June, 1919] 



Fauna of Rock Bottom Pofids 



447 



4. Perca 



flavescens 



5. Apomotis 

 cyanellus 



Dragon-fly nymph, 1 . 



May-fly nymph, 1. 



Blue-Green algae, small amount. 



Chironomus larva;, 4. 



Beetle, 1. 



Filamentous algce, small amount. 



Plankton. The plankton results are given in Table 10, on 

 this page. This represents conditions two feet below the surface. 

 The species of Rotifera which are listed separately in Table 9 

 have been grouped together in this table. To wings, taken over 

 a period of two years in the course of class work, gave results 

 which are a sufficiently close approximation to those given here 

 to indicate that these afford a representative idea of plankton 

 conditions during mid-summer. 



TABLE 10. 

 Plankton Quantitative Results. 



Species 



Ceratium longicorne . 



Nauplius 



Chaedorus sphaericus 



Arcella vulgaris 



Rotifera 



Number in 

 100 Liters 



203,125 

 7,500 

 6,696 

 6,250 

 2,231 



Per Cent 

 OF Total 



90 



3.3 



2.9 



2.9 



.9 



Discussion. A noteworthy feature of this pond is the 

 almost total absence of insects from the greater part of it. In 

 the list of all the forms, given in Table 11, page 449, no adult 

 insects and only a few larval stages are credited to the deeper 

 regions. Possibly at least a few representatives of the species 

 named were present in these regions but they were at all events 

 not noticeably common. In explanation of this state of affairs 

 the fish must be considered. The pond was almost as thickly 

 populated, proportionately, with fishes as was Pond I with 

 Notoneda and gyrinids. The stomach contents of the fishes 

 shows that a large part of their food was insects. The presence 

 of appreciable numbers of insects in the shallow pools strikingly 

 illustrates the faunal differences between waters that can be 

 entered by fishes and those that can not. 



Physical factors also had their influence on the character of 

 the association. The smooth perpendicular face of rock offered 

 a bleak and unattractive habitat for many animals normally 



