364
THE ESSEX NATURALIST
entirely to evaporation and subsequent precipitation as drainage
into the pond from the surrounding areas is limited, and seepage
from the bottom is negligible. The pond is well shaded by trees
and nearby buildings, and direct sunlight can reach the surface
only briefly during the day. The highest temperature recorded
while animals were active, was 23°C on August 1, 1964, and the
coolest, during April the same year, was 4°C but as temperatures
were measured infrequently, these do not necessarily represent
the actual extremes experienced. pH varied between 7.4 and 7.9.
The pond supports a poor flora of vascular plants, and could
hardly be described as "weedy". The microflora included few
easily recognised algae and was assumed to comprise largely nanno-
planktonic organisms. The variety of animals present was also
poor for most of 1964. Besides Scapholeberis there were many
culicid larvae, a few chironomids and a thriving population of the
snail Limnaea pereger. No fish were present, but a newt was seen
on one occasion. It was evidently a habitat which suited
Scapholeberis; the water surface was placid and well shaded, and
the environment was effectively devoid of predators.
However, the population did collapse in September 1964, and
although the actual reasons for its demise are unknown, it is
possible to speculate on these. The eventual disappearance was
preceded by the advent of ephippial females (and assumedly,
therefore, males, although the latter were never seen), a fact
implying the onset of adverse conditions. There had been
no significant change in the weather during that period,
and it would seem unlikely that this was the onset of the normal
overwintering phase. It is conceivable that some rapidly repro-
ducing pathogenic organism was the cause, or even that it was
induced by exhaustion of available food, but both explanations
seem unlikely. Clearly, however, some environmental change had
taken place. The event was preceded by the apparance of a
population of Daphnia magna which, until then, had not previously
been seen in the pond. This organism established itself, and the
population flourished until December. It is possible that Daphnia
magna was better adapted to existing environmental conditions,
and ousted the Scapholeberis.
In 1965 Scapholeberis individuals had begun to reappear by the
third week of April, in company with Daphnia magna and a
population of cyclopoids, the latter having been in evidence for
several weeks previously. Numerically Scapholeberis did not
increase to anything but a fraction of it former status, although
small numbers persisted until July.
General Morphology
Linear measurements were determined by means of a calibrated
micrometer eyepiece, length being measured from the tip of the
head spine to the tips of the posterior spines of the carapace. The
largest individuals were parthenogenetic females, and the maxi-
mum length recorded was 0.95 mm, during June 1964. Average