NORSK ENTOMOLOGISK TIDSSKRIFT - Norsk entomologisk forening

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lunatum pupated and the fact that the imago
did not emerge and that the larva of Patrobus
septentrionis and several larvae of Bembidion
argenteolum died after some time may well
have other causes than submergence, since a
high death rate for larvae (especially first
stage) and pupae is observed in captivity
under normal conditions,
DISCUSSION
The insect fauna at the spring flood should
be poor quantitatively as well as qualitatively,
only comprised of the imaginal hibernators
surviving the winter (Wiren 1954), However,
many Bembidion spp, on the Gaula were
abundant prior to the spring floods in late
May 1965, in mid-May 1967 and on MiUselva
in 1967 at the end of May (Andersen in prep.).
Other typical bank-living beetles (Dyschirius
spp., Bledius spp., Tachyusa spp., Stenus spp.,
and Atheta spp.) were present, too. Thus on
the Gaula, Malselva, and other rivers of
similar type, the adults of many or most
beetles are exposed to the spring floods. The
number of dead insects should greatly surpass
the number of living ones in the cold water
during spring floods (Wiren 1954). The author
states that the insects are in a cold stupor
with little ability to save themselves. Only in
high air temperatures should some arthropods
survive, by slowly climbing objects floating
upon the river. A large death-rate during
floods is also supposed by Joy (1910). This is
not supported by the present studies on the
Gaula and Malselva, as the imaginal populations
of most of the Bembidion species
seemed intact even after the long spring
flood period on the Gaula in 1965 (Andersen
in prep.). The same also seemed to be the case
for most other arthropods confined to river
banks. The methods making it possible for
Coleoptera to survive during inundations are
discussed in the following.
Generally, the obligatory shore- and bankliving
species do not seem to differ from
other ground-living beetles, either in ability
THE EFFECT OF INUNDATION 127
to survive on a calm water surface, or in
swimming capacity. Thus Agonum assimile
survived at least as long as Bledius sp. and
any Bembidion spp. The first is not restricted
to shores and banks, whereas this is the case
for the species of the two latter genera used
in the experiment. Furthermore Bembidion
schueppeli, confined to shores and banks, lived
insignificantly longer than Pterostichus niger,
living in woodland and fields.
Joy (1910) studied the locomotion method
of various Coleoptera. Dianous coerulescens
Gyll., Stenus spp., and Anchomenus albipes
Fabr. (= Agonum ruficorne Gze.) are able
to 'skim' rapidly over the water surface by
reducing the tension of the surface membrane
behind them, using a substance secreted from
the abdomen. Otherwise Coleoptera are swimmers,
but some rather poor (mostly Staphylinidae).
According to the same author (Joy
1910), small species are much affected by
capillary attraction. However, it seems obvious
that the morphology, above all else, determines
the swimming capacity of the species. Species
with a plump body and short legs, for example
Chrysomelid, Curculionid, and Scarabaeid
beetles, or with a very long body and short
legs as several Staphylinidae, for instance
Bledius spp., are at best very poor swimmers,
in contrast to the typical runners with an
elongate body and long legs, e.g. many Carabidae
and Staphylinidae. Species of both
these groups are richly represented among
the fauna on shores and banks, as well as in
other habitats.
According to Palm & Lindroth (1936), Wiren
(1954) and Lehmann (1965), shore-living species
are able to orientate straight to the
nearest bank. In a way this is evident
also from the present experiments, and even
for those without any connection to water.
However, when the observer is close to the
animals they orientate without regard to the
bank, towards the observer. Further studies
are needed to analyse the orientation mechanism,
but the present observations indicate that
the species studied, as with Stenus spp.