Vol. XXVl] ENTOMOLOGICAL NEWS 7 



had built its case and was feasting on the diatoms about the 

 door of its house. This incident seems to express, perhaps 

 in exaggerated form, the one-sided benefit resulting from the 

 relationship between epiphytes, epizoa and the nymphs. Kam- 

 merer ('08), who maintained that the algal growth may bring 

 to the nymph such benefits as a richer supply of oxygen, the 

 keeping aloof of parasites, protection through color, et cetera, 

 thinks this relationship is one of symbiosis. The relation 

 seems to me to be a natural one resulting from proximity; 

 because, if the populations living upon the nymphs be com- 

 pared with those of the mud and Water weeds which the 

 nymphs inhabit, they will be found practically the same. The 

 abundance of the population in both cases depends to some 

 extent upon the inactivity of the nymphs. 



III. THE FOOD OF THE NYMPHS. 



From November 14 to July 10, dragonfly and damsel-fly 

 nymphs were taken from Cascadilla Pond directly to the labo- 

 ratory, where the contents of their stomachs were mounted 

 in water upon slides and examined under the microscope. If 

 they could not be examined immediately, the heads of the 

 nymphs were removed from the bodies and the alimentary 

 canal gently pulled out, but not severed, so that the contents 

 could be preserved in formalin, and the head and body remain 

 connected for later identification. Eight nymphs contained 

 no fragments of food, but four of these had eaten much sand. 

 The presence of large quantities of diatoms* was puzzling 

 until it was noticed that they appeared during the cold months 

 when Chironomid larvae are the chief source of food, and, in 

 one or two cases, they were seen to be protruding from par- 

 tially digested larvae. The cases of the Chironomids, on which 

 diatoms grow abundantly, may account for some of the shells 

 and probably the sand present was that of Chironomid cases 

 from which the larvae had been entirely digested. Bits of 



*Dr. A. H. Morgan has suggested that some of the diatoms are un- 

 doubtedly taken in with the mayflies since they are abundant in the 

 food and epiphytes of mayfly nymphs. 



