WATER-SURFACE PLANTS AND ANIMALS. 227
polished and highly water-repellent chitin and their two hinder
pairs of legs, especially the last pair, are much flattened, with the.
tarsal joints arranged to fold up like a fan. Such powerful
swimming organs are not only required for the extremely active
gyrations which these animals indulge in when on the surface
and to overcome their buoyancy when under the water, but
probably also to enable them quickly to break through the sur-
facc-film when they wish to dive, as they do in a flash when
alarmed. Their eyes also are most remarkable, for they are divided
into two widely separated parts, one on the upper and one on the
lower surface of the head. Such an arrangement one cannot
but suppose must be related to the double life of these creatures,
partly on and partly below the surface, and it is only natural to
conclude that the lower eyes give warning of what is happening
below when the animals are careering about on the surface.
Experiments to determine this supposition would be very wel-
come.
Coming next to those animals which, although living con-
stantly under water, nevertheless utilise the force of surface-
tension for support, we find that we have to deal with many
different groups such as the Insects, Entomostraca, Mollusca,
Planarians, Hydrozoa and almost certainly also the Rotifera and
Protozoa.
The Insects are represented chiefly by aquatic larvae, nearly
all of which belong to the Diptera, although a few beetle-larvae
may also perhaps be regarded as belonging to the water-surface
association, the test being, as already suggested, whether they
not only attach themselves occasionally to the surface-film for
breathing purposes, but remain suspended for considerable
periods feeding all the time. The larvae of the gnats, such as.
species of Culex, Anopheles, etc., undoubtedly secure a large
part of their food while suspended, although the actual methods
both of suspension and feeding differ markedly in the Culicine
and Anopheline types of larvae. In Culex and its allies the larva
has a rather long breathing tube, or siphon as it is usually called,
arising from the eighth abdominal segment, the end of which
is provided with five movable flaps. When the larva is under
the water these flaps are folded together in the form of a cone,
thus closing the aperture of the siphon, but when the animal
comes to the surface, the tip of the siphon is struck against