324 
BULLETIN OF THE BUREAU OF FISHERIES. 
On coining to the surface to breathe it swims upward, tail first, then reverses its body and thrusts its 
abdomen above the surface film, but having no cerci it can not cling to this film for any length of time. 
Its usual procedure is to cling to some support with its legs while breathing. Its exertions when eating 
demand a constant air supply, and it usually seizes its prey and swims to the nearest water plant. It 
then backs up the stem, dragging its prey after it until it can thrust its abdomen above the surface. 
It can thus eat and breathe at the same time, and it is engaged in doing both nearly all the time. There 
is nothing in its makeup which sanctions an eight-hour day or one day of rest in seven. It works con- 
stantly from the earliest streak of dawn until the last rays of twilight disappear and keeps this up until 
it is fully matured. It is the very personification of voracity and gluttony and will eat any kind of an 
insect adult or larva that it can overpower. Like the Hydrous larva it partakes freely of its own kind, 
even of its brothers and sisters, and of those hatched in any egg case only one or two survive, and they 
have eaten all the others. This wholesale cannibalism is the salvation of the other smaller denizens 
of the ponds; without it they would speedily disappear, but with it they obtain a chance to live. In 
the ponds the favorite food of these larvae is mayfly larvae and damselfly nymphs; they also eat Pelocaris 
femorata, a bug of very questionable reputation, and the nymphs of back swimmers and water boatmen, 
both of which are very harmful to young fish. 
This is one of the larvae mentioned by Comstock (1912) that swim with their prey to a leaf or stem 
near the surface. Resting on this support it raises its head out of the water and, holding it vertically 
upward, crushes its prey to pulp between its powerful jaws, letting the juices run down its open throat. 
It is never content with the juices alone, however, as is the dystiscid larva, but always swallows the pulp. 
Description of the newly hatched larva . — This larva is 3.85 mm. long, including the mouth parts, and 
0.7 mm. wide. Head trapezoidal, considerably narrowed posteriorly; frontal sutures straight; epi- 
cranial suture longer than in the glaber larva and shorter than in the mixtus larva ; frons flat and narrow 
triangular, the base of the triangle (anterior side) a fourth shorter than the sides, the apex acute. An- 
terior margin of head not concave but approximately straight; teeth of labro-clypeus small, the two 
outside ones the largest, the two next to them much smaller and the same size as the one in the center, 
the remaining two mere points, sometimes wholly lacking. There are two spines near the center of 
the frons set well back from the frontal margin and six others in a transverse row just behind the frontal 
margin, and a single spine on each lateral margin in front of the center. There are no spines on the lateral 
expansions of the epistoma except at the outer frontal corners, where there are two, one behind the other. 
The first eye (farthest anterior) is the largest; outside of its posterior end is a seta, another stands 
behind the fourth eye, and a third behind the sixth eye, and there are two more widely separated near 
the fronto-antennal suture. The colored area on either side of the head behind the eyes carries three 
setae. 
The basal joint of the antenna is two-thirds as long again as the two terminal joints, and the tip 
of the latter nearly reaches the point of the mandible. The finger like appendage at the end of the ter- 
minal joint has only a single segment. The mandibles are the most slender of the three Tropisternus 
species here described, are curved but little, and are very acuminate. The distal tooth on the inner 
margin is also the most slender, its bifid tip is the most acuminate, and the teeth on its inner margin 
are the longest and sharpest in the three species. The middle tooth on the left mandible is long, slender, 
and acuminate; it is not double but is inserted in the groove nearer its ventral margin. The proximal 
tooth is minute, triangular, acute, and single; it is inserted on the ventral margin of the groove, and 
its edge is not produced transversely. On the right mandible there are two small proximal teeth, 
side by side, one on either margin of the groove, and a trifle larger than the proximal tooth of the left 
mandible. The maxilla is more slender than in the other two species, but otherwise similar to them. 
The labium also is slender but short, and the tips of its palps only reach the distal third of the maxil- 
lary stipes. The mentum has nearly straight sides, its anterior angles are produced but little, and are 
rather bluntly rounded. The ligula is conical, conside&bly tapered, and apparently jointed near 
the tip. The terminal joint of the palpus is nearly five times as long as the basal and ends in the 
usual armature of sense cones and setae. 
The mature larva . — General form spindle-shaped and considerably depressed, with tufts of silky hairs 
along the sides of the abdomen and other smaller ones scattered sparsely over the body. When fully 
grown the larva has a length of 10 to 12 mm. and a width across the third abdominal segment of 3 to 
3.5 mm. Each of the abdominal segments is divided into a narrower anterior and a wider posterior 
