Butterflies of
Britain & Europe
Scotch Argus
Erebia aethiops
ESPER, 1777
Family - NYMPHALIDAE
subfamily -
SATYRINAE
Tribe - SATYRINI
subtribe - EREBIINA
introduction
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habitats |
lifecycle |
adult behaviour
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Erebia
aethiops, male, Arnside Knott, Cumbria |
Introduction
The
Scotch Argus is widely distributed in central and eastern Europe, but is
absent from Ireland, Spain, Portugal, western France, peninsular Italy,
Scandinavia and the Mediterranean region. Its range extends to northern Turkey
and across temperate Asia to western Siberia.
The sexes are
virtually identical on the upperside - males have a large patch of dark
androconial scales on the forewings, but this can only be seen under certain
lighting conditions. The underside hindwings of the male are dark reddish brown,
with a broad grey post median band. In females the ground colour beneath is a
paler earthy brown, banded with olive.
In mainland Europe there are several other
Erebia species which closely resemble
aethiops. These include
ligea, euryale, medusa
and triaria.
The various species can be distinguished by examining the number and
configuration of the ocelli. The patterns on the undersides also differ. A good field
guide is essential for determination.
There are no similar species in Britain,
although aethiops
could be confused in flight with the Ringlet
Aphantopus hyperantus.
The Arran Brown Erebia ligea is similar to
aethiops but has
prominent white markings on the underside hindwings. It was allegedly found in
Scotland in the 19th century, but all records are regarded as highly dubious -
unfortunately early entomologists often made false claims about the butterflies
they collected, habitually importing European specimens and claiming they had
captured them in Britain.
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Erebia
aethiops, male, Arnside Knott, Cumbria |
Habitats
The
Scotch Argus forms large colonies often numbering dozens, hundreds or
sometimes thousands of individuals in the best habitats.
In Britain it
is confined almost exclusively to Scotland. In England it formerly occurred at
several sites in Cumbria, Yorkshire, Northumberland and County Durham; and
probably also in Lancashire, but
is now only found only at 2 sites - Arnside Knott in Cumbria, and Smardale Gill
in Yorkshire.
Unlike most members of
the genus Erebia,
the Scotch Argus is not a montane species. It occurs at elevations below 500m,
in warm, sheltered and damp situations where the grasses Molinea caerulea
or Sesleria
caerulea
grow in tall, lush, dense tussocks.
In Scotland the butterfly is
commonly found in valley bottoms, and at damp grassland / woodland mosaic
habitats, particularly those along riverbanks, or around the edges of lochs.
Colonies also occur on warmer, more sheltered areas of lightly wooded moorland, on scrubby
damp heaths and hillsides on the islands of the Inner Hebrides, as well as on
the mainland.
The
2 sites in northern England are very different in nature - at Arnside Knott it occurs
on limestone where rough grassland is sheltered by woodland; and at Smardale
Gill it
is abundant along a disused railway line running through a limestone grassland / woodland
mosaic.
Lifecycle
The butterflies emerge in late July and fly throughout August.
The eggs
are attached singly to grass blades and stems, at the base of
tussocks of purple moor grass Molinea caerulea
in Scotland, or blue moor grass Sesleria caerulea
in
Cumbria and Yorkshire. A much wider range of grasses is used
in mainland Europe including
Bromus,
Brachypodium,
Festuca,
Poa,
and
Dactylis.
At some sites in Europe eggs are laid on the sedge
Carex sempervirens.
The eggs are spherical, finely
ridged, and straw coloured. After a few days they develop a
pattern of dark purplish speckles, finally turning grey just
before the larva hatches.
The
caterpillars hatch
in late August, nibbling a circular groove around the upper egg
shell, creating a "lid" through which they exit. They then eat
the remaining shell, which contains vital nutrients, and having
finished their meal rest for a while before starting to eat
tender young grass blades.
They
only feed for a few weeks before entering into hibernation at
the base of grass tussocks where they remain until April when
they awaken to resume feeding. When fully grown they
are dull umber in colour, with dark green stripes and speckles along the back
and sides. Like most Satyrine larvae they are crepuscular by
nature, feeding in the early evening and again at dawn.
The fully grown larva leaves the
foodplant in mid June, and wanders a short distance to pupate in a small
silk-lined cavity excavated amongst mosses.
The pupa is pale brown, with the thorax, wing cases and eye
covers pale purplish in colour.
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Erebia
aethiops, male, Arnside Knott, Cumbria |
Adult behaviour
The
butterflies dislike hot weather, and at such times sit motionless
on bushes or amongst grasses, with their wings firmly closed. They are most active in warm
sunny conditions when the males can be seen gently bobbing about
just above the tall grasses, fluttering and weaving tirelessly in
search of potential mates. If there is a slight drop in
temperature, or cloud temporarily obscures the sun, they settle to
bask on bracken, bramble leaves or other low foliage; but quickly
disappear deep into grass tussocks if it becomes too cool, or if
there is any threat of rain.
Copulation takes place in late morning amongst grasses, and lasts
for about an hour. Females are quiescent until mated, after which
they can be seen flying in and out amongst grass tussocks in hot
sunshine to oviposit.
Both sexes nectar
avidly at almost any available flowers including thistles, knapweeds, bramble,
hawkweeds, heather and meadowsweet; fluttering from flower to
flower, and usually holding the wings half-open while feeding.
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