422 ZOOLOGY. 



to prevent its repulse, and escapes from it by its cephalic 

 extremity. Moreover, the motion of the blood does not 

 depend altogether on this organ, for there have been dis- 

 covered in several insects moveable valvules, whose pulsations 

 determine in this liquid rapid currents, and strange to say, it 

 is in the limbs that this apparatus is lodged. 



527. The blood become venous by its action on the 

 different tissues of the economy, is thus unable to reach, a 

 determinate part of the body, to place itself in contact with 

 the oxygen, and thus to recover its vivifying qualities. If 

 the respiration were accomplished in the ordinary way, by 

 the aid of lungs or by the external surface of the body, it 

 would have been consequently extremely incomplete ; but the 

 disadvantage which would seem necessarily to have resulted 

 from this great imperfection in the so important function of 

 the circulation does not in reality exist. Nature has made 

 amends for the rapid and regular transport of the blood by 

 conducting the air itself into all parts of the body by means 

 of a multitude of canals which communicate with the exterior, 

 and are infinitely ramified in the substance of these organs 

 (Fig. 57). 



Fig. 444. Male Lampyris Fig. 445. Female Lampyris. 

 (Glow-worm) 



These air-bearing tubes, designated, ^s we have already 

 said ( 133), under the name of trachea, present a complex 

 structure ; they are composed generally of three tunics, of 

 which the middle one is formed of a cartilaginous filament 

 rolled into a spiral, like the elastic of a strap (bretelle]. 

 Sometimes they are simple, but at other times they present a 

 certain number of large swellings, in the form of soft vesicles, 

 which perform the functions of a reservoir of air (Fig. 57). 

 The apertures by which the air penetrates into the trachea are 

 called stigmata; they resemble in general a small button- 

 hole, but these often have valves which open and shut like 



