THE STUDY OF FRESH-WATER ANIMALS. 187 



The position of the breathing tube close to the tail is 

 noteworthy. A close examination of the larvae as they 

 hang suspended on the surface film will show that mean- 

 time they are not otherwise idle. Under the head a pair 

 of brush-like structures may be seen in constant action, 

 sweeping the water and sucking towards the mouth the 

 smaller organisms falling within the currents they create. 

 While the larvae, by taking advantage of the supporting 

 properties of the surface film, hang without effort breath- 

 ing there, they are still able to feed in the pool. 



When the larva enters the pupal stage it ceases to feed, 

 but must continue to breathe. It is therefore interesting 

 to find that it is now supplied with a pair of respiratory 

 funnels and that these are placed just behind the head, so 

 that the pupa rests with its thorax at the surface of the 

 water (Fig. 72). It should be possible for pupils to tell 

 correctly the real significance of this change of position. 

 When the adult insect emerges it must pass directly into 

 the air without getting wet. In the new position taken up 

 by the pupa this is quite easy. The thorax splits just 

 behind the respiratory tubes upon the back, and through 

 the opening the gnat rises into the air above. Of course 

 the process should be watched in the aquarium. 



Water Beetles. The larva of the water beetle (Fig. 66 3L ) 

 is equipped with a pair of finely fringed appendages at the 

 tip of the tail. These are pushed through the surface film, 

 and by their means the insect is suspended. It has a pair 

 of breathing openings at the tip of its body, which are thus 

 brought into communication with the atmosphere. There 

 are spiracles too upon the sides of the body, but these are 

 closed until during pupation. When the adult beetles are 

 watched they will be seen to rise passively to the suiface 

 from time to time, the tail end slightly tilted upward. 

 When doing this they are renewing their air supply. They 

 are lighter than water (as are the larvae also), and the tail 

 end more so than the rest. 



The tips of the wing covers on being pushed up into the 

 air are slightly diverged, and it will be seen that the beetle 

 carries a supply of air beneath them. The dorsal surface 



