AQUATIC INSECTS 877 



water are either bugs or beetles. Of those aquatic insects having 

 complete metamorphosis, the pupa is strictly aquatic in caddisflies 

 only. The adaptations of the immature stages have chiefly to do 

 with their respiratory apparatus, and this is most extraordinarily 

 diverse. This will be discussed later. Suffice it here to say that 

 gills of several sorts may be developed upon either the outer or 

 inner surfaces of the body, and those on the outside may be dorsal 

 or ventral, and may be developed upon the head or on any seg- 

 ment of the thorax or abdomen : thus they bear all the usual- signs 

 of independent and adaptive origin. 



Finally, it is to be noted that insects have not invaded the water 

 very far. Nearly all of them have stopped at the shores or in shoal 

 water; .only a few have established homes for themselves in deep 

 water. Only the phantom larvae of Corethra have become free 

 swimming and are regular plankton constituents; possibly a few 

 others also, for a Umited distribution-period immediately follow- 

 ing their hatching from the eg'g. The press of life on land result- 

 ing from the evolution of the highly successful hexapod type of 

 organization, with great adaptabihty, brief life cycle and excellent 

 reproductive capacity, may have resulted in the crowding into the 

 water of those moisture-loving forms whose structures were best 

 adapted to meet the new conditions. The severity of the competi- 

 tion on land is most evident to the careful observer; every nook 

 and comer has its insect inhabitants and every scrap of nutritious 

 food is eagerly sought by a host of competitors. It is easy to 

 conceive that a great variety of forms already accustomed to living 

 by the water side, finding food more abundant in the water than 

 out of it, might, if adaptable, become modified for entering the 

 water for a greater or less depth and for remaining there a greater 

 or less time. 



And, as a matter of fact, adaptation of the adults has proceeded 

 only a little way. Some adult insects, as certain caddisflies and 

 damselflies, enter the water only to lay their eggs, and they remain 

 enveloped by a layer of adherent air while beneath the surface. 

 Some Hve constantly in the water but maintain communication with 

 the surface by means of a long respiratory tube, as does Ranatra. 

 The most nearly aquatic of adult insects are the bugs and beetles that 



