CLASS INSECTS. 385 



digestive tube, and mingles with the blood. This last liquid is 

 watery and colourless ; it is not enclosed in vessels, and is found 

 spread about in the interstices which the organs have between 

 them, or present in the substance of their tissue. Insects also 

 have no regular circulation. We perceive, indeed, in certain 

 parts of their bodies, currents even sufficiently rapid, but the 

 nourishing liquid does not describe a circle so as to return 

 constantly to its point of departure. There exists in fact, in 

 these animals, only the vestiges of the circulating apparatus 

 ( 112). We observe near the dorsal surface of the body a 

 longitudinal tube (Figs. 336 and 337), which performs alter- 

 nate movements of contraction and of dilatation, analogous to 

 those of the heart in the superior animals. But this dorsal 

 vessel seems to give off no branch. The nourishing liquid 

 penetrates into it by lateral openings, furnished with valvules 

 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 thf 

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

 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. 48). m 



These air-bearing tubes, designated, as we have already 

 said ( 133), under the name of tracheae, 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 

 c c 



