parach "lor phenol in concentrations as low as 0.0005 mq/1 (Hasler and Wisby. 

 1950). 



There are still few data on the avoidance reaction to toxic substances 

 in aquatic invertebrates. Studies have utilized freshwater insects, 

 spiders, leeches, and marine crustaceans. The forms of behavior pre- 

 venting toxic exposure to animals are varied. Beetles usually crawl out 

 of toxic solutions along the walls of the experimental vessels and occa- 

 sionally even fly out of the solutions. Hymenoptera jump out and fly 

 away, and water spiders escape by running along bits of grass placed into 

 the vessel. Animals incapable of rapid movement manifest defensive be- 

 havior in other ways. The larvae of many flies and some butterflies use 

 their cases as means of protection from toxicants. Some caddis fly larvae 

 in toxic media will build thicker case walls. Chironomids bury themselves 

 deeper into the mud. Molluscs close the valves of their shells tighter 

 and for a longer period (Alekseev and Flerov, 1972). 



The most dangerous toxic substances (pesticides) are, however, poorly 

 avoided by invertebrates. Again, the medicinal leech will serve to illu- 

 strate this fact. A comparison of avoidance of substances belonging to 

 different classes of chemical toxicants (Table 2) shows that pesticides 

 are either not avoided at all (chlorophos), or avoided only at their 

 lethal concentrations (polychlorpinene) . These substances are evidently 

 not "unpleasant", causing pain for the leech. Herein lies the insidious- 

 ness of pesticides. In contrast, phenol and "Lotos-71" were actively 

 avoided by the animals. 



The avoidance reactions are of great importance for adaptation. For 

 practical purposes, it is essential to know the range of concentrations 

 in which these reactions are manifested. However, questions arise rela- 

 tive to the extrapolation of experimental results into the natural system. 



There are some observations on migrations of marine fish from the 

 areas polluted with petroleum wastes. Data also exists on impoverishment 

 of species composition of communities which may be explained, not only by 

 the death of some species, but also by escape of others from the polluted 



TABLE 2. TOXICITY OF SOME SUBSTANCES FOR MEDICINE LEECH AND THEIR 

 THRESHOLD CONCENTRATIONS (MG/L) PRODUCING AVOIDANCE REACTION 



128 



