36 The Medium 



tracheae of the insects with sufficient speed to meet respiratory 

 needs. No doubt partly for this reason relatively few kinds of insects 

 have succeeded in establishing themselves in the aquatic environment. 

 Most of these remain in the water for only a portion of their lives— 

 usually the larval stage. Insects living completely submerged possess 

 tracheal gills or other special devices for obtaining oxygen without 

 taking water into the tracheae. Many fresh-water species are adapted 

 to come to the surface for air. In the marine environment Halohates 

 lives on the surface of the ocean as a "water strider," and several 

 dipterans inhabit shallow areas as larvae, but only one insect, the 

 midge Pontomyia nutans, is known to complete its entire life cycle 

 submerged in sea water. 



Existence of Plankton 



The relatively high density of the water medium not only tends 

 to buoy up parts of the body but also in some instances supports the 

 whole body and thus allows certain organisms to float at various 

 depths in the free water. This fact makes possible the existence of 

 plankton— plants and animals that live suspended in the ocean and 

 inland water bodies and that drift about either because they are non- 

 motile or because they are too small or too weak to swim effectively 

 against the currents (Figs. 2.5 and 2.6). The term plankton is de- 

 rived from a Greek word meaning "wanderer," and many organisms 

 in this category, or plankters, spend their whole lives drifting in 

 the water. Both animal and plant plankton is found in practically all 

 natural waters, frequently in enormous abundance and variety. 

 Other categories of life in the aquatic environment are the benthos, 

 which consists of the organisms living on or in the bottom material, 

 and the nekton, which is composed of the strong-swimming animals. 

 The benthos and the nekton have their counterparts on land, but the 

 permanent plankton represents an important category of life that is 

 totally absent from the air environment. 



Certain planktonic animals and plants live permanently suspended 

 in the water by actual flotation. This method of support is not pos- 

 sible in the air. Pollen grains, seeds, and spores, commonly spoken 

 of as "floating" in the air, are not actually doing so, but are sinking 

 at a slow rate— often retarded by various feathery structures. In the 

 water actual flotation is possible for forms that contain air cavities or 

 light materials such as fats or oils. The brown alga Sargassiim is pro- 

 vided with gas-filled bladders, and the Portuguese man-of-war, a 

 siphonophore, has a pneinnatic "sail." Many fish eggs float by virtue 



