REPRODUCTION AND DEVELOPMENT 



103 



heavily shaded while others v^^ere not, the diurnal rise of tempera- 

 ture was more marked in some streams than in others. The 

 breeding activity was most advanced in the warmer waters. Within 

 a given section of the stream breeding was more advanced in 

 places where the leeches were densely aggregated, presumably 

 because the opportunity for fertilization was greater, or because 

 there was some physiological response to the presence of numbers 

 of other leeches. Moreover, 2-year-old leeches began breeding 

 about one month earlier than 1 -year-olds. 



Erpohdella testacea is an annual species, breeding at 1 year of 



Fig. 53. Diagram of the life history of Helobdella stagnalis in a 

 calcareous artificial lake. The width of the line is roughly- 

 proportional to the density of the population. Dotted areas 

 indicate breeding leeches ; arrows indicate the release of young 

 by the parents. From Mann, 1957. 



