X] THE BODY-WATT, AND MUSCLES 211 



which are antagonistic, but pull very nearly in the same direction, 

 can both elevate and depress the wing. (This diagram is not in 

 any sense intended to shew the true shape and positions of the 

 structures indicated, but merely to explain their action.) 



The Diaphragm. 



According to Amans [1], who discovered it, the diaphragm is an 

 elliptical membrane stretched across the body-cavity of the larva, 

 between the fourth and fifth abdominal segments, in front of the 

 Malpighian tubules. It is inserted externally, above and below, 

 on to the inter-segmental muscles, and internally on to the circular 

 muscle-tunic of the mid-gut. It shews muscle-fibres stretching in 

 different directions. Amans considers that it plays an important 

 part in respiration, defaecation, and especially in the play of the 

 mask. The diaphragm is slightly inclined to the vertical plane. 

 A contraction of the abdominal segments causes it to become 

 taut, and consequently the blood exerts a forward pressure on the 

 narrow cavity of the mask. This causes the movable hooks to 

 shoot out suddenly, as explained on p. 79. 



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