THE INTERNAL ANATOMY OF INSECTS 



119 



As the air-sacs lessen the specific gravity of the insect they proba- 

 bly aid in flight; as filling the lungs with air makes it easier for a man 

 to float in water ; in each case there is a greater volume for the same 

 weight. 



5. Modifications of the open type of respiratory organs in 

 aquatic insects 



There are many insects in which the spiracles are open that live in 

 water ; these insects breathe air obtained from above the surface of 

 the water. Some of these insects breathe at the surface of the water, 



Fig. 136. Part of a trachcal gill of the larva of Corydalus; T, trachea; /, 



tracheoles. 



as the larvas and pupae of mosquitoes, the larvas of Eristalis, and the 

 Nepidas; others get a supply of air and carry it about with them 

 beneath the surface of the water, as the Dytiscidae, the Notonectidae 

 and the Corisidae. The methods of respiration of these and of other 

 aquatic insects with open spiracles are described in the accounts of 

 these insects given later. 



b. THE CLOSED OR APNEUSTIC TYPE OF RESPIRATORY ORGANS 



That type of respiratory organs in which the spiracles do not 



function is termed 

 the closed or 

 apneustic* type; it 

 exists in naiads and 

 in a few aquatic 

 larvae. 



i. The Tracheal 



Gills 



In the immature 

 S ects mentioned 

 above, the air in 

 the body is purified by means of organs known as tracheal gills. 



* Apneustic: apneustos (&Trvev<rTos), without breath. 



