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THE POPULAR SCIENCE MONTHLY 



The change of air is produced in the tracheal system by the en- 

 largement and contraction, or bellows-action, of the abdomen, the seg- 

 ments sliding in and out with a telescopic movement, very perceptible 

 in the larger insects. 



Many insect larvae live in water, but are still air-breathers. The 

 mosquito is an example. Here the eighth joint of the abdomen sends 

 off a long tube, crowned at the extremity with feathery bristles, which 

 are brought to the surface of the water to seize a. drop of air. With 

 this the larva descends and the repellent action of the setae prevents 

 the water from dissolving it. Some water-larvae have tubes several 

 inches long. Other larvae, destitute of tubes, have the spiracles at the 

 posterior end of the abdomen. In the pupa state the mosquito has 

 two respiratory tubes on the back of the thorax. 



Fig. 19. Gill-Lungs of Ampfllaria. a, Ampullaria irmtlarum (D'Orb.). a, long respiratory 

 siphon ; b, section in the direction of the arrow b ; h, the upper lung-cavity ; k\ branchial cav- 

 ity with the right and left gills ; the cavities communicate by a passage in the center of the 

 dividing wall. 



The aquatic larvae of the dragon-flies {Idbellulidce) have, as already 

 mentioned, hair-like processes, "villi," of the lining of the rectum, 

 which absorb air from the water and convey it into the tracheal sys- 

 tem. It is difficult to determine if such larvae should be regarded as 

 aquatic or aerial. Certainly in their habits, and in their method of 

 procuring oxygen, they are aquatic and possess gills. But the oxygen 

 is not immediately given to the blood in the villi, as in true gills, but 

 is conveyed through tracheae just as in adult insects. On the latter 

 account these larvae are generally considered aerial. 



