176 THE VITAL PTTNCTIONS. 



system of circulating vessels in tliat insect, of which the for- 

 mer is only a dctaclied part, is sliown in Fig. 33S, where 

 the course of the Ijlood is indicated h}' arrows; A, repre- 

 senting the currents in the antenna:; w, those in the rudi- 

 mental wings; and t, those in the tail; in nil which parts 

 the vessels form loops, derived from the main vessels of the 

 trunk. In some larvre tlie vascular loops, conveying tliesc 

 collateral streams, pass only for a certain distance into the 

 legs; sometimes, indeed, they proceed no farther than the 

 haunches. The currents of hlood in these vessels have not 

 a uniform velocity, heing accelerated hy the impulsions 

 they receive from the contractions of the dorsal vessel, 

 which appears to he the prime agent in tlicir motion. 



As the insect advances to maturity, and passes through 

 its metamorphoses, considerahle 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 ap- 

 pearing to retire into the more internal parts. In the wings, 

 on the other hand, where the development proceeds rapidly, 

 the circulation becomes more active; and even after tliey 

 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 dis- 

 tinctly visible:^ but afterwards, as the wings become dry, 

 it ceases there also, and is then confined to the vessels of the 

 trunk. In proportion as the insect approaches to the com- 

 pletion of its development, these latter vessels also, one after 

 the other, shrink and disappear, till, at length, nothing which 

 had once appertained to this system remains visible, except 

 the dorsal vessel. But, as \vc observe this vessel still con- 

 tinuing its pulsatory movements, we may fairly infer that 

 they are designed to maintain some degree of obscure and 

 imperfect circulation of the nutrient juices, through vessels, 



• These currents in the wing- of the Semblls hilineata have been described 

 and delineated by Carus, in the Acta Acad. Cxs. Lcop. Carol. Nat. Cur. vol. 

 XV. part ii. p. 9. 



