LIFE HISTORY OF HYDROUS ( HYDROPHILUS) TRIANGULARIS. 
27 
and did not return again. The final color of the adult beetle is a rich olive black, 
very shiny above and piceous beneath, while the antennae, palpi, and tarsi are a 
deep mahogany brown, often black. 
EXTERNAL CHARACTERS. 
The adult beetle has the outline of an elongated oval, 34 to 40 mm. long and 
15 to 16 mm. wide. The thickness of the thorax is 8 to 9 mm. The dorsal surface 
is not as convex as in the species ovatus. The prosternal prominence, into which the 
front end of the sternal spine fits, is closed in front, and there are more or less dis- 
tinct triangular yellow spots along the ventrolateral margins of the abdomen. 
These color spots are more distinct on the second, third, and fourth segments than 
on the others and are partly due to the fact that the skin is here pubescent and 
partly to a lighter color in the skin itself. 
This genus is distinguished first by its size, the species being fully twice as large 
as any other North American hydrophilid. Then, the last four joints of the nine- 
jointed antennae are of peculiar shape and form a distinct club. Along the cen- 
ter of the ventral surface of the thorax there is a continuous ridge that projects in 
front into the prosternal prominence already mentioned and is prolonged behind 
into a large, acute spine. The tarsi of the middle and hind legs are strongly com- 
pressed and fringed with hairs, making powerful swimming organs. 
The pro, meso, and metasterna, the coxse of all three pairs of legs, the 
trochanters and bases of the femora of the first legs, and all of the first segment of 
the abdomen are covered with a dense yellow pubescence, becoming blackish in 
older specimens. This serves to hold the air film that covers the ventral surface 
when the beetle is under water. The scutellum is almost an equilateral triangle, 
the base being scarcely shorter than the sides. Each elytron has four rows of 
coarser punctures, the outer row being double, and there is an additional partial 
row along the distal portion of the sutral margin. Between these the surface is 
covered with minute wrinkles and finer, scattered punctures. There are also 
scattered punctures along the base of the upper lip, on the lateral margins of the 
head in front of and inside of the eyes, along either side of the mid line of the head, 
on the lateral margins of the pronotum, and in an oblique line on either side 
extending from behind the eyes backward and inward and not quite reaching the 
center. There is no definite arrangement of the punctures at any of these places, 
but they are very distinctly visible. 
ANTENNA. 
These are attached just in front of the eyes and curve around below them, 
being concealed in dorsal view. They are nine-jointed; the basal joint is longer 
and wider than the four that follow it and is curved. The second joint is one- 
fourth the length and three-fifths the width of the basal joint; the next three joints 
are the same size, half the length, and the same width as the second joint. The 
sixth joint is deeply bilobed; the ventral portion is enlarged and flattened into a 
spoonlike lamina, which extends outward beneath the seventh joint and covers all 
of it except the very tip; the dorsal portion is a short fingerlike process. Each 
of the seventh and eighth joints has a long process arising from its anterior margin 
