ECOLOGY 145 



than 95% of the offspring being lost. In Erpohdella and Glossi- 

 phonia, where most individuals lived for 2 years, there v^as subse- 

 quently a mortality at the rate of one third to tvv^o thirds of the 

 population per annum (Table 10). In Helohdella, where there are 



lOOOx 



500 



100 



3 6 9 12 15 18 21 24 27 



Months after hatctiing (calculated from 1st May) 



Fig. 90. Survival curve for 1000 newly hatched young of 

 Glossiphonia complanata. From Mann, 1957. 



two generations a year, the rate of mortality is correspondingly 

 higher. 



Looking over our very incomplete knowledge of the ecology of 

 leeches, it appears that for the blood-sucking ectoparasites the 

 most important single factor influencing distribution and abun- 

 dance is the presence or absence of an appropriate host organism. 

 In temperate climates, however, the predatory macrophagous 



