16 



the body, while certain insects are provided with tracheal 

 gills, in the aquatic larval stage. In the case of the larva of 

 the pond-fly and of a few other species, the modifications 

 of the tracheal system adapt the insect to a completely 

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

 these instances, which are the exceptions rather than the 

 rule, the insect derives oxygen from the water in a manner 

 exactly similar to that employed by fishes, which breathe by 

 gills. ' 



Fig. 13. Larva of mosquito. 

 (Fiom V. S Dept Agric.) 



The aeration of the blood is accomplished in a manner 

 exactly opposite to that in higher animals. The air penetrates 

 to all parts 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. 



NEKVOUS 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.) 



