186 THE STUDY OF FRESH-WATER ANIMALS. 



rather difficult to find, but all the other stages can be 

 readily followed in the school aquarium. 



Pupils, having followed the life-history, should make a 

 set of drawings representing all the stages. 



Some of the structural adaptations of the gnat will be 

 found described below (pp. 186, 187). 



For another life-history study see p. 86, " The Life 

 History of the Frog." 



ADAPTATIONS IN FRESH-WATER ANIMALS. 



Two of the most important adaptations worthy of a little 

 study in school are those for breathing and locomotion. 

 This is especially the case with insects, which are abundant 

 in fresh waters. And naturally so, since insects living in 

 water are in a sense " out of their element." The 

 presence of insects in fresh waters is probably due to the 

 success of those ancestral types which invaded this region 

 in the search for food. 



Respiratory Adaptations. 



All living things respire. They require oxygen to main- 

 tain life. Let us inquire how some of the inmates of a 

 pool obtain it. 



Gnats. G-nats are air-breathers throughout the whole 

 of their existence. Although the larval and pupal stages 

 are spent entirely in the water, the insect has never 

 acquired the power to use dissolved oxygen. Note how the 

 larvae hang in large numbers at the surface of the pool. 

 Near the tail they have a long tube with five folding plates 

 at the tip. These plates converge to a point, which the 

 larva pushes through the surface film of the water. The 

 plates are then spread outward so as to lie upon the film 

 and support the larval gnat. Whilst the creature hangs 

 here its respiratory system is opened through the medium 

 of this tube to the atmosphere above, enabling breathing to 

 go on. 



