RESPONSES OF POIKILOTHERMS TO VARIATIONS 25 



by radiation if its own temperature differed little from 

 the environment. 



Poikilotherms respond to thermal alterations in the 

 environment mainly in four ways: (1) torpidity, (2) 

 migration, (3) acclimatization, and (4) tolerance. 



Torpidity. — Metabolic processes vary with tempera- 

 ture, being slowed as an animal becomes colder and 

 quickened when it is w^armed. The limits at which an 

 animal can carry on life processes lie between the freez- 

 ing point of protoplasm and the coagulation of certain 

 proteins by heat. As animals approach the limits of 

 their toleration, certain of them may suspend their ac- 

 tivities and go into a state of torpidity. Protozoa encyst ; 

 many insects become torpid during cold and dry weather ; 

 snails may seal up their shells ; many fishes hibernate or 

 aestivate during extreme climatic conditions; certain 

 amphibians and reptiles hibernate during winter, but 

 apparently do not hibernate if kept warm the year 

 around. 



Migration. — Many insects are known to migrate dur- 

 ing changing climatic conditions. Fishes, such as herring 

 and sardines, in summer live near the surface and store 

 fat, but in winter descend to greater depths and lose 

 fat. Marine fishes are sensitive to differences of 

 0.2 deg. C. Ward (1921) states that the migration of 

 salmon is primarily controlled by temperature, the fishes 

 choosing the cooler branches of the rivers as they move 

 upstream. Amphibians such as Necturus move into the 

 deeper and colder water during the summer months, and 

 many frogs regularly alternate between aquatic and 

 terrestrial habitats as the seasons change. 



Acclimatization.— Dallinger (1880) showed that by 

 raising the temperature of a culture of flagellates, he 

 could raise their limits of tolerance from 23 deg. to 



