2 NATURAL HISTORY OF AQUATIC INSECTS 



rear, and rolling forward to the front. All the very 

 simplest animals and plants are aquatic, and a fair 

 degree of complexity of structure is implied in the 

 mere fact of residence out of water. 



Shallow waters are easily penetrated by the sun's 

 rays, and are therefore often occupied by green 

 plants, whose nutrition depends upon the decom- 

 position of carbonic acid by sunlight. Upon these 

 green plants plant-feeding animals establish them- 

 selves, and many predatory animals come in turn to 

 devour the vegetable-feeders. The shallow waters 

 are probably richer in living things of all kinds 

 than any other part of the earth's surface. But the 

 organic products of the plants and animals of the 

 shallow waters may be transported by moving water 

 to great distances. The great depths of the ocean 

 and the bottom of our large lakes have been explored 

 by the dredge, and are found to support a popula- 

 tion of their own, which subsists upon a nutritive 

 sediment originally formed at or near the surface. 



The conditions of life in fresh waters are materi- 

 ally different from those afforded by the sea. The 

 chemical difference between fresh and salt waters 

 is of small importance in itself — so small that 

 many animals have been able to adapt themselves 

 gradually to a change from one to the other. A 

 degree of saltness far greater than that of sea-water 

 is perfectly compatible with animal life, as is shown by 

 the fact that Crustacea and Insects are often found 

 quite at home in brine-pits and vats. There are even 

 a few animals, like the Salmon, which can pass sud- 

 denly from fresh water to salt, or from salt to fresh. 



