234
THE ESSEX NATURALIST.
can also often be seen wriggling their way up the glass sides of an
aquarium well above the surface of the water and doubtless they
can do the same up the stems of water plants. Being unable to
penetrate the surface-film, they carry a drop of water about with
them which binds them to any surface they may be moving over.
As a final example of one of the many ways in which surface-
tension is taken advantage of by water-surface organisms, may be
mentioned the case of the small beetles Dianous coerulescens,
Stenus guttula, &c. Although normally living among the damp
vegetation at the margin of streams, ponds, &c., they occasionally
find themselves on the surface of the water, especially in times of
flood. Under these circumstances they can not only make use
of the surface-film for support, but by emitting some peculiar
secretion which locally reduces the surface-tension, they can move
about on the surface in quite a rapid manner and presumably
without muscular effort. Whether any of the more typical water-
surface-animals make use of the same principle has not been
recorded, but the possibility should be borne in mind when watch-
ing them.
Leaving now the consideration of the various ways in which
water-surface organisms are affected by and respond to the
surface-tension factor in their environment, a few words may be
devoted to the methods of observation of these phenomena. For
work in the field much of course can be done by close scrutiny of
the surface of various waters simply with the naked eye, but, when-
ever possible this method of observation should be supplemented
by the use of field-glasses magnifying, say, six or eight diameters.
With such glasses it is not only possible to follow individual
specimens of such lively creatures as Whirligig beetles and Pond-
skaters in their movements, even at some distance from the
margin, but also to see the form of the capillary depressions
which they produce in the surface-film. For indoor work, ob-
servations may be made with a pocket lens, the organisms being
placed in shallow dishes or, for the smaller forms, in watch glasses.
If the surface of the water be looked at very obliquely with the
lens, so that it reflects a large amount of light and appears as a
mirror, the slightest break of the surface due to the formation
of capillary curves by the organisms can be detected.
The nature of the capillary curves, whether ascending or
descending, can be demonstrated in two ways. By the use of a