MOSQUITOES AND CATERPILLARS 89 



the surface when breathing by hanging from it. Water- 

 striders and the numerous little flies which run quickly and 

 safely about on the surface of the water are supported by 

 the film. Their feet make little dents or depressions on the 

 water's surface, but do not break through. 



It is probable that the movements of the feeding-brushes 

 also help to keep the wriggler at the surface, as the wrigglers 

 seem to be able to balance themselves, i. e., keep from sinking, 

 in the water by these movements. 



Observing the larvag or "wrigglers" from day to day it 

 will be noted that they increase in size, that is, are growing. 

 They breathe and feed and swim and grow. And some 

 keenly observant pupil may see that they occasionally cast 

 their skin, or moult. That the larvag do moult one or more 

 times is certain; how many times, however, has not yet been 

 found out for many kinds. 



The pupae. — After several days — just how many each 

 pupil should determine for himself — the long slender larvEe 

 enter upon another stage in the mosquito's life called the 

 pupal stage, and the young mosqmtoes are now called pupa. 

 In this stage the head end is large and bulbous, the hind body 

 is usually curled underneath the head, and the creature 

 spends most of its time floating at the surface. It can swim, 

 and does so when disturbed, by a peculiar straightening and 

 folding of its body. When it stops swimming what happens 

 to it? In what way must the pupa differ from the larva in 

 its relation to the density of water ? 



Examine with a hand lens one of the pupae in a watch- 

 glass of water. Note the two tubes or horns which project 

 upwards from the back or dorsal part of the bulbous head 

 end of the body, and the pair of flaps at its posterior tip. 

 What are the dorsal tubes for? With what do they cor- 

 respond in the larvae ? The mouthless pupa takes no food 

 and usually floats quietly at the surface. Why then does it 

 swim at all? What is the use of the flaps at the end of the 



