2IO 



BUI.I.ETIN OF THE BUREAU OF FISHERIES. 



amongst the rubbish, while Brythemis, Celithemis, Tramea, etc., and all the damselfly 

 nymphs hide in the matted vegetation. Such lurking places also serve as admirable 

 ambushes whence to secure their prey. 



3. Diving BektivEs, Water Scorpions, and Aquatic Hemiptera. — These 

 retaliate by eating the nymphs before they are large enough to defend themselves. 

 An adult Dytiscus beetle was seen in pond 7 eating a small Brythemis nymph, which 

 would partly compensate for the beetle larvae of the same species that are found in the 

 table on page 201 . Dr. Muttkowski has observed both Dytiscus and Zaitha feeding 

 upon nymphs and noted that after capturing the nymph they invariably stick their beak 

 first into its head. Garman (1917, p. 441) has recorded that "among aquatic Hemip- 

 tera the genera Ranatra, Belostoma, and Notonecta, and probably others feed upon 

 damselfly nymphs." The fact that the water boatman, Notonecta, attacks the 

 nymphs of dragonflies was also recorded by W. J. Lucas (1908, p. 16). 



4. Fresh-water Hydra. — Another enemy of the nymph is found in the common 

 hydra; the green species, H. viridis, does not probably reach a size sufficiently large 

 to overcome even a newly hatched nymph, but the brown species, H. fusca, can and 

 does eat small nymphs. Two leaves of Potamogeton illinoiensis, which contained 

 a large number of Bnallagma eggs that were just hatching, were brought into the 

 laboratory August 11, 191 7, and placed in a small aquarium. On going over them 

 with a hand lens to remove the nymphs already hatched, a large brown hydra was 

 found eating one of the tiny nymphs. It was attached to the under surface of the 

 leaf, nearly in the center of a large cluster of the Bnallagma eggs, and could reach 

 many of the nymphs with its tentacles as they emerged. If this species of hydra 

 became at all plentiful in a fishpond it might kill a large number of the young nymphs. 



5. Nematodes. — Good-sized specimens were found in the stomachs of several 

 nymphs of both Anax and L. luctuosa, and Needham (1898, p. 86) found the intestine 

 of a nymph parasitized by large Gregarines a millimeter in length. An Bnallagma 

 nymph examined July 27, 191 7, contained a dozen large Gregarines, and several others 

 contained one or two apiece. These intestinal parasites probably never become numer- 

 ous enough to actually kijl their host, but their presence may weaken the nymph and 

 make it more susceptible to its other enemies. 



6. Parasitic Mites and Flies. — Some of the Ischnura and Bnallagma nymphs 

 were found infested with small mites between the wing pads and around the bases 

 of the legs; 10 of these were taken from a single Ischnura nymph. 



Mrs. Aaron (Lamborn, 1890, p. 50) mentioned another small red mite "which 

 skims rapidly over the water in search of an Odonat egg, upon which it either deposits 

 an egg or excavates it for immediate nourishment." She also saw one of the parasitic 

 Diptera ovipositing on the egg of Diplax. In these two cases, of course, the larvae of 

 the mite and the fly when they hatch feed upon the dragonfly's egg. 



Needham (1903) reported that many hymenopterous parasites prey upon the eggs 

 of IvCstes, which are inserted in plant tissues above the water line, where they are exposed 

 to such attacks. He succeeded in rearing three species of the parasite, belonging to 

 different genera. 



Brandt (1869) similarly reported rearing another parasite, Polynema ovulorum, 

 from the eggs of Agrion (Calopteryx) , and added that half the eggs were sometimes 

 destroyed in this way. 



