. 



750. Lateral internal dorsal muscle (figs. 8, 9 A). — A broad extra- 

 pericardial muscle on the lateral part of the tergum above the upper 

 ends of the posterior lateral muscles, inserted in line with the intra- 

 pericardial dorsals on the anterior margin of the third tergum. 



i^i. Paradorsal muscle (fig. 9). — This muscle lies against the side 

 of the tergum below the extrapericardial dorsal muscle, external to 

 the internal lateral muscle {139). It is inserted posteriorly on the 

 anterior margin of the third tergum. 



75^, 13^. External dorsal muscles. — Two small oblique or trans- 

 verse muscles lying external to the internal dorsals in the posterior 

 fold of the segment, where they arise on the posterior part of the 

 second tergum. One extends dorsally, the other (fig. 9 B, 13s) cen- 

 trally to their insertions on the anterior margin of the third tergum. 



