CIRCULATION IN INSECTS. 179 



class. It explains why insects, after they have attained their 

 imago state, and the circulation is nearly obliterated, no 

 longer increase in size, and require but little nourishment 

 for the maintenance of life. This, however, is a state not 

 calculated for so long a duration as that in which the deve- 

 lopment is advancing; and, accordingly, the period during 

 which the insect remains in the imago condition is generally 

 short, compared to that of the larva, where a large supply of 

 nutriment, and a rapid circulation of the fluids, concur in 

 maintaining the vital functions in full activity. Thus, the 

 Ephemera, which lives for two or three years in the larva 

 state, generally perishes in the course of a few hours after 

 it has acquired wings, and reached its perfect state of ma- 

 turity. 







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 se- 

 cond set consists of two pairs of branches, one going- apparently to the tongue, 

 the other to the antennas 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 considejably shortened, but 

 is stronger and more consolidated in its structure. Its course is likewise al- 

 tered; 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 tho- 

 rax and abdomen in the perfect insect. It then ascends in the fourth seg- 

 ment, 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 ex- 

 tremity of the dorsal vessel, corresponding to Fig. 341; and Fig. 344, a simi- 

 larly magnified view of the same portion of the vessel seen from above, cor- 

 responding 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. 



