302 
BULLETIN 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 abdo min al segments with ventral yellow spots on either side. Head with the base and a 
broad M -shaped mark black; thorax with a narrow 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 comer 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 Gyrinidse, 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 
larvse 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 (Wickham, 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 leaf, 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. 
