127 

 fa 



PART IV. 



VITAL FUNCTIONS 



CIRCULATION OF THE BLOOD. 



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

 insects analogous 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 without 

 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. 



378. Along the backs of insects there is a tubular or- 

 gan, called the dorsal vessel. This extends the whole 

 length of the back, and is found in every stage of their 

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

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

 backward and forward, by the alternate contractions and 

 dilations of the muscles of the vessel, producing a kind of 

 pulsation. 



379. 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 



380. In some insects, whose bodies are transparent, the 

 whole circulation may be distinctly seen by means of a 

 microscope. 





