62 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



from direct observations. It is only when metamorphosis takes place 

 in late July or August that the neotenic cycle is any use at all. If the 

 tadpoles metamorphose in June, as they sometimes do, the secondary 

 larvae must swim vainly in the pond and die without finding a host. 



lO 20 30 

 MAY 



9 19 29 

 JUNE 



9 19 29 

 JULY 



8 *"l 8 28 

 AUGUST 



Fig. 21. Mean Numbers o£Polystoma per Tadpole in Lower 

 Parkfield Pond, 1947 



In this pond, with a very large number of tadpoles surviving the first few 



weeks, the infestation was low at first. Metamorphosis was delayed, and a 



large influx of secondary larvae took place in August. 



On the other hand, when metamorphosis is late, the losses from the 

 gill chamber are so large that in these ponds there would be few primary 

 larvae left. The neotenic cycle then provides a means of injecting a 

 supply of larvae into the tadpole population just as the original lot are 

 disappearing. It now seems certain that the wide variations in dates of 

 metamorphosis that I reported are so much part of the normal hfe of 



