52 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 85 



there is a pair of slender, longitudinal paratergal muscles (figs. 20, 

 21 A, B, p) lying just above the line of the spiracles and the lateral 

 tracheal trunks (Tra). Anteriorly in each segment the paratergal 

 muscles are crossed internally by a group of internal lateral fibers 

 (//) arising ventrally on the intersegmental fold, and diverging dor- 

 sally and posteriorly to their attachments on the dorsum. These in- 

 ternal lateral muscles of the caterpillar lie internal to the lateral tra- 

 cheal trunks. The dorsal transverse muscles of the caterpillar (figs. 

 20, 21 A, fd) arise in groups immediately dorsad of the paratergal 

 muscles from the posterior margins of the intersegmental folds. 



VNC 



Tra 



Fig. 20. — Ventral musculature of fourth abdominal segment of a caterpillar, 

 Estigmcnc acraca. 



li, internal lateral muscles ; p. paratergal muscle : td. origins of dorsal trans- 

 verse (cardiac) muscles; Tra. lateral tracheal trunk; z'i. internal ventral mus- 

 cles; VNC, ventral nerve cord. 



The complexity of the body musculature of the caterpillar appears 

 to demonstrate that the muscle system of insects has no limits imposed 

 on its possibility of diversification both by multiplication and by re- 

 arrangement of its fibers, since there is no reason to believe that the 

 intricate i)attern of the caterpillar muscles represents in any way the 

 primitive plan of insect musculature. In the other organization of the 

 lepidopterous larva there is little to suggest a primitive condition. The 

 head and motith parts present the tvpical fundamental structure of 

 these organs that has been developed in adult Ptcrygota, and on this 

 basic structure have been l)uilt up the man\- special featiu'cs of the 



