278 
BULLETIN 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 completely reversing the bodylike 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 corner. The lateral margins of the head are fringed with scattered and very 
unequal hairs. 
The antennse 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 
