278 



BUULETIN OF THE BUREAU OF FISHERIES. 



Habits of the larva. — These larvae are excellent swimmers; the legs are not very heavily fringed with 

 swimming hairs, but their length compensates for this. There is also an excellent swimming fringe along 

 the lateral margins of the last two abdominal segments. Ordinarily they use their legs in moving through 

 the water, but when alarmed or wishing to move more rapidly they wriggle the whole body. By lashing 

 the abdomen up and down they can dart with considerable rapidity. 



When coming to the surface to breathe, their body is inclined with the head upward , but on reaching 

 the surface they assume a horizontal position instead of completelyreversing the body like the hydrophilids. 

 Sometimes they assume this horizontal position quite a distance below the surface and then float slowly 

 upward. At the surface the base of the head and the prothorax as well as the last abdominal segment 

 rest in the surface film, while the center of the body is arched downward. When out of the water, the 

 larva crawls along slowly, dragging its abdomen, but it can jump vigorously by lashing with the last two 

 abdominal segments. 



The larva eats tadpoles of the leopard frog, nymphs of mayflies and dragonflies, and the larvae of 

 other water beetles, even those of Dytiscus and Hydrous. This is one of the three genera whose larvae 

 eat fish, and when full-grown the larva is so large and powerful that it has no difficulty in killing fish of 

 considerable size. They are also cannibals, and whenever two come together they fight until one or the 

 other gains the victory. The winner then sucks the juices from the body of his unfortunate competitor, 

 leaving nothing but the tough skin. Such skins are often found in the ponds during the summer and 

 may be recognized by the rents made at intervals where the sharp mandibles of the victor pierced them. 



If several larvae are put in the same jar on a collecting trip, the chances are that only one will be alive 

 on reaching the laboratory, and this is just as true when the jar contains numerous smaller and weaker 

 larvae of other kinds as when it contains only the dytiscids. This chronic cannibalism is the salvation of 

 the other denizens of the pond, since it furnishes a very efficient means of reducing the number of Cybis- 

 ter larvae. 



Description of the larva. — General form elongate and spindle-shaped (fig. 51, opp. p. 291), tapering 

 both anteriorly and posteriorly. A full-grown larva is 75 mm. long and 7.5 mm. wide through the 

 second and third abdominal segments. 



The head is contracted posteriorly to half its anterior width, forming a short neck. The thorax 

 widens posteriorly, the hind margin being more than twice the width of the front margin. The abdomen 

 is made up of eight segments, the first five about the same width, the last three strongly tapered, and the 

 last two with wide lateral fringes. The lateral margins of the first six abdominal segments, and to a 

 lesser degree those of the mesothorax and metathorax, project as a rounded longitudinal ridge. The 

 base of this ridge forms a well-defined groove on the dorsal and ventral surfaces of the second, third, 

 fourth, and fifth abdominal segments, with traces on the other segments. 



The head, the thorax, and the legs are reddish-yellow, inclined to orange on the dorsal surface and 

 flecked with brown pigment. The abdomen is brown on the dorsal surface, yellow along the lateral 

 ridges and on the ventral surface, and brown in the ventral grooves bordering the lateral ridges. 



The head is widest across the anterior border, which is emarginate and very irregular, with a long 

 and narrow conical process on the mid line, a shorter and wider one on either side of it, and rounded 

 lateral corners. Each of the three processes is tipped with a dense fringe of hairs, and there is a tuft of 

 hairs inside each lateral comer. The lateral margins of the head are fringed with scattered and very 

 unequal hairs. 



The antennae are filiform and nine-jointed, the relative lengths of the joints being 32, 44, 15, 14, 12, 

 13, 13, 17, 4; the second, third, and fourth joints bear one or two setae each. The mandibles are sickle- 

 shaped, perforated near the tip, the perforation opening into a tube that extends along the inner margin 

 of the mandible and opens at the base on the dorsal surface. The inner margin of the mandible has a 

 fringe of short, stiff, blunt spines along its center, leaving the base and tip smooth. At the distal end of 

 this fringe the entire surface of the mandible is covered with long hairs. At the base of the mandible 

 on the ventral surface is a flattened, knoblike process, which fits into a socket in the chitinous covering 

 of the head. 



The maxillae (fig. 26) are reduced to a single filiform ramus of 10 joints; the basal joint carries three 

 setae, the second joint one, the fourth and fifth joints tufts of small setae near their tips, and the seventh 

 and ninth joints one seta each. At the base of the appendage on the ventrolateral surface is a small 

 process bearing a tuft of setae. The labium is small and its anterior margin is deeply emarginate, bearing 



