SOME OBSERVATIONS ON THE CLADOCERAN 365
length of mature females when recorded at the end of April and
during August were found to be 0840 mm and 0.856 mm respec-
tively.
Ephippial females, though smaller, were very similar to the
contemporary parthenogenetic females in their proportions.
All the individuals examined microscopically showed the
typical coloration pattern, with the ventral portions of the valves,
the head, tail spines and antennae darkly pigmented. As
Scapholeberis normally swims with the ventral side uppermost,
the animal appears black to the naked eye, and therefore less
obvious against the dark background of its natural habitat.
Some Aspects of the Life of Scapholeberis
During the day Scapholeberis spends much of its time suspended
from the surface film. It is maintained in this position by means
of slightly elongated setae, on the straight ventral edges of the
valves. As Scourfield (1894) pointed out the setae can be seen
actually to pierce the surface, and in this way the physical pro-
perties of the film are utilised.
Scapholeberis is capable of rapid progression whilst in the
suspended position. As with the majority of Cladocera, movement
is effected by means of the antennae, which beat simultaneously
in an antero-posterior plane, and do not themselves penetrate the
surface film. The animals frequently cease swimming movements,
but continue to remain in the suspended position. However, they
are able to interchange between surface and lower waters, and
indeed, do so frequently. If swimming movements are halted by
an animal not in contact with the surface, it does not float to the
top, but slowly sinks. Therefore, being slightly heavier than
water, individuals must swim, in an upward direction, in order
to regain contact with the surface film.
The speed at which Scapholeberis swims was investigated in
vitro. Several individuals were placed separately into petri dishes
and, using a stop-watch, the rate of antennal beat was ascertained.
The frequency, whilst the animal swims at the surface, was found
to be in the order of 0.9-1.6/second. Swimming is not, however,
sustained and takes place in bursts of, at most, thirty consecutive
strokes, with intermediate 'resting' phases. The speed attained
whilst in contact with the surface film was found to be 2-3 mm/
second. As might be expected, progress when not in contact with
the surface is faster, with no apparent increase in antennal effort.
The digestive tracts of individuals examined in May and June
were bright yellow-green, implying that food consisted mainly of
microscopic chlorophyll-containing organisms. Later, between
July and September, the guts appeared brown-black, the source
of colour being probably due to the animals having fed largely
upon the abundant detritus particles available at that time. Under
the low power of the microscope, particles could be seen to be
drawn in between the valves by the beating of the thoracic limbs,
and was more clearly demonstrated with particles of a yeast-