Butterflies of
Britain & Europe
Northern Brown Argus
Aricia artaxerxes
FABRICIUS, 1793
Family - LYCAENIDAE
subfamily -
POLYOMMATINAE
introduction
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habitats |
lifecycle |
adult behaviour
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Aricia artaxerxes, male,
Sutherland. Photo courtesy Tony Mainwood © 2008 |
Introduction
The
Northern Brown Argus, also known as the Mountain Argus or Scotch White-spot, is
distributed patchily across Europe, occurring as a montane species in the
Pyrenees, the Cantabrian mountains, Massif Central, the Alps, and the Balkans,
but is absent from the Tatra and Carpathian ranges.
It
also occurs at lower elevations in northern England, Scotland, and in
Scandinavia.
Unlike it's close relative the Brown Argus
Aricia agestis,
it is single brooded, emerging in June and July, at which time agestis
is "between broods".
In
Britain the ranges of the 2 species do not overlap, but in Europe they often
share the same habitats, and there is a degree of overlap in the flight periods.
It can also be confused with the dark females of several European
"blues", so a well illustrated field guide is essential to determine the
species, which each differ in the arrangement of the spots and lunules on the
underside wings.
The
populations in northern England are known as form salmacis.
They closely resemble
agestis
on both wing surfaces. Scottish specimens however have a prominent white spot in the
discal cell of the forewings, and the white spots on the undersides are devoid
of black centres.
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Aricia
artaxerxes, male, Sutherland. Photo courtesy Tony Mainwood © 2008 |
Habitats
This is primarily an
alpine and arctic species - a relic of the time when butterflies recolonised
Britain following the retreat of the last Ice Age about 12,000 years ago. As the
climate warmed, artaxerxes became restricted to
northern England and Scotland where temperatures were cooler, and at the same
time the Brown Argus Aricia agestis colonised the
warmer parts of Britain further south.
In Scotland it is found
in a variety of habitats including cliff tops, south-facing hillsides, open
areas within woodlands, and grassy embankments. It occurs at altitudes as high
as 450m.
In northern England form
salmacis is found at well drained grassland habitats, usually on limestone
pavements or hillsides where there are patches of scree or other small areas of bare ground.
The
butterflies are very sedentary in behaviour, with the result that all colonies
are highly localised, hence there is little interchange of genetic material
between adjacent populations. Despite this, most colonies currently appear to be
stable, although global warming is certainly a long term threat to the
butterfly's future in Britain.
Lifecycle
The butterflies emerge from mid June to early July depending on
location, and lay their eggs singly on the upper-side of leaves
of common rockrose
Helianthemum
chamaecistus
( whereas
agestis
oviposits on the underside ).
The young caterpillar feeds in a
similar manner to
agestis,
nibbling at the underside of the leaves, but leaving the cuticle
intact, creating a distinctive pattern of tiny holes visible from above.
It hibernates from September to
April, amongst withered or dead leaves at the base of the
foodplant
Helianthemum
chamaecistus.
When fully grown in May it is pale green, with a pale pinkish
stripe below the spiracles. The mature caterpillar feeds openly
on the upper surface of the leaves, and is attended by ants.
The
chrysalis is formed in late May on the ground, and is also attended by ants,
which cover it with soil and fragments of vegetation.
Adult behaviour
At it's habitats in northern
England this species nectars at bird's foot trefoil, thyme and
many other low growing flowers, but in Scotland I have often seen
it nectaring at
tall thistles.
The butterflies
are active only in warm sunshine, fluttering amongst grasses and
basking periodically on low herbage or patches of bare ground.
I have not
observed courtship or copulation in this species, but it is likely
to follow a similar pattern to that of agestis, in which
both sexes roam
freely, and copulate within a few seconds of meeting.
In dry weather
the adults roost overnight on grass heads, but during wet spells
they go deep into herbage, roosting on the underside of leaves.
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