176 THE VITAL FUNCTIONS. 



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

 mer is only a detached part, is shown in Fig. 338, where 

 the course of the blood is indicated by arrows; A, repre- 

 senting the currents in the antennae; w, those in the rudi- 

 mental wings; and T, those in the tail; in all which parts 

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

 trunk. In some larvae the vascular loops, conveying these 

 collateral 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 velocity, 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 ap- 

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

 on the other hand, \vhere the development proceeds rapidly, 

 the circulation 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 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 we 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 Semblis bilineata have been described 

 and delineated by Carus, in the Acta Acad, C<es. Leop. Carol. Nat. Cur. vol. 

 xv. part ii. p. 9. 



