

PART IV. 



VITAL FUNCTIONS. 



CIRCULATION OF THE BLOOD. 



IT is but recently, that movements in the fluids of in- 

 sects analagous to the circulation in the larger animals 

 has been discovered. At the present time however, all 

 naturalists agree that such a circulation does exist. 



It will be remembered that insects are entirely with- 

 out lungs, and that the respiratory function is carried on 

 in them by means of minute tubes on each side of their 

 bodies called spiracles or stigmata. 



Along the backs of insects there is a tubular organ, 

 called the dorsal vessel. This extends the whole length 

 of the back, and is found in every stage of their devel- 

 opement, from the larva to the perfect state. It con- 

 tains a fluid which appears to have a wave-like motion, 

 backwards and forwards, by the alternate contractions 

 and dilations of the muscles of the vessel, producing a 

 kind of pulsation. 



This organ performs the office of the hearts of other 

 animals, its contractions throwing out a portion of the 

 fluid it contains, into all parts of the insect, even into its 

 wings, from which it again returns to the dorsal vessel, 

 as the blood does to the heart. 



In some insects, whose bodies are transparent, the 

 whole circulation may be distinctly seen by means of a 

 microscope. 



