64 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



of such a parasite, if it lives all the time in the animal, must be greater 

 than that of its host, for to its own causes of death must be added those 

 of its host. The existence of the neotenic, which is a multiplying stage, 

 cannot explain the paradox, for this phase is only effective in late 

 tadpoles, and only needed because so many larvae are lost in the pro- 

 tracted life of the tadpole. The probable explanation is that the most 

 dangerous stage of tadpole hfe is the first few weeks. In tlais period, 

 the trematode eggs are lying, comparatively safe, at the bottom of the 

 pond, and only hatch to infest the hosts when the worst of the risks are 

 over. This is another point at which the hfe history of the parasite 

 confirms the observations on the tadpoles themselves. 



I close this short account of epizootics of P. integerrimum with some 

 unpubhshed observations on the factors causing the transformation 

 into neotenics. 



It is known from the work of Zeller and of Galhen that this trans- 

 formation has some connexion with the age of the tadpole when it is 

 first infested. The phenomenon as described by Galhen himself was 

 not so sharp as the brief derivative accounts in various textbooks make 

 it appear, nor did Gallien say that the parasites attach themselves to 

 the external gills. But he did use the presence of external gills as an 

 indication of the stage of development of the tadpoles in his experi- 

 ments. In my field observations, only one abnormally small and 

 undeveloped tadpole was found infested at this early stage. In every 

 other case, the tadpoles were well past this stage before the first of the 

 parasites appeared, and yet many neotenics resulted. If Gallien' s 

 original results are examined, it is clear that he explicitly and fully 

 described similar transformations, but in the laboratory these were 

 exceptions. In the field, on the contrary, they are the rule, and in- 

 festations at the age of the external gill stage occur, if at all, as a rare 

 abnormahty. This shows the importance of distinguishing between 

 laboratory experiments and observations based on events in the field. 



Since the hypothesis that age was the important factor seemed not 

 to be quite right, I supposed that the sex of the frog harbouring the 

 mature fluke was important. Female frogs are later arriving at the 

 ponds than males, so that the eggs of the parasites that they contain 

 would hatch later. If the parasites from male frogs became neotenics, 

 but those from females did not, an explanation consistent with the 

 older observations might appear. I therefore harvested the eggs of 

 adult parasites from the two sexes of frogs separately, and by retarding 



