226 THE VITAL FUNCTIONS. 



by the time it is ready to enter into the pupa state. 

 When it has undergone that change, it decreases 

 to 22, and afterwards to 10 or 12 ; and during the 

 subsequent period of hybernation, it almost entirely 

 ceases. When the insect is developed into its per- 

 fect state, and excited in flight, this nvimber rises 

 to about 130 ; but when it is in a state of repose, it 



served in the chrysalis (Fig. 340) : in all the stages they may be 

 divided into three sets ; the first is given off immediately after the 

 vessel has passed beneath the ganglion, and consists of two lateral 

 trunks, the united capacity of which is equal to about one-third of 

 that of the aorta ; they descend, one on each side of the mouth, and 

 are each divided into three branches. The second set consists of two 

 pairs of branches ; one going apparently to the tongue ; the other 

 to the antennae. The third set is formed by two branches, which 

 pass upwards, and are the continuations of the aorta ; they divide 

 into branches, and are lost in the integuments, and structures of 

 the anterior part of the head. 



The pulsatory action of the dorsal vessel is continued along its 

 whole course, and seem? to terminate at the division of the vessel 

 into branches. During the metamorphoses of the insect, this vessel 

 becomes considerably shortened ; but is stronger, and more conso- 

 lidated in its structure. Its course is likewise altered ; from having, 

 in the caterpillar (Fig. 339) passed along, nearly in a straight line, 

 it begins in the chrysalis (Fig. 340), to descend in the fifth segment, 

 and to pass under what is to become the division between the thorax 

 and abdomen in the perfect insect. It then ascends in the fourth 

 segment, and descends again in the second ; so that when the in- 

 sect has attained its perfect form, (Fig. 341) its course is very 

 tortuous. The vessels which enter it are situated in the abdomen, 

 and pass in laterally among the muscles, chiefly at the anterior part 

 of each segment or valve. Fig. 342 is a superior, or dorsal view of 

 the same vessel, in the perfect state of the insect, which shows still 

 more distinctly the vessels entering it laterally, intermixed with the 

 lateral muscles. Fig. 343 is a magnified lateral view of the anterior 

 extremity of the dorsal vessel, corresponding to Fig. 341 ; and Fig. 

 344, a similarly magnified view of the same portion of the vessel 

 seen from above, corresponding to Fig. 342. Fig. 345 shows the 

 mode in which the valves are formed by a duplicature of the inner 

 membrane in the perfect insect. 



