NO. 2 THORACIC MECHANISM OF A GRASSHOPPER SNODGRASS 25 



fifth abdominal sterna, and two lateral groups on each side which go 

 to the parasternal plates of the second abdominal segment. In the 

 Cicadidae ventral muscles extend from the metathorax to the second 

 abdominal sternum. In the Tenthredinidae, according to Weber 

 (1927), a pair of muscles extends from the metaf ureal arms to the 

 second sternum of the abdomen, and in the honeybee Morison (1927) 

 describes two corresponding pairs of muscles going from the meta- 

 furca to the anterior margin of the second abdominal segment. Inas- 

 much as these muscles, which represent the ventral muscles of the first 

 abdominal segment, have no connection with the first abdominal ster- 

 num, it is evident, as Morison points out, that their insertions, normally 

 on the intersegmental anterior edge of the first abdominal sternum, 

 have been secondarily transferred to the f ureal apophyses of the meta- 

 thoracic sternum. In the Ephemerida, however, Diirken (1907) 

 records the presence of a pair of muscles attached anteriorly on the 

 bases of the metasternal apophyses and posteriorly on the anterior 

 margin of the first abdominal sternum. These muscles would appear 

 to correspond with the furco-spinal muscles, which are present in the 

 prothorax of the grasshopper (fig. 35, 61). 



Even a brief review of the comparative musculature of larval and 

 adult holometabolous insects thus shows that there takes place during 

 metamorphosis a rearrangement in the attachments of the ventral 

 muscles of the thorax, and, in some cases, of those of the first abdomi- 

 nal segment, as a result of which most of the persisting fibers lose 

 their intersegmental connections and acquire segmental attachments on 

 the furcal apophyses of the thoracic sternal plates. 



The larval condition of intersegmental muscle attachments is clearly 

 a more primitive one than that of the adult. The adults of insects with 

 incomplete metamorphosis resemble those of holometabolous forms in 

 having the principal ventral muscles attached on the furcal arms. 

 Therefore, we must suspect that the latter condition is one secondarily 

 acquired in all pterygote insects, and that it has come about during 

 the evolution of the thorax as a specialized locomotor region of the 

 body. Since the transposition of the ventral muscles takes place in 

 the prothorax as well as in the other two thoracic segments, we cannot 

 attribute its inception to the development of the wings. As yet, how- 

 ever, we may draw only tentative conclusions concerning the evolution 

 of the ventral musculature of the thorax, since our knowledge of the 

 nymphal muscles in hemimetabolous insects and of both the larval and 

 adult muscles in the more generalized holometabolous forms is very 

 incomplete ; but the facts known point strongly to the transformation 

 suggested above. 



