94 LEECHES 



leeches and is not normally found in small enclosed bodies of 

 water where the temperature may rise excessively. Hirudo and 

 Theromyzon on the other hand do inhabit such places and must be 

 able to survive contact with warm blooded hosts. 



When the rate of locomotion of Hemiclepsis was measured at 



20 1 ^ 



+ 





J 1 I ! ! 1 1 ] L 



(0 12 14 16 18 20 22 24 26 28 30 32 34 36 38 



Temperature, °C 



Fig. 49. Rate of locomotion of Hemiclepsis in relation to 

 temperature. From Herter, 1932. 



many temperatures it was found to increase in an almost linear 

 manner from to 34°C after which it declined (Fig. 49). According 

 to Table 4 locomotion ceases at 36'5°C. 



4. Reactions to Mechanical Stimulus 



One of the most characteristic reactions of leeches is their 

 tendency to creep into crevices, i.e. their strong positive thigmo- 

 taxis. In nature freshwater leeches are most commonly found 

 under stones or in the leaf axils of plants. The reaction is not 

 entirely one of light avoidance as is shown by the experiment in 

 which leeches were given the choice of remaining in the open or of 

 creeping under a sheet of glass where there was little reduction 

 of Hght intensity. Table 5 shows that the majority preferred to 

 lie under the sheet of glass. Here again there is a distinction 

 between the predators of invertebrates, such as Glossiphonia and 

 Helobdella, and the blood-sucking parasites of mammals, for the 



