CIRCULATION IN INSECTS. 243 



from the main vessels of the trunk. In some 

 larvse the vascular loops, conveying these colla- 

 teral streams, pass only for a certain distance 

 into the legs ; sometimes, indeed, they proceed 

 no farther than the haunches. The currents of 

 blood in these vessels have not a uniform velo- 

 city ; being accelerated by the impulsions they 

 receive from the contractions 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 

 parts. In the wings, on the other hand, where 

 the developement proceeds rapidly, the circula- 

 tion becomes more active ; and even after they 

 have attained their full size, and are yet in a 

 soft state, the motion of the blood in the centre 

 of all the nervures is distinctly visible : * but 

 afterwards, as the wings become dry, it ceases 

 there also, and is then confined to the vessels 



* These currents in the wing of the Semblis bilineata have 

 been described and delineated by Cams, in the Acta Acad. Cses. 

 Leop. Carol. Nat. Cur. vol. xv. part ii, p. 9. 



