222 



THE VITAL FUNCTIONS. 



sels form loops, derived from the main canals of 

 the trunk. In some larvae the vascular loops, con- 



337 



»-* 





338 



veying these collateral streams, pass only for a 

 certain distance into the legs; sometimes, indeed, 

 they proceed no farther than the haunches. In all 

 cases, and more particularly in the wings, these 

 channels of circulation surround the tracheae ; so 

 that the blood is brought into immediate approxi- 

 mation to the air. The currents of blood in these' 

 vessels have not a uniform velocity ; being accele- 

 rated by the impulsions they receive from the con- 

 tractions of the dorsal vessel, which appears to be 

 the prime agent in their motion. 



As the insect advances to maturity, and passes 

 through its metamorphoses, considerable changes 

 are observed to take place in. the organization of 

 the circulating system, and in the energy of the 

 function it performs. The vessels in the extreme 

 parts, as in the tail, are gradually obliterated, and 

 the circulation in them, of course, ceases; the 

 blood appearing to retire into the more internal 



