PARASITES AND PREDATORS 



6i 



On 13th August there was a relatively large peak, which must have 

 been due to secondary larvae, for by this tune all the primary larvae 

 would have fomid hosts or have died. Fig. 22 is an extreme example of 

 the importance of secondary infestation in late-metamorphosing 



O 20 30 

 APRIL 



o 20 30 



MAY 



19 29 

 JUNE. 



Fig. 20. Mean Numbers of Polystoma per Tadpole in Upper 

 Parkfield Pond, 1947 



The black dots and solid line indicate neotenics, the open circles and thin Une 



the larvae. Note the two larval peaks and some sign of a third. Note also 



that the scale of this diagram is different from Figs. 21 and 22, because the 



parasites were far more numerous than in the other cases. 



tadpoles. The infestation in this pond, Dagger Lane 1947, was so low 

 early in the season that none were found until i6th June, but, during 

 July, enough secondary larvae had arrived to give a moderate infesta- 

 tion. Since, however, there were by then not many tadpoles, the total 

 number of infested litde frogs must have been very small. 



I think that these graphs and others in the references cited show that 

 the comphcated life history of the parasite is an adaptation to the wide 

 differences in the time of metamorphosis which I have already described 



