Ii8 NATURAL HISTORY OF AQUATIC INSECTS CH. 



similar n'ccl may be found in the Gnat larva, and 

 also in the fish-like Amphioxus. 



Respiration must be carried on chiefly through the 

 integument, for there are no spiracles. There is 

 however an imperfect tracheal system, consisting 

 mainly of two tubes which run the whole length of 

 the body. In the greater part of their length these 

 tubes contain no air, but in the thorax and again 

 towards the end of the abdomen, each of them dilates 

 and forms a good-sized air-sac (fig. 31). In the living 

 larva, as seen by the microscope, these air-sacs appear 

 black, owing to spots of pigment as well as to the 

 difference in refractive power between water and air. 

 The imprisoned air is more important from its 

 h}'drostatic function than as a means of respiration. 

 The four air-sacs serve in fact as floats, and maintain 

 the larva without effort in a horizontal position near 

 the surface of the water. 



In a fresh-hatched larva the tracheae and air-sacs 

 are altogether empty of air. How are the air-sacs 

 subsequently filled .-' We have no definite informa- 

 tion on this subject. Gaseous air can be discharged 

 from solution in a watery fluid either by rise of 

 temperature or by diminution of pressure. Bring a 

 glass of water fresh from the pump into a warm room. 

 Bubbles of air shortly appear on its sides. Open a 

 soda-water bottle which was corked under consider- 

 able pressure. The diminished pressure due to open- 

 ing occasions a copious discharge of gas. It is hardly 

 conceivable that the evolution of gas in the body of a 

 small aquatic Insect can be due to rise of temperature. 

 But there is no serious difficult}- in supposing that it 



