60 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



mature fluke. These larvae enter the tadpoles in the same way, and at 

 metamorphosis travel to the bladder with any undeveloped larvae. 

 They never give rise to any neotenic forms. The neotenic larvae die 

 and are expelled either in July or at metamorphosis if this is earHer. 



Nomenclature 



Five kinds of larvae are involved in these investigations — frog larvae, 

 and four kinds of trematode larvae. To prevent confusions, I refer to 

 the frog larvae, as usual, as tadpoles, and to the neotenic larvae as 

 simply neotenics. This leaves the term "larvae" for the small forms. 

 If these are the progeny of the mature fluke that hves in the bladder, 

 they are called "primary larvae." There are two broods of these. The 

 progeny of the neotenics are called "secondary larvae." There are no 

 morphological differences between the primary larvae and the secondary, 

 but if the course of the epizootic is followed it is usually clear which 

 kind is contributing to the total infestation. 



Nearly all the work on the tadpoles so far described has been carried 

 out in a small part of south-east England, and over a period of only 

 four years. There is no reason to suspect that the results I have reported 

 are not typical, but it would be interesting to have some wider con- 

 firmation. P. integerrimum has confirmed the direct observations. 



Fig. 20 shows the course of the epizootic in Upper Parkfield Pond 

 in 1948. The first collections, on loth and 20th April contained no 

 polystomes. The first eggs hatched in the following week, so that on 

 27th, large numbers of parasites were present, and some had already 

 transformed themselves into neotenics. On 12th May, the numbers of 

 both kinds had much diminished, because the parasites live in a 

 precarious position, where the rushing water in the gill chamber is 

 hable to carry them away. At about this time, however, the second 

 brood of larvae appear and, on 28th May, the numbers of primary 

 larvae reached a new peak, but the neotenics were not numerous. The 

 numbers of both types then fell to a new low point on 19th June. A 

 slight rise then took place, probably due to the emergence of secondary 

 larvae. This is much better shown in the other graphs, which depict 

 the state of affairs in ponds in which metamorphosis is delayed until 

 July or August. Fig. 21 is an example. In this pond. Lower Parkfield 

 1947, as will be recalled, there was an enormous number of tadpoles. 

 There were so many, in fact, that the parasites distributed themselves 

 over so many hosts that the average number in each was always low. 



