Water 



33 



oxygen content in the media of high salinity, but possibly also related to salt 

 excretion. -^" 



ADAPTATION TO FRESH WATER 



All fresh-water invertebrate animals are hypertonic to the medium. None 

 (except some eggs) are impermeable to water. Hence probably all fresh-water 

 animals have a mechanism for excretion of urine hypotonic to the blood. Some 

 separation of water from salt is made by the excretory organs. The body 

 surface may retain salt but no fresh-water animal, so far as is known, excretes 

 a totally salt-free urine. The salt necessarily lost in the urine must be replaced 

 by food or by active absorption. 



Aquatic Insects. Insects are essentially terrestrial animals but in many 

 orders some members have invaded fresh water, at least for lar\al life. There 

 are some larvae in salt lakes, and a few venture into brackish water. There are 

 marine chironomids. A brackish-water species of Corixidae or water boatman 

 {Sigura litguhris) has been compared with two fresh-water species (S. distincta 

 and S. fossarnm). ^'-^ The brackish-water insect is found over a salinity range 

 of 0.5 to 1.8 per cent. The curves relating internal and external concentrations 

 resemble similar curves for the regulating fresh-water Crustacea. The blood 

 concentration of Sigiira lugiihris is constant in media ranging from 0.1 to 

 about 1.5 per cent salinity; in media below 0.17 per cent salinity the blood 

 concentration declines, and in media above 1.5 per cent salinity it rises. The 

 fresh-water species does not regulate so well, but it increases its blood concen- 

 tration by only 30 per cent in going from 0.1 to 1.8 per cent salinity. There 

 is no information regarding the mechanism of this regulation. 



The blood and coelomic fluid of various aquatic insects are similar in osmotic 

 concentration to those of other fresh-water animals. Insects differ from other 

 animals in the small fraction of their osmotic pressure due to chlorides and 

 the large fraction due to organic substances, particularly amino acids. The 

 chlorides in body fluids of Ciilex and Aedes account for only 35 to 40 per cent 

 of the total osmotic concentration. -'^^ 



The only aquatic insects which have been examined carefully as to their 

 osmoregulation are dipteran larvae, particularly mosquitoes and midges. The 

 details of their adjustments are discussed by Krogh "'^ and by Wiggles- 

 worth. -^° The so-called anal papillae of gills which had been thought to be 

 respiratory organs actually have no respiratory function. -■*^' '^^ Isolated 

 papillae, however, swell in tap water and constrict in Ringer solution. If an 

 Aedes argenteiis larva is ligated ahead of the opening of the malpighian tubes 

 into the gut and then put into hypertonic glucose the hind part shrinks, 

 whereas without the Hgature the whole larva shrinks. If the animal is Hgated 

 just behind the opening of the malpighian tubes and placed in fresh water 

 the posterior region becomes swollen, whereas the anterior part does not. By 

 several such ligation experiments Wigglesworth found that normal larvae 

 swallow very little water, that the anal papillae are the part of the body most 

 permeable to water, and that water is excreted bv the malpighian tubes (Fig. 



17). 



When Aedes larvae are transferred to various media a constant osmotic 

 pressure is maintained in an external concentration lower than or cqui\alent 

 to 0.65 per cent sodium chloride;-^^- ~^^- -■''■• in media of higher concentration 



