20 



Usinger: Introduction 



E 7 in Volts 



cm -0. 10 +0.10 0.20 0.30 0.40 0.50 0.60 



4-4 1 1 1 I I L_l 



B U U C 



Intro, fig. 23. Glyptotendipes larvae (Chironomidae) in tubes 

 showing stages in the spinning of the food-catching net (Walshe, 

 1951). 



shown that "there is a sharp and narrow transition 

 from the more or less oxidized substances at the 

 ooze-water interface to the strongly reduced sub- 

 stances in the deeper layers of the sediments." A 

 measure of the oxidizing or reducing power of solu- 

 tions has been devised (called the redox potential), 

 and Mortimer discovered that the redox potential, 

 E7 (=E at pH 7), was approximately 0.6 volts in 

 oligotrophic lakes or eutrophic lakes at the time of 

 complete circulation. Within the sediments, however, 

 the redox potential declines very rapidly, reaching a 

 minimum at a depth of about 5 cm. (intro. fig. 24). 

 The effect of Chironomid water circulation on this 

 system is not known but must be considerable. 



Open-water communities are the neuston or organisms 

 associated with the surface film (above and below), 

 the plankton or small floating organisms (and other 

 material) whose movements are more or less dependent 

 on currents, and the nekton or larger free-swimming 

 organisms. Insects play a dominant role in the neuston. 

 Their supremacy is challenged in the nekton only by 

 fishes. Periphyton organisms are a chief source of 

 food for "grazing" and "browsing" insects, and 

 plankton forms the staple diet of the sieve feeders 

 or strainers. Others, of course, prey on these primary 

 converters of plant and microscopic animal food. 



Marine Marshes, Estuaries, and Intertidal Habitats 



The oceans are, of course, the largest aquatic habitat 

 in the world, and it is interesting to speculate as to 

 what would have happened if insects had successfully 

 invaded this habitat. Their failure to do so cannot be 

 attributed to salinity, because a few of them inhabit 

 inland lakes with salts more concentrated than in the 



0- 



4 . 



6- 



8- 



10 - 



12- 



14- 



16 



1 1 1 1 1 1 1 



-0.10 +0.10 0.20 0.30 0.40 0.50 0.60 



Intro, fig. 24. The redox potential at and below the mud-water 

 interface in two lakes in northern England: E, Ennerdale Water 

 (oligotrophic) and ES, Esthwaite (eutrophic) (after Mortimer in 

 Ruttner, 1953). 



ocean. Competition from previous inhabitants may 

 have been a factor, but fishes and crustaceans did 

 not deter them in fresh water. The most plausible 

 explanation is that practically all insects, as men- 

 tioned previously, are dependent at some point in 

 their life cycle on surface air, and most are terrestrial 

 or aerial in the adult stage. This effectively limits 

 them to shallow waters not too distant from shore and 

 may account for their absence from the open ocean 

 and their scarcity in the deeper parts of large fresh- 

 water lakes. Only one group, the marine water striders 

 (Halobates), lives in the open ocean, and they are 

 su-face dwellers rather than true aquatics. 



However, the situation is quite different along sea 

 coasts and in coastal marshes and estuaries. A small 

 and usually inconspicuous but varied insect fauna 

 exists in such places and includes such notorious 

 pests of mankind as the salt-marsh mosquitoes (Aedes) 

 and sand flies (Culicoides). 



The four principal types of habitat along the Cali- 

 fornia coast are given in the following classification 

 and examples are cited of the insect inhabitants of 

 each. 





