SOME OBSERVATIONS ON THE CLADOCERAN 367
carmine suspension. According to Ruttner (1962), Scapholeberis
feeds upon neustic plants and on detritus floating on the water
surface, but this may not be exclusively so. Rather, the diet is
probably supplemented by similar foods in suspension in the
water.
Particular attention was paid to changes in physical appear-
ance of individuals throughout the period of study. Generally the
head spine was longest during May and early June—progressively
shortening through to September. The posterior spines of the
carapace were also subject to variation in length, reaching their
maximum in August. Figure 1 shows selected parthenogenetic
females sampled on the dates shown.
Green (1963) has studied a population of Scapholeberis in
Hampton Court Long Water and published figures revealing
similar variations. He proposes that the changes in the length of
the head spine corresponds with food availability, while the tail
spines vary with temperature.
Ephippal females, although smaller, were similar in proportions
to the contemporary parthenogenetic females. I am unable to
report upon males, but Green says that, when they appeared,
they possessed only very short head spines, but that the tail spine
was relatively long.
Parthenogenetic females of Scapholeberis are said to carry up
to sixteen developing embryos, but in this habitat females were
seen only occasionally to carry even five parthenogenetic eggs and
then only in April and May. The numbers carrying four also
diminished during the summer and, in general, the percentage of
females actually carrying eggs at all tended to diminish, though
as Table 1 shows, there was a significant increase in the number
of females carrying parthenogenetic eggs in the August collection.
It was intended also to estimate the life span of partheno-
genetic females by isolating egg bearing females in a fine muslin
net so that the organisms were allowed access to surface, and
presumably would be subject to the same environmental changes
experienced by the rest of the population. Three such attempts
resulted in the early death of the isolated animals. This may
have been due to the mesh excluding too much potential food,