34 



BULLETIN OF THE BUREAU OF FISHERIES. 



larvae were obtained from a Hydrous larva just as the latter was about to pupate 

 after remaining several days inside its pupal chamber. Evidently some hymen« 

 opter had succeeded in laying its eggs in this larva while it was hunting for a suitable 

 spot for its pupal chamber. The eggs had hatched into maggots and these were 

 busy devouring the tissues of the beetle larva. It is doubtful whether this larva 

 would have been able to pupate, but even if it did the maggots would have con- 

 tinued their destruction and there would not have been enough of the pupal tissues 

 left for the final transformation into the adult beetle. These Hydrous larvae 

 usually escape such attacks, because of the shortness of the interval between coming 

 out of the water and burrowing into the earth, but evidently they do get caught 

 sometimes. 



ENEMIES OF THE ADULT BEETLES. 



Dyche (1914, p. 149) has stated that the common bullfrog is especially fond of 

 the larger water beetles, Hydrous and Dytiscus. Thirty frogs were captured from 

 the ponds on the State hatchery grounds near Pratt, Kans., in April, 1910, and their 

 stomach contents were examined. No effort was made to distinguish the two 

 genera of beetles from each other. Twelve of the frogs had eat,en these beetles, 

 and amongst them had consumed 27 specimens, over two apiece. Another large frog 

 captured the following year contained four of these beetles (p. 155). Dyche added 

 that more than half the food mass of frogs taken from natural lakes and ponds in 

 Kansas was frequently made up of insects, and that these large water beetles usually 

 formed a considerable percentage of such insect food. Unfortunately the bull- 

 frogs also eat many small fish and hence could not be utilized to keep down the 

 beetles. 



Two little green herons were shot on one of the fishponds August 10. On 

 examining their stomachs the remains of adult Hydrous beetles were found in one 

 of them. 



McAtee and Beal (1912, pp. 19 and 24) stated that various beetles, chiefly 

 aquatic, compose 23.3 per cent of the food of the horned grebe (Colymbus auritus)j 

 and that the black tern {Hydrochelidon nigra surinamensis) feeds extensively on 

 dragon-fly nymphs, dytiscid beetles, and crawfishes. A bird that eats dytiscid 

 beetles eats also hydrophilid beetles in all probability, and it is reasonable to sup« 

 pose that other water birds, with reference to whose food we have no data, eat these 

 beetles in considerable quantities. 



During the seasons when the beetles migrate from one locality to another 

 they are often attracted by the electric lights in large towns and cities at night 

 and can be found in the morning upon the pavement or earth beneath. Blatchley 

 (1910, under HydropJiilus triangularis, p. 255) said: Sometimes attracted by 

 thousands to electric lights in Indianapolis and the larger cities." 



William J. Gerhard, of the Field Museum of Natural History of Chicago, 

 has records (unpublished) of the finding of Hydrous and hydrophilids under the 

 electric lights of Chicago before the use of arclights was discontinued 10 years ago. 

 These records show that the finding of Hydrous in varying numbers was a regular 

 occurrence every spring toward the last of May, and when we remember the other 

 large cities which are as favorably situated as Indianapolis and Chicago we are 



