202 



BULI.ETIN OI^ THE BUREAU OF FISHERIES. 



General Character of the Diet. — On the whole there is a remarkable similarity 

 in the diet of the various species. Eleven of the food items appear in all five of the 

 columns and nine more are found in four out of the five. There are only three foods 

 confined to a single species and three others that are restricted to two columns. 



Perhaps the most noticeable items are those which begin and end the list. Two 

 species of vSnails are very common in all the ponds, and upon these every kind of nymph 

 examined had been feeding freely. Not only had more than half of the nym.phs partaken 

 of these snails, but in several instances no other food was found in their digestive tract. 



With reference to the algse, it is of course understood that inasmuch as the nymphs 

 catch their prey among the algge, they would be expected to swallow some of the latter. 

 Hence its presence was not noted unless in sufficient quantity to make it reasonably 

 certain that it had been taken voluntarily. Like the snails, in a few instances it con- 

 stituted the sole article of diet. 



The other popular foods were the mayfly larvae and the small Crustacea, the latter 

 being consumed in large quantities. 



Cannibalistic tendencies are shown by the presence of odonate nymphs in all five 

 columns; and in four out of the five, nymphs were found which had eaten others of their 

 own species. 



There is a good showing of beetle larvae and adults, and of adult water boatmen, 

 ail of which are injurious to young fishes. 



Specific Differences in the Diet. — Notwithstanding the wonderful agreement 

 just mentioned, a careful examination reveals also striking differences in diet. Every 

 one of the Anax nymphs was fully grown, with well-developed wing cases, and was 

 captured in pond 4, which contained only adult buffalofish. Of course the nymphs 

 could not eat these fish, and hence the absence of fish in their diet is a matter of necessity 

 rather than choice. (See p. 206.) Neither did the fish eat the nymphs, however, and 

 this probably accounts for the exceptional abundance of the latter, as evidenced by the 

 exuviae obtained. The bulk of the food d these nymphs was made up of mayfly larvae 

 and snails, but it is worthy of note that they also ate large quantities of Dytiscus 

 larvae, the water boatman Corixa, and small crayfish. The proportion of odonate 

 nymphs in their food was much greater than for any of the other species examined. 

 One very large specimen had eaten nothing but crayfish, and its stomach was packed 

 full of their shells and claws, which had turned red in color like a boiled lobster. The 

 Dytiscus larvae were identified by their heads and mandibles, Corixa by the peculiar 

 color pattern of the elytra, and the Haliplid beetles also by the color pattern of the 

 elytra. All three of these foods were especially abundant in this pond. 



The bulk of the food of the Libellula nymphs consisted of snails and various 

 entomostraca, Cypris being a particularly toothsome tidbit; there were. also quite a 

 number of damselfly nymphs and a good representation of the beetle larvae. A beetle 

 larva belonging to the genus Peltodyes was found to be common on the algae in the 

 ponds, and its body was covered with long, jointed, bristlelike processes. The broken 

 fragments of these processes were found in the gizzards of seven nymphs, in three of 

 which the larvae were the only food eaten. 



In the gizzards of five of the nymphs v/ere rounded masses of the spicules of a 

 ffesh-water sponge. Sponges are common in two of the ponds, and on them live a species 

 of Sigara, which is a minute water boatman, and one of the caddisworms, Eeptocerus. 



