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 oj the open tyfe 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 tracheal gill of the larva of Corydalus; T, trachea; t, 



tracheoles. 



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

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

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

 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. 



7. The Trachea! 

 Gills 



In the immature 

 Fig. 137. Part of a tuft of tracheal eills of a larva of . 



Corydalus. insects mentioned 



above, the air in 

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



*Apneustic: apneustos (TT wares), without breath. 



