CIRCULATION IN INSECTS. 247 



plains why insects* after they have attained their 

 imago state, and the circidation is nearly oblite- 



339) : 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 capa- 

 city 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 seems 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 

 consolidated 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 di- 

 vision 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 insect 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. 



