Butterflies of
Britain & Europe
Silver-studded Blue
Plebejus argus
LINNAEUS, 1758
Family - LYCAENIDAE
subfamily -
POLYOMMATINAE
introduction
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habitats |
lifecycle |
adult behaviour
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Silver-studded Blue
Plebejus argus, male, Hawkhill, New Forest,
Hampshire, England |
Introduction
The
Silver-studded Blue gets its English name from the row of
silvery-blue submarginal "studs" on the underside hindwings. These vary in size
between individuals and from site to site, and in some specimens can be entirely
absent. The butterfly was formerly known as the Lead Argus - a reference to the
dull steely blue colour of the male's forewings, which are quite unlike the more
vivid hues of Common Blues and Adonis Blues.
The butterfly
is widely distributed
across Europe
but absent from central Spain, southern Portugal, Scotland, Ireland and northern
Scandinavia. Beyond Europe it's range extends across temperate Asia
to northern China and Japan.
In Britain it cannot be confused with any other species, but elsewhere within
it's range it can very easily be mistaken for the Idas Blue Plebejus idas.
The 2 species can only reliably be distinguished by examining the tibia on the
foreleg of the male - in argus
this is spined, in idas
it is not.
The isolated population at Great
Ormes Head, an island off the north coast of Wales, differs in many ways from
the typical form, and were once considered to be a distinct subspecies
caernensis. The butterflies are noticeably smaller
than the normal form, females have extensive blue scaling, and the adults emerge
in late May - about 3 weeks earlier than other populations.
Other closely related and very similar species occurring in Europe include
Reverdin's Blue Plebejus
argyrognomon, and
the Zephyr Blue P. pylaon.
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| Silver-studded Blue
Plebejus argus, male, Prees
Heath, Shropshire, England |
Habitats
Unlike other British
blues, this is primarily a heathland butterfly. Although some populations can be enormous,
comprising of hundreds or even thousands
of butterflies, this is a very localised species,
which rarely flies more than a few metres from its emergence site.
It breeds on heaths that are damp but not boggy, primarily in Hampshire, Dorset
and Surrey; strongly favouring sheltered areas that have regenerated following
burning, clearing, or heavy cattle grazing.
Colonies also exist on
coastal dunes in Cornwall, south Devon and the Gower peninsula of south Wales.
Populations which once existed on chalk grasslands have all become extinct, but
there are still large populations on limestone cliffs at Great Ormes Head (an
island off the coast of north Wales ),
and in the nearby Dulas valley where it was artificially introduced. Small populations
also occur in limestone quarries at Portland, Dorset.
Lifecycle
In
warmer parts of Europe the butterfly is double brooded, emerging in May and
August, but in Britain there is only a single generation, which emerges in late
June.
At
most heathland sites the eggs are laid singly in July at the base of young shoots of
cross-leaved heath
Erica tetralix,
heather
Calluna vulgaris, bell heather
Erica cinerea or gorse Ulex
europaeus. On the Suffolk Brecklands however eggs are often laid on the
underside of bracken fronds - these possess nectaries whose sole function seems
to be to attract ants. Although unproven, it is possible that the butterflies
have evolved this egg-laying strategy so that the eggs gain protection from
being eaten or attacked by parasitoids, by virtue of the presence of the ants.
On limestone sites the
eggs are laid
on the stems of bird's foot trefoil
Lotus
corniculatus
or rockrose
Helianthemum chamaecistus, very
close to the base of the plants.
In
all cases the eggs are always laid in proximity to nests of the black ant
Lasius niger.
The tiny larva develops
within the egg in the late summer, but doesn't hatch until the following March.
It feeds by day on the flowers and tender leaf tips of the foodplants, and is constantly attended by
ants. The ants milk "honey" that the caterpillar produces from an eversible
gland. In return for their meal they protect the larva from wasps, spiders
and carnivorous bugs.
When ready to pupate, the larva is driven or carried into
the ant's nests. The pupa is attended by the ants until the butterfly is ready
to emerge in late June or early July, at which time it crawls out of the nest
and makes it's way up a stem where it settles to expand and dry it's wings.
The
butterfly / ant relationship is not a true example of symbiosis - it has been
demonstrated that
captive larvae which are prevented from having contact with ants invariably die;
but the ants are perfectly capable of surviving without the butterfly.
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Silver-studded Blue
Plebejus argus, male, Ocknell,
New Forest, Hampshire, England |
Adult behaviour
The
steely blue males are easily seen,
as they
flutter incessantly over heather or grasses in search of females, stopping occasionally to
nectar at bell heather, bird's foot trefoil or rockrose. Females
are much harder to find, as they are far more sedentary and duller
in colour, being earthy brown with an indistinct series of orange
sub-marginal lunules.
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Plebejus argus, male at roost
on cross-leaved heath, Shropshire, England |
When the sexes meet, copulation
takes place almost immediately, with no observable pre-nuptial
ritual. Mated pairs can sometimes be found basking with wings in
the characteristic three-quarters open position. They remain in
cop for about an hour.
Overnight or in overcast
weather conditions the butterflies roost
on cross-leaved heath, heather, or less commonly amongst grasses
or on bushes; adopting the typical head-downward posture shown in
the above photograph.
On sunny days the butterflies are active until
sunset, and at certain sites can sometimes be found basking in
groups of 30 or 40, congregating on bushes or heather clumps to
soak up the last remnants of sunlight before going to roost or the
night. In the morning these same groups bask communally for about
half an hour prior to taking flight.
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