ANATOMY OF INSECTS INTERNAL 



35 



In aquatic insects various respiratory devices have been developed. 

 Many of them (May-fly, dragon-fly, stone-fly, and mosquito nymphs) 

 bear tracheal gills which consist of a leaflike expansion, or a tuft of 

 thin filaments, into which the tracheae extend and divide into a fine 

 network. The oxygen of the water passes through the gill mem- 

 brane into the air of the tracheae, and thus the air of the tracheal 

 system is purified. No true gills, that is, gills carrying blood 

 vessels, like those of fishes, are found in insects. Other aquatic 

 insects carry a thin film of air with them, either by means of a 

 thick coating of fine hairs to which air bubbles 

 adhere, or beneath the wing-covers. The tracheae 

 are sometimes prolonged into tubes which pro- 

 ject beyond the tip of the abdomen and extend 

 to the surface of the water or mud in which 

 these insects live. 



From the above description it is evident that 

 insects possess the best-developed type of respir- 

 atory system, extending as it does to all the 

 tissues of the body, giving them a constant 

 supply of fresh air and carrying off the waste 

 gases. With an ample food supply this makes 

 possible a rapid oxidation of the tissues, and 

 undoubtedly is one of the chief reasons for the 

 wonderful muscular activity, working power, and 

 endurance of insects. 



The structure of the respiratory system is of great practical im- 

 portance in combating insect pests. Many insects which cannot 

 be destroyed with arsenical poisons are killed by contact insecti- 

 cides in either a spray or a dust form. These contact insecticides 

 destroy the insect by entering or clogging the spiracles or tracheae. 

 Oils are particularly valuable because they spread and pass readily 

 through the hairs which guard the spiracles. Soap solutions leave a 

 gummy deposit, when the water evaporates, which clogs the tracheae. 

 Finely divided dusts, such as fine tobacco dust, pyrethrum, and 

 even air-slaked lime or road dust, will clog the spiracles of many 

 insects. Insects living in grain, stored products, and other inac- 

 cessible places are often destroyed by the use of poisonous gases, 

 such as carbon bisulphide and hydrocyanic acid gas, which quickly 



FIG. 40. Diagram 



of tracheae in head 



of cockroach 



t, trachea, or air tube. 

 Note branches to all 

 the mouth-parts and 

 the antennae. (After 

 Miall and Denny) 



