STRUCTURE AND CLASSIFICATION. 



33 



indicated, lies just under the back, and in a cavity formed by a 

 series of peculiar triangular muscles known as the ' ' wings of the 

 heart ; ' ' and these serv^e to prevent undue pressure upon it. 

 The organ is simply a long tube, open at both ends, and divided 

 into a variable number of chambers, into which the blood is 

 admitted at the sides, at intervals along its length. The heart- 

 beat consists of a wave of contraction beginning at the posterior 

 end, forcing the blood forward and out into the head. Some- 

 times one wave is completed before another one starts, and 

 sometimes two or even three waves may be on the road at one 

 time. Sometimes the beat is exceedingly sluggish, and some- 

 times, in active insects, it is very rapid. After the blood has 

 been forced out of the heart it first bathes the head parts, and 

 then makes its way between the muscles and other organs through 

 the body cavity and into the appendages. Part of it bathes the 

 alimentary canal, where it receives the products of digestion, and 

 these are carried everywhere and assimilated by the various 

 tissues, the blood eventually finding its way back to the heart, 

 to begin its journey anew. The fact that there is no closed 

 system of blood circulation is peculiar, and in this insects differ 

 from all the higher animals. It necessitates a very decided 

 modification of the respiratory or breathing system, and this 

 forms the subject of the next chapter. 



CHAPTER V. 



THE RESPIRATORY SYSTEM. 



The respiratory or breathing system of insects is adapted to 

 the system of blood circulation. It is important from the eco- 

 nomic, as well as interesting from the scientific, stand-point, for 

 upon this rests the basis for the application of contact insecticides. 

 ■ Insects have no lungs, and nothing which at all corresponds to 

 them. They have no means of taking in air at the head or 

 through the mouth ; but breathe from the sides, where there is a 

 series of breathing-pores, or spiracles, through which the air is 

 taken into the body cavity. Typically, one pair of spiracles 



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