AQUATIC INSECTS 



893 



(Fig. 1363), there are developed three more or less leaf like gills upon 

 the tip of the abdomen, and these are traversed by fine tracheae, 

 and doubtless assist in getting air, although not entirely essential 

 to that end. In the larger dragonfly nymphs there is developed 



FIG. 1363. The nymph of Ischnura verticalis. 



within the abdomen a respiratory chamber made out of the hinder 

 portion of the modified alimentary canal. Through the action of 

 the abdominal muscles, the water is alternately drawn into , this 

 and expelled again. This chamber is lined with multitudes of 

 tracheal gills, and abundantly supplied with tracheae, constitut- 

 ing the most perfect aquatic respiratory apparatus developed in 

 insects. 



Transformation occurs in most Odonata very close above the sur- 

 face of the water. The larger species transform for the most part 

 at night: the damselflies, at any time. The period of half an hour 

 or more required for drying the wings before sustained flight is 

 possible is a time of great peril in the life of the dragonflies. It is 

 a time of opportunity, however, for the collector of life history 

 material. 



WATER BUGS (Order Hemiptera) 



A small part of this great order is aquatic ; a number of families 

 are well adapted for life in the water; a few run over the surface 

 and a few others live habitually on the wet shores and forage in the 

 flotsam and drift of the waves. Adults and nymphs are of similar 

 habits and are generally sufficiently alike in structure for ready 

 identification, the metamorphosis being slight. All are distin- 

 guished from the members of other groups by the possession of a 

 jointed puncturing and sucking proboscis that is directed backward 

 beneath the head. The families are so diverse in structure that 



