PARASITES AND PREDATORS 



63 



tadpoles that the Hfc history of the parasite has had corresponding 

 modifications stamped upon it through the ages. I feel confident that 

 similar differences must exist widely. 



The numbers of parasites are distributed in a curious way. A tadpole 

 usually has either very few parasites, or else very many, intermediate 

 values being much rarer than would be expected from a random 

 distribution (Chapter 10, Appendix i h). Moreover, there is no 



UJ 



a 

 < 



HI '^ 

 Q. 



or 



CO 1 



2 



Z 



zO 



< 



lU 



— / 



::::::7: 



30 1020 30 

 APR MAY 



9 19 29 

 JUNE 



9 19 

 JULY 



Fig. 22. Mean Numbers o{ Polystoma per Tadpole in 

 Dagger Lane Pond, 1948 



Infestation was very low at first. Metamorphosis was rather late, but the 

 few neotenics evidently gave rise to enough larvae to infest the tadpoles in 



July. 



relation between the size of the tadpoles and infestation, a fact that 

 disposes of the idea that the parasites, even the neotenics, are harmful 

 (Chapter 10, Appendix i c). They probably hve on mucus and 

 debris, and, if the tadpole has an operculum more transparent than 

 usual, the neotenic provides the exceptional spectacle of an undisturbed 

 internal parasite at work. In the Hght of the microscope lamp, the 

 tadpole being held upside down in water, it can be seen ceaselessly 

 grazing on the internal surfaces of the operculum, its eyes shining like 

 motor-car headlights. 



P. integerrimum lays fewer eggs than its host. This is a remarkable 

 achievement for a parasite that must die if its host dies. The death rate 



