Norwegian Journal of Entomology - Norsk entomologisk forening

Hågvar & Aakra: Spiders active on snow in Southern Norway
regarding certain dominant species. The most
numerous species recorded by Huhta & Viramo
(1979), Macrargus rufus, was represented by only
one individual in the present material. Tmeticus
affinis (Blackwall, 1855) was abundant in the study
of Huhta & Viramo (1979) and Gnaphosa sticta
Kulczyn’ski, 1908 in the material of Koponen
(1989), but these were absent in the present
material. In fact, among the fourteen species in
the latter study, collected far North at Kevo,
only Bolepthyphantes index, Zornella cultrigera
and Cryphoeca silvicola were taken in Southern
Norway. This probably mirrors the local character
of the spider fauna, and for instance Gnaphosa
intermedia is a Northern species restricted to
Lapland (Koponen 1976).
Is the snow surface colonized from
below or from above?
Theoretically, spiders may colonize the snow
surface either from the subnivean air space or
from trees and other high vegetation. Huhta &
Viramo (1979) found that the majority of the
species in their material live permanently on the
ground or in low field vegetation, so they must
have colonized from below. The presence of some
tree-living species was explained by involuntary
falling, but this could not be the main reason for
species encountered regularly on snow.
At Kevo, Koponen (1976, 1989) compared the
spider fauna on snow with pitfall catches below
the snow. He concluded that most spider species
found on the snow were abundant also in the
subnivean pitfall traps, so the snow surface
activity was an extension of the normal winter
activity below the snow. However, many species
active below the snow did not appear on the
snow surface. In Norway, Waaler (1972) caught
Macrargus rufus and Tenuiphantes cristatus in the
subnivean space. The ecological meaning of the
subnivean spider activity is poorly understood. In
Canada, Aitchison (1989) has suggested that active
subnivean spiders may represent an important
food source for overwintering shrews.
Also in the present material, most species are
typically living on the ground or in the field
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vegetation, and must have colonized the snow
surface from below (see Huhta & Viramo (1979)
for habitat preferences). Even at great snow
depth, a small air space is usually created along
penetrating stems and bushes, so winter active
invertebrates have a certain possibility to migrate
between the snow surface and the subnivean
environment.
A study of the invertebrate fauna on branches
of spruce during five winter months near Oslo
by Hågvar & Hågvar (1975) gives an idea of
spider species that might have colonized the
snow surface by dropping down. Among 701
spiders from 15 taxa on spruce branches, only
4 taxa were recorded on snow: Philodromus sp.
(dominant group on branches, representing 33-
45 % of these spiders in different months),
Dictyna sp. (4-15 %), Pityohyphantes phrygianus
(0-10 %), and Araniella cucurbitina (0-3 %).
Tree-living individuals of these taxa were all
juveniles. The dominance of Philodromus sp. on
spruce branches may explain the relatively high
numbers of subadults on snow in April, especially
in warm weather. The observation of “flying”
individuals and many specimens on snow, even
in areas without penetrating vegetation, indicates
a mass exodus from trees during the first, warm
days in late winter. Huhta & Viramo (1979) and
Koponen (1989) also reported Philodromus sp.
from the snow surface in Northern Finland. We
assume that the limited number of the three other
taxa on the snow surface was due to accidental
falling from trees.
Our conclusion is therefore that the majority of
snow surface active spiders in Southern Norway
colonize from the subnivean air space, a few
species drop in small numbers involuntarily
from trees and subadults of Philodromus sp. may
colonize the snow surface in late winter due to a
mass exodus from trees during warm weather and
suitable wind conditions.
Remarks to some species
In Norway, Østbye (1966) found a specimen of
Centromerus incillium (L. Koch, 1881), which had
constructed a net in a crevice in the snow surface