92 LEECHES 



V the chromatophores were themselves sensitive to Hght and could 



respond to changes in light intensity without the mediation of the 

 central nervous system. 



3. Reactions to Heat 



Those leeches such as Hirudo and Theromyzon which sack the 

 blood of warm-blooded animals are stimulated to attach themselves 

 to an object warmed to 3 3-35° C. Placohdella costata is said to be 

 attracted to such an object from a distance of 15 cm (Mannsfeld, 

 1934). If the temperature is raised above 35°C Hirudo will 

 eventually release its hold and in a number of trials the average 

 temperature at which this occurred was 41-5°C. Theromyzon 

 releases its hold at a similar temperature but parasites of fish let go 

 at about 31°C while those which attack invertebrates release their 

 hold from the tube at under 30°C (Table 4 column 5). Land 

 leeches {Haemadipsa) when hungry will sit erect on their posterior 

 suckers. If they are then subjected to a current of warm, moist air 

 they will move towards the source of it (Stammers, 1950). A man's 

 breath or the wind moving past his hand (if this is held a few 

 inches from the leech) is sufficient to produce this reaction 

 (Matthews, 1954). 



When placed in a temperature gradient Hirudo congregated in 

 water of 21°C and Kaiser (1954) concluded that it was capable of 

 distinguishing between temperatures differing by only 1-5°C. 

 When subjected to temperatures higher than those normally 

 occurring in the natural habitat leeches show a distinct series of 

 behaviour patterns. The first is a shock reaction, comparable 

 with that obtained with strong tactile stimulation, in which the 

 longitudinal muscles are strongly contracted. This occurs at 

 temperatures between 26° and 39° according to species. In the 

 next phase the leeches coil and uncoil vigorously, throwing them- 

 selves into a variety of contortions. At still higher temperatures 

 locomotion becomes impossible and finally paralysis sets in. The 

 temperatures at which the four stages were reached by various 

 German freshwater leeches are shown in Table 4 columns 1-4. 

 Piscicola showed the greatest sensitivity to rising temperatures 

 while the greatest tolerance of high temperatures was shown by 

 Hirudo and Theromyzon. Piscicola is closely related to marine 



