204 NATURAL HISTORY OF AQUATIC INSECTS ch. 



tiidinal air-tubes of the body, run through the tail side 

 by side. In cross-sections made through the respira- 

 tory tube they appear 

 kidney-shaped, the con- 

 cavities facing each other, 

 but in front of the base of 

 the respiratory tube they 

 become cyHndrical. When 

 the tail is completely ex- 

 tended the trachea; are 

 straightened out, but dur- 

 ing contraction of the tail 

 they are thrown into nu- 

 merous and regular coils, 

 which lie in the hinder 

 part of the body. 



" Retraction of the tail 

 is largely effected by six 

 muscles which are attach- 

 ed to the junction of the 

 respiratory tube and the 

 channelled sheaths. When 

 these muscles contract the 

 respiratory tube is with- 

 drawn bit by bit into the 

 channelled sheath, and 

 ultimately into the outer 

 sheath (see Fig. 71), the 

 external surface of the 

 , , , channelled sheath bccom- 



FiG. 73.— Diagram of internal structure 



of larva of Eristalis, showing the Jncr at thc SamC timC foldcd 

 retracted head, the tracheal system, fc> 



??'^T^wMi5^''°^'r"'"""''^°^ ''"'''''' inwards. This may go on 



