SENSE ORGANS AND BEHAVIOUR 83 



position where they are reasonably safe from predators. In the 

 latter condition they appear to avoid light, to seek the contact 

 stimulus afforded by creeping under a stone or into a leaf axil and 

 to be relatively insensitive to chemical or vibration stimuli. In 

 the hungry condition they may come out into the light, change their 

 colour and be roused to activity by vibrations in the water, by 

 scents emanating from a food organism or by a passing shadow. 

 The foundations of our knowledge of leech behaviour were laid 

 by the careful descriptive work of Gee (1912) who studied the 

 American erpobdellid Dina microstoma and the glossiphoniid 

 Helohdella stagnalis. The study was carried much further by 

 Herter in a series of papers published between 1928 and 1942 

 on the behaviour of certain German freshwater leeches. These 

 papers are listed in the bibliography and will not be mentioned 

 individually in the account which follows. 



2. Reactions to Light 



In general leeches are strongly photonegative in their behaviour, 

 some species more so than others. Table 3 shows the result of a 

 series of observations in which leeches were given the choice of a 

 lighted or a shaded part of an aquarium. On each of 20 days the 

 position of each leech was noted and the shading was moved to 

 the opposite side. In the experiments where only one leech was 

 present in each experimental tank (columns 1 and 2) the majority 

 of leeches chose the shade every time. Those that did not were 

 Hirudo the medicinal leech, Theromyzon the duck leech, and 

 Hemiclepsis a parasite of fish and amphibia. Hungry Theromyzon 

 were much more frequently photopositive than satiated specimens, 

 suggesting that in these blood-sucking species it is the need to 

 obtain a meal which modifies their mormal light avoiding reactions. 

 Where there were several leeches in each experiment the percentage 

 of leeches taking up positions in the lighted zone was considerably 

 increased (columns 3-5). 



Herter and later Denzer-Melbrandt (1935) gave leeches a choice 

 of four intensities of illumination which may be referred to as 

 lighted, lightly shaded, heavily shaded and dark. The percentage 



