WATER BEETLES IN RELATION TO PONDFISH CULTURE. 



293 



larva swims with an even and rapid motion, propelling itself by short strokes of the legs, twisting and 

 turning its body in different directions. During its progress it thrusts its tail frequently to the surface 

 for breathing and seems unable to remain long under water. While at the surface it holds itself in place 

 by depressing the cerci down upon the surface film. When disturbed, it snaps its body, often to quite a 

 distance, by a sudden contraction of the longitudinal muscles of the abdominal segments. The same 

 convulsive movement is made when the larva is placed in a preservative. It is aided considerably by 

 the fringes of swimming hairs along the lateral margins of the last two abdominal segments. Sometimes 

 a flexion of these two segments upon the segment just in front of them can be seen, and at other times a 

 flexion of the abdomen upon the thorax, but usually no such movement is visible, because of its abrupt- 

 ness. 



When swimming about, the last two segments of the abdomen are almost always carried at an eleva- 

 tion of 45° dorsally, as though constantly reaching for the surface. At the same time the abdomen 

 between the thorax and the penultimate segment is curA^ed downward. Whenever swimming ceases 

 the buoyancy of the posterior end of the body carries it rapidly upward in a sinuous path, due to the 

 curve of the abdomen and the angulation of the last two segments. There is seldom any need of positive 

 effort in reaching the surface. Whenever there is the larva approaches the surface either tail first or head 

 foremost indifferently; in the latter case it stops just before reaching the surface and inverts its body. 



This larva feeds close to the surface upon whatever it can catch and snaps continually at the Noto- 

 necta and Belostoma nymphs that are swimming there. After seizing its prey it swims about, holding it 

 in its mandibles, and jerking its head and prothorax from side to side much as a terrier shakes a rat. 

 This movement evidently assists it in thrusting its mandibles through the hard skin of its prey. After 

 the mandibles are fixed satisfactorily it remains quietly suspended from the surface film and sucks the 

 juices out of its prey. When it wishes to obtain a new hold, it again swims about and shakes its prey and 

 then quiets down, repeating this until all the juices have been extracted and nothing but the shell of 

 the body is left. 



In the aquarium they were fed with dragonfly and damselfly nymphs, mayfly larvse, the nymphs of 

 Belostoma and Notonecta, and the larvae of Tropisternus. Out of these they always selected the Tropi- 

 sternus larvae first and ate them until they were all gone, then took the Belostoma nymphs, and finally 

 the Notonecta nymphs. They refused to touch any of the other food, and they showed no tendency to 

 fight with one another no matter how hungry they became. Some of the Tropisternus larvae were larger 

 in diameter, though not as long as the Thermonectes larvae, but the latter had no difficulty in killing 

 them because of superior agility. 



Description of the larva. — General form spindle-shaped, narrowed strongly both anteriorly and pos- 

 teriorly; length 17.5 mm., greatest width, through the second and third abdominal segments, 2.5 mm. 

 (fig. 52, opp. p. 291). Head yellow on the dorsal surface with the following parts reddish-brown: The 

 mandibles, the tips of the antennae, the tips of the labial and maxillary palps, the anterior margin of the 

 frons, irregular figures along the median line, across the head, and along the lateral margins behind the 

 eyes; eyes black. Thorax and abdomen reddish-brown on the dorsal surface, white on the ventral sur- 

 face except the last two abdominal segments, which are reddish-brown like the back. On the dorsal 

 surface there is a curved yellow diagonal line on each of the first six abdominal segments, about half-way 

 between the mid line and the lateral margin and extending backward nearly across the segment. A 

 second spot, outside of the first and more inclined, appears on the second, third, fourth, and fifth 

 segments. 



Chitin sclerites cover the entire tergum of the thorax and abdomen and extend onto the pleurum a 

 a short distance on either side, so that their lateral margins are not visible in dorsal view. These margins 

 and also a projection which extends forward beneath the preceding sclerite are considerably darker in 

 color. Inside of the lateral margin is a second narrow dark line on the mesothorax and metathorax and on 

 the fifth and sixth abdominal segments; there are two such lines on the first four abdominal segments. 



The sclerite on the mesothorax does not reach the spiracle, that on the metathorax just covers it, and 

 those on the abdomen segments extend slightly beyond it. The sclerites of the last two abdominal seg- 

 ments extend entirely around the body, and the spiracles of the seventh segment open on the ventral 

 surface. The ventral margins of the pleura of the first six abdominal segments are fringed with short silky 

 hairs. On the sixth segment this fringe curves outward and becomes continuous with the much longer 

 48790°— 23 5 



