WATER BEETLES IN RELATION TO PONDFISH CULTURE. 



281 



At this stage the pupa is at least one-half longer and narrower than subsequently, and its first act 

 is to stretch itself vigorously. The abdomen is arched upward until it becomes nearly a half circle, 

 and the two ends of the pupa thrust the discarded larval skin against the walls of the pupal chamber. 

 The hind legs are but a trifle longer than the first and second pairs, and the tarsi of all three pairs are 

 deeply cleft. The legs, antennae, and mouth parts stretch with the rest of the body and quickly assume 

 their normal proportions, and the clefts at the tips of the tarsi gradually fill out and disappear. The 

 abdomen shortens and widens, and the compound eyes become entirely pigmented. By the third 

 day the pupa turns over and rests upon the dense bristles at the anterior margin of the prothorax and 

 upon the posterior spines, the body being arched strongly upward. 



The general shape of the pupa at this stage is that of an elongated ovoid, just twice as long as wide, 

 considerably narrowed, and rather pointed anteriorly. The front margin of the prothorax projects 

 over the base of the head and is armed with a dense row of short and stiff bristles, and there is another 

 row, much less dense, along the hind margin. On the mid line of the dorsal surface, near the anterior 

 margin of the prothorax, and at the center of the mesothorax and metathorax are small areas covered with 

 bristles. Each of the first six abdominal segments is raised into a dorsal ridge near its posterior margin, 

 the ridges being highest on the third and fourth segments and diminishing both anteriorly and pos- 

 teriorly. Each ridge carries a single row of bristles. The ventral surface of the seventh segment is 

 covered with a hard, smooth plate, armed with scattered short bristles. At the posterior end of the 

 body are two stout curved spines; the lateral margins of each abdominal segment and the ventral sur- 

 face are armed with scattered short setae (fig. 33, p. 283). 



The antennae are twisted around beneath the femora of the first legs; the labial palps reach nearly 

 to the posterior margin of the bases of the second legs. The legs are short and rather stout and are bent 

 in such a manner that their tips meet on the mid line; the tip of each leg in all three pairs is emarginate. 

 The wings are also very short, hardly reaching the fourth abdominal segment. 



The color of the pupa is white, tinged with light brown on the back, the eyes brown, the bristles 

 reddish. In later development the head and prothorax and the last two abdomen segments become 

 reddish brown, while the elytra are tinged with dark brown, almost black. The spiracles are on the 

 dorsal surface and close to the anterior margin of each segment, the anterior ones slightly larger than 

 the posterior. 



Habits of the adult. — In this genus is shown the highest development of those modifications which 

 are adapted to swimming and diving, and as a result the beetle displays great rapidity and agility when 

 in the water. On being removed to the land, however, it makes only a poor attempt at walking and 

 can not run at all or jump. 



In breathing Brocher (1911a) claims to have discovered that the last pair of spiracles are the only 

 ones that possess an apparatus for filtering the air. Furthermore, and of greater importance, the last 

 two pairs of spiracles are provided with a special tubular trachea, which extends forward to the meta- 

 thorax. Hence, he concludes that inhalation takes place through the last pair of spiracles, exhalation 

 through the other abdominal spiracles. 



The adult beetles have been accused along with Dytiscus and Hydrous of catching and eating 

 small fish, but the testimony against Cybister seems mostly derived from analogy. Although this 

 species is abundant in at least three of the fishponds they have never been known to attack a fish either 

 in the ponds or in aquaria in the laboratory. They have been seen eating dragonfly and mayfly 

 nymphs, together with those of Corixa, Notonecta, and Benacus. 



Description of the adult. — ^The newly emerged adult is a beautiful yellowish white without any 

 pigment except in the legs, which are reddish, and the black eyes. General shape obovate with the 

 posterior end pointed. Color brown with a decided greenish tinge; a broad yellow band along the mar- 

 gins of the thorax and elytra; the front of the head, the two front pairs of legs, and spots on the ventral 

 surface at the sides of segments three to six of the abdomen are also yellow. The thorax and elytra of 

 the female are covered with impressed lines of varying length and not parallel, even anastomosing in 

 places. The thorax and elytra of the male are smooth and shining. 



The front tarsi of the males are attached at an angle to the tibiae and point backward toward the body. 

 The first three segments of the tarsus are enlarged into a disk, which together with the tibia forms a 

 triangle, whose apex is proximal and base distal. In dorsal view the tarsus is apparently joined to the 

 base of the tibia instead of its tip. On the bottom of the disk are four transverse rows of small, equal- 



