302 



BUULETIN OF THE BUREAU OF FISHERIES. 



Description of the adult. — General form broadly obovate, 14 to 15 mm. long, 7 to 8 mm. wide; dull 

 yellowish-brown on both dorsal and ventral surfaces, the thorax and elytra with yellow margins and the 

 posterior abdominal segments with ventral yellow spots on either side. Head with the base and a 

 broad M -shaped mark black; thorax with a najrow black line near each lateral margin of the disk and 

 two wider transverse ones. In all the females obtained the elytra were broadly sulcate or grooved; in 

 the male they were smooth with a subapical yellowish cross bar. The front tarsi of the male are broadly 

 dilated; the basal joint bears a large sucking disk at its posterior proximal corner and two very much 

 smaller ones on its anterior margin. The second and third joints bear numerous rows of minute hairs 

 terminating in sucking disks along both anterior and posterior margins, and the posterior ones extend 

 onto the basal joint distal to the sucking disk. These tarsi are thus quite different from those in other 

 genera and help in identification. 



Genus DINEUTES MacLeay. 



Dineutes (MacLeay, 1825, p. 30). 



This is a genus of fair-sized beetles belonging to the Gyrinidae, or whirligig 

 beetles; they are more or less oval in form and quite strongly depressed. The 

 upper surface is bronzed, shining, and finely reticulate, and in the only species 

 found in the Fairport ponds the ventral surface is black and shiny. The front 

 tarsus of the males is dilated only a little and is clothed with rows of short tubular 

 suckers. The adults are harmless and never injure or attack young fish, but the 

 larvae have been known to kill and eat small fish under somewhat abnormal 

 conditions. 



Dineutes americanus Say. Figures 84-94. 



Dineutes americanus (Say, 1825, p. 107). 

 Cyclinus assimilis (Kirby, 1837, p. 78). 



Dineutes assimilis (Wickhain, 1893a, p. 330, pi. 9; 1894, p. 39). 



Eggs. — The eggs were found in clusters varying from 7 to 40 in number on the under surface of the 

 leaves of Potamogeton illinoensis in pond 2D. They were white in color and were arranged diagonally 

 at an angle of 45° with the mid rib of the leaf, all on one side near the base. They were regular elongated 

 ellipsoids, 1.85 mm. long and 0.64 mm. in diameter. The egg shell consists of a thin and smooth inner 

 membrane covered with a much thicker layer composed of short cylinders placed end to end and packed 

 so closely together as to become more or less hexagonal. The cylinders are of different diameters and 

 their outer ends are rounded into hemispheres. This thick prismatic layer covers what may be called 

 the top of the egg, namely, the surface next to the l^af, runs down on either side and onto the bottom 

 surface, but leaves quite a wide strip through the center of this surface uncovered. The two edges of 

 the thickened layer meet at the posterior end of the egg but not at the anterior end. Each egg is glued 

 separately to the leaf by a transparent colorless cement, which extends the whole length of the egg and 

 projects slightly beyond the ends. The egg is apparently not flattened at all on the top next to the leaf. 

 The larva is folded lengthwise inside the eggshell in the same manner that the mature larva afterwards 

 folds itself inside the pupal chamber. The dorsal surface of the thorax and the first five abdominal 

 segments are next to the surface of the leaf, the rest of the abdomen is folded up against them and the 

 head is folded against this posterior abdomen with the antennae and mouth parts fully extended. The 

 lateral gills are closely appressed to the sides of the body. 



When the egg hatches, the shell splits lengthwise on the bottom through the space uncovered by 

 the prismatic layer from end to end. The larvae all batch at about the same time and crawl around on 

 the surface of the leaf amongst the empty eggshells for several hours before swimming away. The 

 hatching occurs five or six days after the eggs are laid. 



The newly hatched larva. — As soon as it is once straightened out the newly hatched larva is from 5 to 

 5.5 mm. long, and the head, which is the widest part of the body, is 0.75 mm. in diameter. This larva 

 is like the fully matured one, except that the prothorax is as wide as the mesothorax and metathorax, 

 and the first two pairs of lateral gills are plumose like the others. 



