304 



BULLETIN OF THE BUKEAU OF FISHERIES. 



Habits of the larva. — This larva swims with a sinuous motion of the whole body up and down after 

 the manner of a flatworm. There are no swimming fringes on the legs, but instead the eight posterior 

 pairs of lateral gills are heavily fringed on both sides, and thus serve for locomotion as well as for breath- 

 ing. By lashing them up and down the larva can move either forwards or backwards with great rapidity. 

 On the land it crawls quite rapidly; being unable to lift the posterior part of the abdomen above the 

 ground, it aids locomotion by using the last segment with its hooks like the posterior prologs of the 

 caterpillar, arching the abdomen upward like the so-called inch worm. It can also jump quite a dis- 

 tance by snapping its body in the same manner as the Thermonectes larva. "When two larvae come 

 together out of the water, the first instinct is self-preservation, and each jumps back as far as it can. 



In an aquarium the larva rests nearly always on the bottom and not on the water plants, and the 

 abdomen maintains a constant trembling motion up and down, which is evidently its mode of breath- 

 ing. Hence, it never needs to come to the surface for an air supply. 



It is not very voracious but will defend itself fiercely when attacked and will catch anything that 

 is unfortunate enough to crawl against it. It sometimes attacks and kills small fish as is related on 

 page 250. It shows a preference for the nymphs of damselfleis and mayflies and the larvae of Corethra 

 and Chironomus. 



Description of the larva. — The larva has the appearance of a small centipede, owing to the presence 

 of lateral gills along the abdominal segments (fig. 84). When full grown it measures 25 to 30 mm. in 

 length and 3 to 3.5 mm. in width. The body is made up of the head, 3 thorax and 10 atJdomen 

 segments, and is seven times as long as wide and strongly flattened. The general color is white, lighter 

 on the ventral surface and in the lateral gills, and faintly tinged with yellowish-brown on the dorsal 

 surface. The eyes are black, but the following parts are dark brown — the mandibles, a triangular spot 

 between the bases of the antennae and the eyes, the neck on both the dorsal and ventral surfaces, the 

 posterior margin of the dorsal plates on the first thorax segment, and a small spot on the dorsal surface 

 of the base of each leg in the shape of an inverted V- 



The head is elliptical in outline, one-third longer than wide, flattened dorsoventrally, but with 

 both surfaces convex, almost squarely truncated anteriorly with a small rostrum on the mid line, bordered 

 on either side by a short pointed tooth. The antennae project from the dorsal surface of the head behind 

 the outer corners of the bases of the mandibles. Each is four-jointed, the three terminal joints filLform 

 and diminishing a little in length and diameter, the basal joint much wider and shorter. The mandibles 

 are alike, each is slender, curved, acute, and suctorial, the inner tube opening through a slit near the tip. 



The maxillae are a little longer than the antennae and are attached to the ventral surface of the 

 head just inside the bases of the mandibles. The basal joint is long and stout, the second joint about 

 half the length but nearly as wide. To its outer corner is attached the four-jointed palp, the three 

 distal joints about the same length, the proximal one about half as long. At the inner corner is a long 

 curved process, emarginate at the tip, representing the lacinia, and between the lacinia and palp is a 

 two-jointed galea, the terminal joint much longer and narrower than the basal, both joints armed with 

 a long seta. There are also two setae on the dorsal surface opposite the base of the galea, two on the 

 outer margin near the basal joint, and a small curved spine on the inner margin at the base of the lacinia. 

 The labium is peculiar in that the bases of the palps are fused on the mid line; the palps themselves 

 are three-jointed with a small fold or wrinkle between the first and second joints; obviously there is 

 no chance for a ligula. 



As in the Dytiscidae, the eyes are 12 in number, a group of 6 just behind the base of each antenna 

 and all visible in dorsal view. Their arrangement may be seen in iigure 85, p. 303; the two dorsal eyes 

 are close together with their long axes at right angles to each other. The other eyes are separated a 

 short distance laterally and a much longer distance longitudinally, and their long axes are parallel, 

 respectively, to those of the dorsal pair. All the eyes protude sUghtly from the surface, and in the 

 space between them a seta is located. Posteriorly the head is narrowed into a short neck, covered with 

 two narrow plates on the dorsal surface, but naked on the ventral surface. 



The three thorax segments are about the same length, but the first one is a quarter narrower than 

 the other two and carries a pair of dorsal plates. The other segments and the abdomen are destitute 

 of dorsal plates, since this is not a burrowing larva. 



The 10 abdominal segments are represented in length by the nimibers 11, 15, 18, 18, 19, 19, 20, 20, 

 15, 10, and in width by the numbers 28, 31, 33, 35, 34, 34, 33, 30, 21, 8. From the posterior corners of 



