Tracheal System 109 



insect. In Chironomus the posterior thoracic spiracle is 

 clearly nietathoracic, while the anterior spiracle lies in 

 the groove between the pro- and mesothorax. The 

 tendency of the spiracles to shift into the intersegmental 

 grooves in front may be attributed to the necessity of 

 protection for an organ of vital importance. 



In Aphis-larvae the whole series can be plainly seen. 

 Each segment has its own pair of spiracles, that of the 

 prothorax being of peculiar form ; the spiracle in all 

 cases is situated near the middle of the segment. 



The tracheal system of the fly, though very much more 

 extensive than that of the larva, is not so elaborate as in 

 large insects of powerful flight. Its arrangement is as 

 follows : — The anterior or mesothoracic spiracle is con- 

 nected by a short branch with a longitudinal trunk, 

 which sends off several branches to the head, and with an 

 external branch which passes outside the vertical muscles 

 of the mesothorax. There is a pair of good-sized air-sacs 

 between the vertical and longitudinal muscles of the 

 mesothorax. The main longitudinal trunks pass inside 

 the vertical muscles, and are connected in front of them 

 by a transverse branch. They are continued forwards to 

 the head, and in this part of their course lie very near 

 to the dorsal vessel. From each metathoracic spiracle a 

 branch joins the main longitudinal tnink, which gives off 

 at the same place a large descending branch. The trunks 

 are then continued into the abdomen, and receive 

 branches from the spiracles. The abdominal spiracles 

 are so minute that it is hard to say how many of them 

 are open ; probably either four or five, viz. those lying 

 in the intersegmental spaces behind the four or five 

 anterior abdominal segments. 



In the more primitive insects, the reproductive organs Reproduc- 

 are not very unlike in the two sexes, and the general 

 arrangement is comparatively simple. A number of 



