232
THE ESSEX NATURALIST.
contract into little rounded masses which either roll off or are
dissipated by evaporation. When closely-set water-repellent
scales, &c., are present, the water never even comes into contact
with the actual surface as it cannot, as already explained, pene-
trate the fine spaces between the scales, &c. In these circum-
stances the upper surfaces of the plants carry a layer of air down
with them when submerged and so keep dry even under water.
In both cases the dry upper surface, in combination with the
water-logged under surface, which is the general rule, ensures
that the plants shall have a good chance of righting themselves
when they come to the surface after submersion.
The animals living on the water-surface keep themselves dry
by similar means to the plants, namely, either by having smooth
water-repellent coats (e.g. Whirligig beetles and some Spring-
tails) or by being clothed with dense water-repellent hairs, scales,
&c. (e.g. Pond-skaters and some Spring-tails). Should the animals
with the hairy coats be submerged by any chance, they carry down
with them a film of air which gives them a silvery appearance
similar to that so well known in the case of the Water-spider.
The egg-raft of the gnat illustrates both this method of keeping
dry and the power of self-righting. Its upper surface remains
dry, even under water, in consequence of the inability of water to
penetrate between the water-repellent conical ends of the closely
packed eggs, and, as the other ends of the eggs on the under
surface remain constantly wet, the raft can right itself if accident-
ally turned over.
The role played by surface-tension with regard to the distri-
bution of water-surface plants and animals, depends chiefly upon
the force which binds together two wettable surfaces. When
a stick is plunged into a thick growth of Duckweed and then
withdrawn, it comes out covered with the little plants, all with
their dry upper surfaces outwards. The water between their
wet lower surfaces and the stick acts, by reason of surface-tension,
as if it were an adhesive material binding the plants to the stick.
A similar adhesion takes place when quadrupeds and birds enter
the water and in this way water-surface plants may get carried
from one piece of water to another. It is doubtful if any living
water-surface animals are distributed in the same way, but their
eggs (e.g. egg-raft of gnat) are almost certainly so carried about.
And as the water plants are also very frequently the repositories