16 



the bodv, while certain insects are provided with tracheal 

 ^ilK. in the aquatic larval stage. In the case of the larva of 

 the pond-fly and "I ;i few other species, the modifications 

 of the tracheal system adapt the insect i<> ;i completely 

 aquatic existence over a certain portion of its life-cycle. In 

 these instances, which are 'Ik- exceptions rather than the 

 rule, the in--'''-! derives oxygen Prom the water in .1 manner 

 exactly similar to that employed by fishes, which breathe by 

 eills. 



Fig". 13. Larva of mosquito. 

 (Ftom U. S DejH Agric.) 



The aeration of the blood is accomplished in a manner 

 exact ly opposite to t hat in higher animals. The air penetrates 

 to all pails of the body by means of the tracheae, which are 

 bathed by the blood, and comes into intimate contact with 

 the food-substance by means of the minute branches of the 

 tracheae which envelop the alimentary canal. 



Nek vors System. 



The relative positions of the vital systems are also 

 opposite in insects to those in higher animals. In insects 

 the* circulatory system is dorsal, and the nervous system is 



Fig*. 14. Nervous system of an insect. 

 (Redrawn from Theobald, Agricultural Zoology ) 



