8o The Larva of Chironomus 



suited to a state of things in which the functional respira- 

 tory organs are limited to the hinder end of the body. But 

 when, in the course of post-embryonic development, the 

 insect acquires an extensive tracheal system, segmentally 

 repeated, the circulatory apparatus becomes repeated 

 too, and many segments are provided with contractile 

 chambers. 



6. OyganH of Respiration. 



[a) THE TRACHEAL SYSTEM. 



Traciieai TliG oxAj mcaus which the Chironomus-larva is known 



to employ for renewing its supply of oxygen is wriggling 



Fm. ^6. — Tracheal system of larva, in side view. The head and thoracic 

 segments are included. Two tracheal systems, with communicating branches, 

 and two closed spiracles at the extremities of initial branches, are seen. 



about in the comparatively well-aerated water near the 

 surface of the stream. Owing to the circumstance that 

 this exercise is usually taken by night, we have no 

 detailed information as to its frequency or duration. The 

 few observations which we have ourselves made only 

 show that larvae kept in a deep tank with a sediment of 

 mud and decaying leaves are frequently seen to rise to 

 the top, nearly always by night. 



The larva has only a rudimentary tracheal system, 

 which appears late in the larval stage (fig. 56). In Chiro- 



