102 



LEECHES 



latitudes, a fact which might also be accounted for by differences 

 in day length, but it is later in higher altitudes and can be advanced 

 or retarded in the laboratory by appropriate changes of tempera- 

 ture (Leopoldseder, 1931). In a detailed study of the life history 

 of Glossiphonia complanata (Mann, 1957a) it appeared that the 

 onset of breeding was governed by three factors : temperature, the 

 density of the population and the age of the leeches. The popula- 

 tion was living in a network of streams which ran through water- 

 cress beds and carried water from a spring at the foot of chalk hills 

 in Berkshire, England. Owing to the fact that some streams were 



:A 



April 



7 



\ 





- 1 \j'\ 



Moy 



.^^^^£2^ 



•-•■• Leeches without 



ciitellum 

 CKM3 Leeches with 



ciitellum 



M!!!m 



Aug. 



•••-• Leeches without 



ciitellum 

 o-oo Leeches with 



ciitellum 



6 

 c^3So 1 



Sept. 



July 



200 300 



Weight, mg 



Fig. 52. The life history of Erpobdella octoculata. Smoothed 

 frequency polygons representing the distribution of weights of 

 leeches in 6 monthly samples of about 100, between April and 

 September, 1952. The weights are arranged in 10 mg classes. 

 Each point represents the mean value of its own class, the one 

 above and the one below. A, B and C indicate the same three 

 populations in successive samples, and in May represent 1 , 2 and 

 3-year age groups. Those emerging from the cocoon in 1952 are 

 first seen in the samples in September. Note that 2-year-olds 

 breed before 1 -year-olds. From Mann, 1953. 



