368 THE ESSEX NATURALIST
or the restriction upon free movement to lower levels in the pond
proving fatal.
One of the most interesting facts to emerge during the period
of study on this population concerns vertical movements. The
phenomenon of circadian rhythms of migration of certain
planktonic organisms have been established for some time, and
are becoming increasingly well-documented; Scapholeberis, too
apparently undertakes characteristic movements. Of the factors
which may influence movements, light is probably the most im-
portant, and indeed, Scapholeberis exhibits a strong positive
phototaxis. When a beam of light from an electric torch was
shone into the water at night, large numbers of Scapholeberis
were seen to enter the beam, and subsequently to tend to remain
there. It is significant that the animals appeared from low water
levels and not, as might have been expected, from the adjacent
surface waters, although there was a distinct tendency for animals
to come to the surface within the light beam.
At a later date, samples were collected from close to the bottom
and from the surface, over a period of 24 hours, and the results
are set out in Table 2. Samples were taken at the time mentioned,
by means of a 250 ml conical flask. For surface samples, the flask
was immersed just under the surface, and for the deep water, a
suitable rubber bung was placed in the neck of the flask, which was
then held on the bottom, and the bung removed. Each sample
was filtered, and the animals were preserved for counting. The
method, though simple, was adequate.
The figures show that the majority of individuals are at, or
close to the surface in the daytime, but during the hours of darkness
are distributed, probably widely, in the lower levels of the pond.
An attempt was made to demonstrate the different relationships
with the surface film, under conditions of light and dark. Ten
individuals were placed in each of two glass dishes containing pond
water to a depth of 5 cm. One was exposed to daylight, the other
was covered with a black cloth. The dishes were examined after
intervals of one hour. In daylight, nine or ten of the animals
were to be seen in contact with the surface, while in the darkened
dish between four and eight only were to be seen at the surface
at any one time. Although far from conclusive, these demonstra-
tions suggest more random movement in darkness than in daylight.
This would appear to be supported by the fact that, as Table 2
shows, there is a sharper difference between numbers near the
surface and near the bottom in daylight than there is in darkness.