878 FRESH-WATER BIOLOGY 



have developed oar- like hind feet and have become good swimmers; 

 these enter the water to depths of several feet and spend most of 

 their time near the bottom in shoal waters, but they must come to 

 the surface at intervals for air which they carry down with them 

 beneath their wing covers or adherent to the pile of their bodies. 

 A few adult insects also have taken to walking or running on the 

 surface of the water, but these are naturally the most minute forms, 

 as springtails, or those of slenderer build, like little Diptera and 

 water striders; and of this last-mentioned group, some wander far 

 from shore, even upon the surface of the ocean. But there are 

 few adult insects to be found far from the shelter of vegetation, 

 and it remains true that the great press of insect Hfe is at the shore 

 line. 



The case is only slightly different with insect larvae. Most of 

 these have remained near shore. As compared with the adults, 

 their smaller size, less chitinized skin and greater plasticity have 

 allowed much more complete adaptation to aquatic Hfe. There 

 are some larvae, like those of beetles and of many flies, that take 

 air at the surface as do the adult beetles, and there are a few others, 

 that, descending the stems, tap the air spaces of plants far beneath 

 the surface and get oxygen from that unusual source ; but there are 

 also very many that are capable of a truly aquatic respiration, 

 being able to utilize the air that is dissolved in the water. Most 

 of these larvae when newly hatched absorb the oxygen directly 

 through their skins; and a few of them, especially such as live in 

 well aerated water, acquire no better means than this during their 

 larval existence, but most of them develop gills of some sort. 



These gills are delicate outgrowths of the thinnest integument of 

 the body. Two types of gills are usually distinguishable, blood 

 gills, and tracheal gills. The former are more Kke the gills of 

 other aquatic animals; the latter are pecuHar to insects. The 

 blood gills are simple outgrowths of the body wall into which the 

 blood flows. The interchange of gases which constitutes the 

 respiratory process takes place between the blood within the gill 

 and the water outside it by means of direct diffusion through the 

 thin membranous wall. Such gills are very commonly developed 

 in dipterous larvae as paired and retractile appendages of the pos- 



