Butterflies of
Britain & Europe
Chalkhill Blue
Lysandra coridon
PODA, 1761
Family - LYCAENIDAE
subfamily -
POLYOMMATINAE
introduction
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habitats |
lifecycle |
adult behaviour
Chalkhill Blue
Lysandra coridon, male, Magdalen
Hill Down, Hampshire
Introduction
A
freshly emerged male Chalkhill Blue is a particularly beautiful insect, with
shimmering silvery blue wings, and long hair-like scales adorning the body,
giving it a distinctly furry appearance. The wings are highly reflective so the
butterfly appears to be almost silver when seen in bright sunshine.
The female can be confused with the female Adonis Blue L. bellargus,
but the upperside of the latter has a deeper chocolate ground colour.
It also has blue edging to the black sub-marginal spots on the upperside, as
compared to whitish edging in coridon.
The
emergence of the 2nd brood of
bellargus overlaps slightly with
coridon
during August, and both species often share the same habitats.
The Chalkhill
Blue
is distributed across much of Europe, but absent from northern Britain, Ireland,
northern Scandinavia, Portugal, southern Spain and most of the Mediterranean
islands. It's range extends into western Asia as far as
the Ural mountains.
In
Europe there are several species which closely resemble both sexes of the
Chalkhill Blue, and it is essential to consult a well illustrated field guide to
differentiate between them.
Habitats
The
butterfly breeds on chalk and limestone hills, favouring south facing slopes
where the larval foodplant horseshoe vetch grows profusely amongst fine grasses.
The greatest numbers occur on sites that are grazed by sheep, cattle or
rabbits to produce a sward height of about 5 cms. At the best sites the
butterfly can occur in thousands, but it can also survive at quite low densities at sites where grazing has been abandoned and the grasses grow tall. At
such sites they breed in areas where the foodplant grows at the edge
of paths, on patches of scree, or abutting rabbit scrapes.
Colonies are very localised, but not
to the same degree as those of the Adonis Blue. Thus
coridon may be found spread widely across it's sites, whereas
bellargus is usually restricted to tiny strips of
land where the microhabitat is particularly warm.
Stray males are found irregularly up to 8kms from the breeding sites, sometimes in quite
unsuitable habitats such as woodland clearings. Female strays are almost
unknown, and it is extremely unusual for the butterfly to colonise areas away
from their regular breeding sites.
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Chalkhill
Blue
Lysandra coridon, male, Stockbridge Down, Hampshire |
Lifecycle
This
species is single brooded, with a protracted emergence beginning in early July
and continuing into late August, with the last individuals often seen well into
September.
The reticulated, dome-shaped off-white eggs are laid singly on stems of
horseshoe vetch Hippocrepis
comosa or on nearby grass stems in August. During the winter they are
washed off by the rains, and lie on the surface until the spring. The larvae are
fully formed within the eggs in the autumn, but do not hatch until the following
March or April.
The
plump green larva is marked with broken yellow lines along it's back and sides.
It rests beneath stones or chalk fragments during the daytime, and feeds
nocturnally. It can be found easily at dusk, when large numbers of
ants are in constant attendance.
Ants
sometimes carry a larva, placing it closer to their nest, milking it to obtain the sugary secretion
exuded from it's "honey glands". In return for their reward, the ants protect
the larva against wasps and bugs.
The
pupa is a dirty greenish colour, and is formed on the surface of the ground.
Like the larva, it exudes secretions to attract ants, which hide it by
covering it with tiny fragments of earth, and guard it against attacks by other
insects. The gaps between the abdominal segments of the pupa are armed with
opposing sets of microscopic serrations which can be rubbed together to create a
rasping sound. Just prior to emergence the butterfly within the pupa "sings" to
attract ants, which cluster around it, inadvertently protecting it from
predatory insects and arachnids. The ants never attack the emerging butterfly,
seemingly appeased either by the song, or possibly by a pheromone.
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Chalkhill
Blue
Lysandra coridon, male, Old Winchester Hill NNR,
Hampshire |
Adult behaviour
The butterfly, when it emerges in
July, has to break through the soil and find it's way to a stem
from which it can hang to dry it's wings. The adults often emerge
en masse, and
can sometimes be found basking in hundreds on low herbage early in
the mornings.
Favourite nectar sources include
marjoram, stemless thistle, carline thistle, knapweeds, wild
basil, self heal and thyme, although they will also visit bramble,
ragwort, yarrow and hemp agrimony.
They also occasionally gather in small groups on mammal dung.
In the French Alps I
have often seen males congregate in
thousands to drink at patches of mud or urine-soaked soil. Often
there are several other species present at these gatherings
including Essex Skippers, Large Grizzled Skippers and Heath
Fritillaries, but each species polarises towards its own kind,
thus the Chalkhill Blues will cram very tightly together in one
spot, and the Essex Skippers etc will form their own equally
tightly packed groups just a few centimetres away.
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Lysandra coridon, male drying wings after emergence, Stockbridge Down,
Hampshire |
In southern Britain
one of the commonest predators of butterflies is the spider
Enoplognatha ovata, a member of the
family Theridiidae. This small species traps summer butterflies
which fly into the sticky strands of an untidy web which it spins on
grass-heads and wild flowers. I made a brief study of predation at
Magdalen Hill Down in Hampshire in mid-July 2009 and estimated that
a minimum of 5 percent of the population of Chalkhill Blues fell
victim to this spider.
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Theridiid
spider Enoplognatha ovata,
devouring Chalkhill Blue Lysandra
coridon |
As dusk approaches the butterflies
migrate to the base of hills, where they roost over-night on tall
grasses, often with several males sharing a single grass head, all
resting in a head-downwards position.
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Chalkhill Blue
Lysandra coridon, female, Stockbridge
Down, Hampshire |
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