CIRCULATION IN INSECTS. 227 



is reduced to between 40 and 50. At a more ad- 

 vanced period, it again becomes imperceptible.* 



The discovery of the circulation in insects, and 

 of its varying energy at different periods of growth, 

 has elucidated many obscure points in the phy- 

 siology of this important 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 developement is advancing ; and accord- 

 ingly, the period during which the insect remains 

 in the imago condition is generally short, compared 

 with that of the larva, where a large supply of nutri- 

 ment, 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 maturity. 



The provision which has thus been made for the 

 equable distribution of the nutrient juices of insects, 

 may appear to be exceedingly imperfect, when 

 compared with the extensive vascular apparatus for 

 circulation which is met with in all the higher classes 

 of animals. Yet we find it to be fully adequate to 

 the maintenance of the animal functions in a high 

 degree of energy. This arises from its being con- 

 joined with a peculiar conformation, by which the 

 complete oxygenation of the blood throughout the 

 system is effectually accomplished ; an abundant 



* Todd's Cyclopaedia of Anat. and Phys. ii. 982. 



