386 
Psyche 
[December 
HABITS OF THE STAPHYLINID BEETLE 
DIANOUS NITIDULUS 
By P. J. Darlington, Jr. 
Dianous nitidulus Lee. 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. 
