Butterflies of Britain
Family - LYCAENIDAE
Plebejus argus, male, Ocknell, New
Forest, Hampshire ©
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 once 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.
widely distributed across Europe but is absent from central Spain,
southern Portugal, Scotland, Ireland and northern Sweden. Beyond
Europe its range extends across temperate Asia
to northern China and Japan.
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.
In Britain argus
can't be confused with any other species, but elsewhere within its
range it can easily be mistaken for the near identical Idas Blue
Plebejus idas. The two species can only
reliably be told apart by examining the tibia on the foreleg of the
male - in argus this is spined, in
idas it is not.
Other closely related and very similar species
occurring in Europe include Reverdin's Blue
Plebejus argyrognomon, and the Zephyr Blue
Plebejus argus, male, Prees Heath,
Unlike other British blues, this is primarily a heathland butterfly.
Although some populations can be enormous, comprising of hundreds or
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
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,
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.
limestone sites the eggs are laid on the stems of bird's foot
or rockrose Helianthemum chamaecistus,
very close to the base of the plants. The eggs are always
laid close to nests of the 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. The larva is
constantly attended by the ants which milk it to obtain a sugary
substance exuded from an eversible gland on its back. In return the
larva gains protection because the presence of ants deters predatory
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.
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.
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.
male at roost on cross-leaved heath, Prees Heath, Shropshire ©
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
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.