90 



LEECHES 



10 12 14 16 

 Number of trial 



18 20 22 24 



Fig. 48. Habituation in reaction to decrease in light intensity 



(shadow reflex). The percentage drops in light intensity is 



marked against each line. For further explanation see text. 



Based on the data of Kaiser, 1954. 



change from one state to the other is a relatively slow process, 

 usually taking about 1 hr, although the dark brown chromatophores 

 of Piscicola are exceptional and can expand in about 15 min. Most 

 of the leeches in which colour change has been observed are blood- 

 sucking parasites of vertebrates, for example Theroviyzon and 

 Hemiclepsis. These become positively phototactic when hungry 

 and so expose themselves to predators in daylight. Presumably 

 the change in colour makes them less conspicuous under these 



