June, 1919] 



Fauna of Rock Bottom Ponds 



439 



ditions for both of these species. In all the deeper regions the 

 water was thickly populated with Notonecta from surface to 

 bottom and in certain protected areas Gyriniis was equally 

 numerous. 



All of the species found in the pond are given in Table 6, 

 page 440. A glance at the totals for each station will show that 

 there was a progressive reduction in the number of the species as 

 the distance from the parent pool, i. e., Station 1, increased. 

 This indicates a correlation between the age of the various parts 

 of the pond and the number of inhabitants present. It also 

 throws some light on the rate at which forms take possession of 

 new and unoccupied habitats with which there is an unbroken 

 medium of communication from a region already inhabited. 



TABLE 5. 

 Quantitative Plankton Results. 



Species 



Ceratium . . 

 Dinobryon 

 Arcella. . . . 

 Rotifera . . 



Nauplius. . . 



Number per 

 100 Liters 



850,000 



1,325,000 



4,000 



4,500 



15,625 



Per Cent 

 OF Total 



39. 

 60. 



.0018 



.002 



.007 



As might be expected the adult forms were more generally 

 distributed than were the larval stages. This is especially 

 noticeable with the coleoptera. Beyond Station 2 larval beetles 

 were entirely absent. Among the diptera and ephemeridse 

 there was an uneven distribution of species. Larval represen- 

 tatives of these groups found at Station 1 were not present at 

 Stations 2 and 3, and vice versa. Distribution of this sort is 

 the result of entirely new colonization in the different localities 

 and has no relation to migration within the pond. 



In general, of the larger and more active or powerful adult 

 forms in the region of the parent pool, seven out of eleven were 

 found also at three or more of the four stations. Of similar 

 forms present at only one or two stations there was an even 

 division between those found at the parent pool and those that 

 were not. Apparently, then, the adult population of the parent 

 pool was well represented over the entire pond. The cases of 

 localized distribution of adults were due to fresh colonization in 

 the newer parts of the pond rather than to slow or capricious 

 migration from the original pool. 



