STRUCTURE AND DEVELOPMENT OF INSECTS 31 



the head, where it again flows into the body-cavity. Thus various 

 currents of blood are maintained throughout the body, but other 

 than the heart there is no system of blood-vessels, the blood merely 

 filling the body-cavity around and through the various organs and 

 tissues. Constantly flowing around the respiratory tubes or 

 tracheae, the blood is quickly and thoroughly purified, though the 

 exact manner in which this is done is not definitely known. The 

 respiratory system has absolutely no connection with the mouth 

 or pharynx (Fig. 23, ph) , as have the lungs of the higher animals, 

 and if an insect is to be suffocated, it must be done by closing the 

 spiracles. It is in this way that tobacco-dust, lime, pyrethrum, 

 and similar insecticides kill sucking insects, by penetrating the 

 spiracles and choking the trachea! system. Whale-oil soap, 

 kerosene emulsion, and the other "contact " insecticides, or " irri- 

 tants," also stop up the spiracles and thus cause death, but they 

 may act as " irritants," penetrating the skin and thus killing the 

 insect. When insects are killed by means of a gas such as carbon 

 bisulfide or hydrocyanic acid gas, they are asphyxiated by a 

 substitution of these gases for air, the same as are the higher 

 animals. 



The ugh arsenical poisons are generally used as sprays for biting 

 insects, soft-bodied caterpillars and similar Iarva3 are often killed 

 by the use of contact insecticides, which affect them the same as 

 sucking insects. 



The reader will observe that, almost without exception, the 

 remedies advised for different insect pests in the following pages 

 are determined by some peculiarity, either of structure or develop- 

 ment, of the insect to be combated. 



