320 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 37 



whatever dissimilarity exists between the second roots in the pro- 

 thorax and the mesothorax. Five branches of the second root system 

 in the prothorax enter the integument, but corresponding integumen- 

 tary nerves are absent in the pterothorax. 



The musculature peculiar to the prothorax and innervated by the 

 prothoracic second root consists of the following muscles: (i) Ce- 

 phalic muscles of the cervical sclerites (50, 51) ; (2) ventral lateral 

 neck muscle (57). 



Perhaps the only point of interest here is the fact that muscles 

 arising on the postocciputal ridge and inserting on the cervical sclerites 

 are unqualifiedly innervated by the prothoracic ganglion. 



VI. THE DEPRESSOR OF THE TROCHANTER 



The depressor of the trochanter (71) is a five-branched muscle 

 inserting on a strong apodeme arising from the base of the trochanter. 

 Two of these muscle branches arise within the coxa and three within 

 the body, as follows: 71a, anterior surface of coxa; 71b, dorsal area 

 of episternum; 71c, pleural arm; 7id, lateral wall of protergum ; and 

 7ie, posterior surface of the coxa. The second nerve root provides 

 the innervation of the body branches (71b, 71c, and 7id), and the 

 third nerve root is the source of innervation of the coxal branches (71a 

 and 7ie). This dichotomy of innervation occurs also in the ptero- 

 thorax of Dissosteira, and Maki (1936) describes an identical inner- 

 vation pattern in Chauliodcs. 



It is proposed by Snodgrass (1927) that the division of the depres- 

 sor of the trochanter in pterygote insects into a coxal branch and a 

 thoracic branch represents two separate muscle sources. It is interest- 

 ing to note that the innervation pattern of the depressor of the tro- 

 chanter supports the theory of two primitive muscle sources of the 

 depressor of the trochanter. 



VH. THE VENTRAL MUSCLES 



The ventral longitudinal muscles are usually assumed to have 

 evolved from primitive ventral muscles comparable to the primitive 

 dorsal longitudinal muscles. In a pregenital abdominal segment of 

 Dissosteira, there are three pairs of ventral longitudinal muscles, of 

 which two pairs, the median internal ventrals (fig. 5 A, vim) and the 

 lateral internal ventrals (vil) receive innervation from the dorsal 

 nerve as it passes mesad of the muscles. The third pair of muscles, 

 the lateral external ventrals (vel), are innervated by the second ab- 

 dominal or ventral nerve (VN), passing ventral and lateral. 



