P. J. Darlington, Jr.
Habits of the Staphylinid Beetle Dianous nitidulus.
Psyche 36(4):386, 1929.

This article at Hindawi Publishing: https://doi.org/10.1155/1929/63428
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386 Psyche [December
HABITS OF THE STAPHYLINID BEETLE
DZANOUS NZTZDULUS
Dianous nitidulus Lec. is to be found in moss and similar cover along the swift, shady mountain brooks which drain the west side of Mount Moosilauke, near Warren, New Hampshire. All my specimens have been taken early in July. The species may be washed out like its relatives, but possesses a remarkable habit which is likely to take the collector by surprise.
On land the beetles are not particularly active, but they can skim so swiftly over running water that they disappear as if by magic in the changing shadows of the brook surface. I have had great difficulty in observing the "skimming," for I have never succeeded in getting a healthy Dianous into the laboratory, but the only possible explanation of the phenomenon is that the insects fly on the water partly sup- ported by surface tension, as Donacia often does. This ex- plains the fact that they bog down easily and cannot skim when bedraggled. They follow a straight or curving course, never zigzaging, and go several times faster than they can on land, so that their passage sets up a miniature, V-shaped swell. I have never seen a Dianous rise clear of the water as Donacia does, however.
The skimming habit is, of course, an admirable adapta- tion for a riparian beetle which lives along swift, fluctuat- ing streams. It is shared by certain species of the related genus Stenus, particularly by some of the red-spotted forms which approximate Dianous in choice of habitat, but many Stenus are nearly helpless on a water film. The retiring habits and quick escape of D. nitidulus doubtless explain Major Casey's remark (Revision Stenini, page 9) that "When its localities are found, it seems to be numerous, but these are very seldom discovered."
It is barely possible that some habit of this sort, acquired by an adephagous beetle, may have started the evolution of the Gyrinidse, which are almost certainly not derived from dytiscid stock and whose ancestors very likely never pos- sessed true diving habits.
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