WATER-SURFACE PLANTS AND ANIMALS. 225
easy to see that where their legs rest on the water considerable
capillary depressions occur. These depressions are also very
clearly indicated by their shadows when the insects are viewed in
a white vessel in sunlight, for then the shadows of the legs end in
large expansions due to the concave-lens effect of the capillary
depressions. In Gerris at any rate the two depressions formed
by the last pair of legs are much elongated, owing to the tibia as
well as the tarsal joints resting upon the water. In the case of
the first and second pairs of legs, only the tarsi rest upon the
water and as these are very short in the first and elongated in
the second pair, the capillary depressions are rounded and some-
what elongated accordingly. Apparently only the second pair
of legs is used for propulsion. As some of these insects are fairly
large creatures, measuring up to 3/4" or more in length, they illus-
trate in a very striking manner both the resistance which the
surface-film offers to the passage of a body from above and its
power to support a weight. These effects could not, however,
be secured with any certainty unless at least the parts of the legs
touching the surface were water-repellent, for otherwise they would
soon become wet and, instead of capillary depressions, capillary
elevations would be formed which would have the reverse of a
supporting action. In Gerris najas, the only species I have
examined under the microscope, the legs and indeed most of
the body are covered with a more or less dense coating of hairs,
most of which are decidedly curved or even curled at the tip.
No doubt these hairs are water-repellent and, therefore, act as
would a number of fine tubes of, say, paraffin wax, namely, to
prevent the water entering the open spaces between them to any
appreciable extent. Whether their curved shape has any signi-
ficance in this connection is not known, but it is at least very pos-
sible.
The animals which leap about on the surface are confined to a
few species of Collembola or Spring-tails. These are all small
wingless insects (aquatic species from 1.10" to 1.25") of very
primitive organisation, most of which are characterised by the
possession of a very remarkable saltatory appendage on the
abdomen. This, by being brought down suddenly upon any sub-
stratum on which the animals may be resting, causes them to
take leaps into the air almost comparable in magnitude with those
of a flea. Three species belonging to three different genera are