Norwegian Journal of Entomology - Norsk entomologisk forening

Hågvar & Aakra: Spiders active on snow in Southern Norway
Females were generally more numerous than
males in the material of Huhta & Viramo
(1979). Males, however, predominated in their
material of Bolepthyphantes index, Tenuiphantes
cristatus, Macrargus carpenteri, Walckenaera
spp., Tmeticus affinis, and Centromerus spp. The
present material confirms this for B. index, and
also for the three next taxa. Most nets of B. index
referred to in the present study contained only one
female, indicating that it had been constructed by
a female.
Weather conditions
Also Huhta & Viramo (1979) found most spiders
on snow in mild weather, with highest activity
at temperatures several degrees above zero.
Their coldest observation was at –2.6 o C, while
we found spiders down to –7 o C. Several of our
samlings were made in foggy weather, and some
when snowing. The great majority of the spiders
were taken in calm weather with no wind, but
the sky could be either cloudy or sunny. The
most typical situation would thus be a windless
day with temperatures above zero and a wet
snow surface. At temperatures close to zero, a
Figure 3. On warm days during late winter
(April), juveniles and subadults of Pardosa sp.
are often active on the snow surface. The picture
shows an adult Pardosa amentata (Clerck,
1757). Drawing by Kjetil Aakra.
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cloud-covered sky might be an advantage. This
is because the cloud cover would ensure that the
temperature would not fall too rapidly during the
evening, so the animals would have time to retreat
to the subnivean space before being frozen. In late
winter, however, with mild nights, spiders might
use sunny days and stay on snow during night
without facing the danger of freezing.
Pitfall studies in snow-poor areas
confirm winter activity in spiders
On Askøy near Bergen, Aakra (1998) ran pitfall
traps throughout 14 months, including winter
trapping (31 October 1996 to 28 April 1997),
with periodic snow cover of 1-9 cm thickness.
The following four species in the present material
were clearly winter active as the majority, or a
large portion of the total material (both sexes),
were trapped during the winter period: Macrargus
rufus (in fact confined to the winter period),
Helophora insignis, Tenuiphantes alacris, and
T. cristatus. As virtually all, or the majority, of
males in these species (and even some additional
ones) were caught during winter, Aakra (1998)
concluded that they breed during the winter
period. Three additional species from the present
study were taken in winter pitfall traps in Iceland,
in hayfields and pastures with occasional snow
cover (Gudleifsson & Bjarnadottir 2004): Erigone
atra, Savignia frontata, and Tenuiphantes mengei.
Regular winter activity is clearly a normal feature
among several spiders and deserves to be more
closely studied.
Why active on snow?
Except for the catching of prey, feeding and
copulation of Bolepthyphantes index in nets,
the yearly activity of many spider species on
snow is difficult to understand. Huhta & Viramo
(1979) presented some theoretical explanations,
as escapement from water-logged soil during
thaw weather, or simply a continuation of normal
activity in spite of low temperatures. They
stressed that species which have their main period
of activity during winter could represent a special
adaptation to avoid competition with other species
by utilizing the resource offered by winter active
insects.