22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 94 



are evidently torsion muscles, their transverse positions enabling them 

 to give a movement of partial rotation to the terga on each other. 



In the generalized condition the external dorsal muscles are longi- 

 tudinal in position and lie external to the internal dorsals, but they 

 are commonly shorter than the latter ; and have a tendency to become 

 restricted to the posterior part of the segment. In many of the higher 

 insects they become completely reversed in position, since they take 

 their origins on the posterior part of the tergum and extend forward 

 in the intersegmental fold to their insertions on the invaginated an- 

 terior margin of the following tergum. They thus become tergal pro- 

 tractors. The position of the external dorsals of the grasshopper is 

 seen to be intermediate between the more primitive condition and that 

 of complete reversal. The external ventral muscles, on the other hand, 

 are reversed and hence function as sternal protractors. 



1^2, i/j, 17^. The ventral muscles (fig. loA). — The ventral 

 musculature of segment /// is typical of that of all the pregenital 

 segments (figs. 8, 12). The median internal ventrals (fig. 10 A, 1/2) 

 are the principal sternal retractors ; the short lateral internal ventral 

 on each side (i/j) arises at the base of the anterior apodeme (aAp) 

 just before the angle of the sternal ridge (sr), and is inserted on the 

 anterior end of the corresponding apodeme of the following sternum. 

 The lateral external ventral on each side (174) is a sternal protractor, 

 being completely reversed in position, with its origin on the posterior 

 part of the sternum and its insertion anterior on the under face of 

 the anterior apodeme of the following sternum. 



The lateral musculature is alike in segments /// to VII, there being 

 in each of these segments representatives of the following five muscles 

 of segment ///, two of which are internal laterals, and three external 

 laterals. 



17^. First internal lateral muscle (fig. 10 A). — A slender muscle 

 arising dorsally beneath the edge of the lateral dorsal {168), extending 

 ventrally and anteriorly to its insertion on the base of the lateral 

 apodeme of the sternum. 



i/d. Second internal lateral muscle (fig. 10 A). — A broad muscle 

 arising on the side of the tergum just behind 775 and also beneath the 

 edge of the lateral dorsal {168), extending ventrally to its insertion 

 on the lateral margin of the sternum. The internal laterals are the 

 principal expiratory muscles, since their contraction lifts the sternum 

 and contracts the abdomen in a vertical direction. 



177. First external lateral muscle (fig. 10 A). — This muscle arises 

 ventrally on the anterior part of the ventral margin of the tergum, 



