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the pleurum contracts, the spiracles open ; their opening is nearly 

 but not always exactly coordinated with the contractions of the 

 pleurum, but as a rule they are. There were 65 contractions in a 

 minute in a locust which had been held between the fingers about 

 ten minutes. It was noticed that when the abdomen expanded, the 

 air-sacs in the first abdominal ring contracted. 



For expanding the abdomen no special muscles are required, since 

 it expands by the elasticity of the parts. For contracting its walls 

 there are two sets of muscles, viz., special vertical expiratory muscles 

 serving to compress or flatten the abdomen (Figs. 415-418), and 

 other muscles which draw together or telescope the segments. 



It was formerly supposed that when the abdomen contracted the 

 air was expelled from the body and the trachea? emptied ; that, when 

 the abdomen again expanded by its own elasticity, the air-tubes were 

 refilled, and that no other mechanism was needed. But Landois 

 insisted that this was not enough ; as Miall and Denny state : 

 " Air must be forced into the furthest recesses of the tracheal system, 

 where the exchange of oxygen and carbonic acid is effected more 

 readily than in tubes lined by a dense intima. But in these fine and 

 intricate passages the resistance to the passage of air is considerable, 

 and the renewal of the air could, to all appearance, hardly be effected 

 at all if the inlets remained open. Landois accordingly searched for 

 some means of closing the outlets, and found an elastic ring or spiral, 

 which surrounds the tracheal tube within the spiracle." By means 

 of the occlusor muscle this ring compresses the tube, " like a spring 

 clip upon a flexible gas-pipe." "When the muscle contracts, the 

 passage is closed, and the abdominal muscles can then, it is supposed, 

 bring any needful pressure to bear upon the tracheal tubes, much in 

 the same way as with ourselves, when we close the mouth and 

 nostrils, and then, by forcible contraction of the diaphragm and 

 abdominal walls, distend the cheeks or pharynx." 



Thus an important point in the respiration of tracheate animals, 

 whether insects, myriopods, or arachnids, is, as Landois claimed, the 

 closure of the spiracles, in order that pressure may be brought upon 

 the air in the tubes, so that it may pass onward into the finest ter- 

 minations. 



The injection of air by muscular pressure into a system of very 

 fine tubes may, as Miall and Denny remark, appear extremely diffi- 

 cult or even impossible. Graham (Researches, p. 44) applies the law 

 of diffusion of gases to explain the respiration of insects ; but until 

 physical experiments have been made, we may, with Miall and Denny, 

 " be satisfied that an appreciable quantity of air may be made by 



