THE COMMON 3I0SQUIT0 73 



does not sink without effort, as does the larva, but is 

 only able to descend by violent muscular action. It 

 wriggles and swims as actively as does the larva and 

 soon reaches the bottom of the water. As soon as it 

 ceases to exert itself, however, it floats gradually up 

 to the surface once more. As x^ointed out in our con- 

 sideration of the habits of mosquitoes in general, this 

 activity on the part of the pupa is necessary to preserve 

 it from fishes and from its natural enemies. It is very 

 active and difficult to capture. The mortality of the 

 pupa3, however, is not great, except through the efforts 

 of fish and its other enemies, while that of the wrigglers 

 or larvae is apt to be great through drowning. When- 

 ever a larva becomes sick or weak, or for any reason un- 

 able to exert sufficient muscular force to wriggle to the 

 surface at frequent intervals, it drowns. It seems almost 

 like a contradiction in terms to speak of an aquatic insect 

 as drowning, but this is a frequent cause of mortality 

 among mosquitoes. This fact also explains the efficacy 

 of the remedial treatment which causes the surface of the 

 water to become covered with a film of oil of any kind. 

 Aside from the actual insecticidal effect of the oil, the 

 larvae drown from not being able to reach the air. Or 

 they may draw drops of oil into the tracheae, clogging 

 them and preventing the passage of the air. The struct- 

 ure of the larvae and pupae at the different stages is so 

 well shown in the accompanying figures as to render it 

 unnecessary to describe them fully. One peculiar struct- 

 ure of the pupa, however, has already been pointed out 

 in the general consideration of mosquitoes, and that is the 



