NO. 6 



INSECT ABDOMEN — SNODGRASS 



53 



caterpillar head and mouth parts adapted to the needs of the larva. 

 The alimentary canal of the caterpillar is highly specialized in its 

 musculature. The simplicity of the nervous and tracheal systems 



Tra 



S^1|M, 



Fi(i. 21. — Abdominal muscles of a caterpillar, listigniciic acraca, seen by re- 

 moval of inner muscles shown in figure 20. 



A, muscles of right half of fourth segment (IF) after removal of internal 

 ventrals ivi). B, outermost muscles in right half of third segment, showing 

 particularly the leg muscles ; cross-hatched areas represent bases of hair-bearmg 

 tubercles. 



Cx. basal rim of leg ; di, internal dorsal muscles ; Ht, heart ; //, internal lateral 

 muscles; p, paratergal muscles; td, dorsal transverse (cardiac) muscles; Tra, 

 lateral tracheal trunk ; vi, internal ventral muscles. 



is not necessarily an indication of a primitive state ; it is merely the 

 retention of a generalized structure in these organs accompanying a 

 high specialization in others. The presence of appendages on the pre- 



