Ecology and reproductive behaviour of damselflies.
This thesis is concerned with intraspecific competition among damselfly larvae, and on
the behaviour and ecology of adults. Pyrrhosoma nymphula (Sulzer) was the subject
of the work on adult emergence and behaviour at two ponds in Cheshire, while larvae
of Ischnura elegans (van der Linden) were used in laboratory competition studies.
An asymmetry was found in the way interference competition acts when small
and large larvae of I. elegans were contained together with unlimited prey. Small
larvae suffered significantly reduced development rates and size increases at the moult
in the presence of large larvae, but similar reductions were not apparent when small
larvae were combined with like-instar larvae, or when large larvae were combined with
small larvae. In field populations, larvae which have been adversely ..rfected by
competition for food or fishing sites ".", be smaller and emerge later than
competitively successful larvae. Adult size and seasonal occurrence may have a strong
influence on both male and female reproductive success.
The emergence of P. nymphula was studied at two adjacent ponds. The
emergence from both ponds appeared less well synchronised than might have been
expected. There is likely to be a trade-off between the need to emerge around the same
time as conspecifics in order to achieve a full rep.roRuctive potential, and the
expression of temporal variation arising in part frony~petition. Newly emerged
adults from one pond were significantly larger and emerged earlier than those from the
other pond. This may have been due to differences in prey availability and
distribution, and to temperature differences. A decline in size of newly emerged adults
through the season was found at both ponds.
During periods of bad weather, numbers of individuals emerging were
substantially reduced. Those individuals still attempting emergence presumably did so
because of the progression of metamorphosis. Quantification of emergence mortality
attributed to different factors was attempted. Mortality was found to be highest after a
sudden deterioration in the weather. Mean daily mortality, expressed as a percentage
of the total daily emergence, was calculated to be 27.93%. There was no evidence of
density-dependent mortality at emergence.
Recovery of individuals marked at emergence and when mature was found to be
low. For mature adults, this was attributed predominantly to dispersal to nearby water
bodies. There was some indication that there was a higher mortality of smaller adults
in the immature and reproductive stages of the life-history. Fewer females revisited
the ponds than males which led to the estimation of female survivorship being
considerably lower than that of males. A large increase in weight of females from
emergence to maturity suggests that they may amass clutches of eggs over several days
and thus not visit the ponds on every day of their reproductive life. In addition,
mortality of females may be higher than that of males as a result of predation during
For male P. nymphula, size was shown to have no effect on the ~:)Utcomeof
territorial disputes.which were settled by a simple residency asymmetry. SIze was also
found to be unrelated to the total number of observed matings a male achieved, and the
number of days a male spent at the ponds, although the latter may have been obscured
by the low return of marked males. The relation of size to longevity could not be
studied. Success in mating and in territorial disputes was found to be positively
related for resident males. This was probably due to the amount of time spent at the
ponds within a day.
Late emerging female P. nymphula visiting the ponds later in the season laid
smaller clutches of smaller eggs than those arriving early in the season, irrespective of
size. Clutch size is likely to be strongly related to the inter-clutch interval and thus the
pattern of sunny and cloudy days through the season.