96 



sets move the abdominal segments on one another, and can flex the 

 abdomen downwards, as in oviposition. Possibly they also assist, after 

 the manner of the abdominal muscles in mammals, in the expulsion 

 of the ova. They also provide the motive force for respiration. 



THE RESPIRATORY SYSTEM (PLATE XX) 



In insects respiration is carried on by a system of tubes which 

 has many analogies with the arterial system of vertebrates. The 

 tubes open on the surface of the body, and are there in communication 

 with the external air. As they pass inwards they divide into innumer- 

 able branches, which, progressively diminishing in size, conduct the 

 air into the innermost interstices of the tissues for the supply of 

 the body cells. Those tissues in which the metabolism is the most 

 active, as for instance the mid-gut, in which digestion takes place, 

 and the ovaries, receive the largest supply of air. 



The external openings through which the air enters the tracheae 

 or air tubes are termed the spiracles or stigmata. Normally one would 



expect to find one pair to each segment, and this 

 Spiracles . . . . . . 



arrangement is, in tact, tor the most part preserved, but 



some have been suppressed on account of the shortening of the bod}'. 

 In the head, in which no respiratory movements can take place, there are 

 no spiracles, nor are there any in the very much reduced prothorax. 

 The meso-and meta-thoracic segments have always a pair each, that 

 of the former being the largest in the body. In the abdomen the 

 anterior segments have a pair each, but the reduced segments at 

 the posterior end, and those segments which are modified to form 

 the external genitalia, are without them. The number of abdominal 

 spiracles is naturally less in the Cyclorrapha than in the Orthorrapha, 

 and the size of the tracheae from the thoracic spiracles is increased 

 to a commensurate extent. 



The situation of the thoracic spiracles differs a little in the two 

 divisions of the Diptera. In the Orthorrapha, in which there is a distinct 

 membraneous interval between the dorsal and lateral walls, the anterior 

 spiracle is usually situated at the anterior end of this interval and is 

 separated from the chitinous portion of the thoracic wall. The posterior 

 or meta-thoracic spiracle is placed in a similar membraneous area between 

 the lateral plates of the mesothorax and metathorax. In the Cyclorrapha, 

 in which the consolidation of the thorax has proceeded further with the 

 development of a more powerful and rapid flight, the membraneous 



