PARASITES AND PREDATORS 6$ 



the growth of the tadpoles by refrigeration was able to produce 

 parasites and tadpoles at the external gill stage simultaneously (the 

 fact that I had to refrigerate confirms that the parasites do not normally 

 hatch when the tadpoles are at the external gill stage). The little flukes 

 do not accept tadpoles of this age as hosts, but alight, make a few 

 exploratory movements, and at once take off into the water to resume 

 their active swimming. Not until the gills are becoming reduced do 

 they remain on the tadpoles, and even then, do not attempt to go near 

 the gills, but settle for some days anywhere on the tadpole. Pigmenta- 

 tion takes place during this time, and conversion to neotenics was 

 apparently proceeding. Unfortunately for the hypothesis, parasites 

 from both sexes of frog behaved in the same way ! 



Predators 



It is generally stated that tadpoles are eaten by many of the animals 

 in a pond. This statement is probably true, but nothing approaching a 

 detailed accoimt appears to exist. In ecology, one cannot accept as 

 evidence the fact that, in an aquarium, the larvae of water beetles eat 

 tadpoles, because in a pond it might be that the two species live in 

 different places, and never meet. I do not think that this is so, but the 

 example shows the problems that beset the ecologist when he tries to 

 replace guess-work by observation. Because I have seen it happen in 

 a pond, I know that the larvae of dragon flies (Odonata) seize passing 

 tadpoles. Animals are under observation for such a small part of their 

 Hves that even the two instances that I have seen must indicate that 

 this often happens, and these insects may be one of the important 

 predators. I think there must be many others in order to explain the 

 disappearance of 99-100 per cent of the whole population. In Redwell 

 Wood Pond, so small and shallow that I could wade all over it, and 

 with water so clear that I could see everything, the tadpoles disappeared 

 steadily over several weeks, until none were left, but I could never see 

 why. I have no properly founded explanation, but I think some 

 large predator such as visiting ducks or snakes must have been 

 responsible. Most of the animals in a pond are small, and it is difficult 

 to see in them a large enough cause of such wholesale destruction of 

 animals that in their later stages are among the largest in the pond. 



Blair (1927) states that in the New Forest, young medicinal leeches, 

 Hirudo medicinalis, kill many frogs and tadpoles, which he found dead 

 with the characteristic marks upon them. Carpenter (1953) records the 



