306 BUULETIN OF THE BUREAU OF FISHERIES. 



The duration of the pupal period is seen in the following table: 



Entered chamber- 



Pupated. 



Entered chamber. 



Pupated. 



Julys.., 

 July 13. . 

 July 14., 

 Do., 



July 10.. 

 July 15.. 

 July 16.. 



July 20.. 



July 23.. 



Died. 

 July 20. 

 July 21. 

 Do. 



July 28. 



July 20.. 

 Jul J 21.. 



July 22!! 

 Do . 



July 23.. 

 July 22.. 

 July 23.. 



do... 



July 24.. 



July 28. 



Do. 

 July 29 



Do. 



Do. 



Habits of the adult. — This genus includes the largest of the whirligig beetles, and the present species 

 is the most abundant of any found in the fishponds. Although nowhere occurring in such swarms as 

 characterize the closely related genus Gyrinus, the aggregate, nevertheless, forms a very respectable 

 percentage of the pond fauna. 



The locomotion is peculiar, whirling or spinning around a vertical axis, or circling in companies on 

 the surface of the water; they seem never to move straight away in any direction. They swim with 

 great rapidity and dive like other beetles, but spend most of their time on the surface film, and hence 

 with the upper portion of their bodies in the air. To fit them for this mode of life, the under surface 

 of the body is very flat lengthwise but quite convex transversely, and is hence shaped much like the 

 bottom of a canoe. They skim the surface so rapidly and whirl about with such agility that it is extremely 

 difficult to catch them without a net. They can fly well and frequently fly into the laboratory at night, 

 but they can not rise from the surface of the water; they must climb up a grass or rush stem in order 

 to spread their wings. 



The adults are carnivorous like the larvae, but have rather the habits of scavengers, feeding upon 

 dead animal matter that finds its way into the ponds and floats on the surface. Miall (1895, p. 33) 

 quoted the following observation by W. F. Baker: 



I was once watching some Oyrini when a gadfly came flying round my head. I struck at it and knocked it into the water 

 stunned if not dead. Two or three Gyrini seized it and shortly afterwards the whole swarm clustered round. In a short time 

 nothing was left except the wmgs of the fly, which were allowed to drift away. This is the only occasion on which I have 

 seen anything of the sort. I could never get captive Gyrini to eat dead flies. 



That the genus Dineutes responds more readily to artificial feeding is manifest from the following: 

 The adult Dineutes americanus in pond 2F were fed with freshly killed insects, including two kinds of grass- 

 hoppers, two grass moths of different species, two damselflies {Ischnura verticalis and Enallagma hageni), 

 a dragonfly {Leucorhinia intacta), a butterfly (Phydodes tharos), and a mayfly imago {Hexagenia sp.), and 

 they ate them all voraciously. As many would seize the insect as could crowd around it, grasping it 

 with their mandibles. Then they would swim off, sometimes going fairly straight away, sometimes 

 whirling around in wild curves and sometimes diving beneath the surface, but always holding on to 

 their prey and tearing out mouthfuls of the insect tissues. When one bit off more than he could swallow 

 at once, he would hurry away with it, pursued by a hungry crowd, each intent on snatching it from 

 him, and he was lucky if he could keep it long enough to finish devouring it. 



Another habit became conspicuous during the hot weather of the summer. Ordinarily the adult 

 whirligigs are more or less scattered over the surface of the pond, but on hot days they show a decided 

 preference for the shade. Most of the ponds have a small pier running from the shore to the outlet pipe 

 to facilitate the handling of the latter. This pier is made of short lengths of plank with spaces between 

 and casts a shadow which is either solid or streaked with rays of light according to the time of day. All 

 the adult Dineutes in the pond on hot days gathered in this shadow about the middle of the forenoon 

 and remained until late in the afternoon. When the shadow was solid they scattered indiscriminately 

 over it, but when it was crossed with streaks of sunshine they carefully avoided the latter. The accom- 

 panying photograph (fig. 4, frontispiece) shows them thus congregated beneath the pier of pond 5D. It 

 was taken about noon by H. W. Clark. 



Description of the adult. — General outline an elongated ellipse 11.5 to 12.5 mm. long, 4.5 to 5 mm. 

 wide, thus almost two and a half times as long as wide, the last segment in the female and the last two 

 segments in the male projecting behind the elytra. Both dorsal and ventral surfaces distinctly convex; 

 only the first legs visible in dorsal view, the others folded beneath the body. Above black, the elytra 



