Butterflies of
Britain & Europe
Common Blue
Polyommatus icarus
ROTTEMBURG, 1775
Family - LYCAENIDAE
subfamily -
POLYOMMATINAE
introduction
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habitats |
lifecycle |
adult behaviour
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Common Blue
Polyommatus icarus, male,
Ballard Down, Dorset, England |
Introduction
Despite it's name this species can no longer be considered a
common butterfly. It still remains the most widespread "blue" in
Britain, but many colonies in marginal habitats such as woodland
rides and farmland have declined or been lost. The species still
occurs in moderate numbers on chalk or limestone
grasslands, but even in these habitats most colonies nowadays comprise of no more than a few
dozen individuals.
Males
are very consistent in appearance, the uppersides being
violet-blue with plain white fringes. Females vary
considerably - they always have orange submarginal lunules, but some
are almost devoid of blue and strongly resemble the Brown Argus,
while others are heavily dusted with blue scales. The magnificent
Scottish race is known as marsicolore
- its upperside is almost entirely deep violet blue, and the
orange lunules are much enlarged.
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Common Blue
Polyommatus icarus, female, Ballard
Down, Dorset, England |
The undersides of
both sexes of Common Blue are marked with a pattern of
white-ringed black spots and orange crescents. Sometimes aberrant
forms can be found in which the black spots are elongated into a
series of short bars. Other rare forms occur in which the spots
are reduced in size, or entirely absent. In all forms the male
has a greyish ground colour with bluish scales around the base of
the wings. Females instead have greenish scales at the wing bases, and a
pale brown ground colour.
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All butterflies in the subfamily Polyommatinae have
undersides patterned with black spots, including several
European species in the genera
Plebejus,
Polyommatus and
Agrodiaetus.
Several of these can be confused with the Common Blue. The
number and arrangement of the spots is different in each
species however, so close examination of the markings and
the use of a well
illustrated field guide will enable the various species to be
differentiated. |
The Common Blue is
found throughout Europe, from the extreme north of Scandinavia to
the smallest islands of the Mediterranean. Beyond Europe, it's
range extends from the Middle East across temperate Asia to
northern China. It also occurs in north Africa and the Canary Islands.
Habitats
The butterfly
is found
throughout England, Scotland and Wales at sites where bird's
foot trefoil grows in profusion.
It
is most abundant on chalk or limestone grassland but also occurs in lesser
numbers in woodland
clearings,
meadows, heathlands, sand dunes, along
railway
embankments,
riverbanks and undercliffs.
Numbers
are usually highest on south facing
hillsides, but populations at these sites are prone to crash in hot dry summers,
resulting in poor numbers the following spring.
In
Europe the Common Blue occurs in almost all habitats - I have found it on mountains at altitudes up to 2700m, and in numerous
other habitats including arid scrubland,
glades in pinewoods, and on freshwater marshland.
Lifecycle
In southern Britain there are usually 2 generations per year. The
first brood
emerges
in May
and flies until mid June. The second
brood
emerges
in late July
or early August
and
remains on the wing until mid September or sometimes into early
October. There may be a partial third brood at certain particularly warm sites.
In the north of Britain there is often just a single brood, but this depends
very much upon locality and weather conditions.
The
circular, flattened white eggs are usually laid on the upper surface of terminal leaves of bird's foot trefoil Lotus corniculatus,
but greater bird's foot trefoil Lotus uliginosus,
restharrow Ononis repens, black medick Medicago lupulina and other leguminous herbs
are also used.
The
larvae are pale green in colour, and feed diurnally. Like most Lycaenid species
they are often attended by ants, which milk them for sugary secretions.
The larvae in exchange are protected by the ants
from predatory insects. The relationship is not symbiotic however - captive
larvae prevented from making contact with ants survive well and produce healthy
adult butterflies.
Larvae of the 1st brood feed up quickly, producing butterflies in late
summer. Those of the 2nd & 3rd broods ( where they occur ) hibernate when small
and reawaken in
March to resume feeding.
The chrysalis is pale green, with the wing cases tinged with buff. The shed
larval skin remains attached to the tip of the abdomen.
Ants are attracted to the newly formed chrysalis ( probably by pheromones ) and
quickly cover it with particles of soil and leaf
litter. The pupal stage lasts for about 2 weeks.
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Common Blue
Polyommatus icarus, male in
typical basking posture, Ballard Down, Dorset |
Adult behaviour
In
weak sunlit conditions males often bask on low herbage, with wings
held half open. In overcast but warm conditions they sometimes
bask with wings fully outspread.
When the
weather is warm and sunny they fly actively from flower to
flower, nectaring in spring at
plants including
bird's
foot
trefoil,
buttercup, daisy, black medick, hop trefoil, hoary plantain,
speedwell, heath milkwort, field forget-me-not and
comfrey.
Summer brood butterflies particularly favour
fleabane,
ox-eye daisy
and marjoram.
When the sexes meet copulation occurs immediately without any form of courtship ritual. Mated pairs
often sit in prominent positions on grass-heads or on the
flowerheads of plantain.
Both sexes roost
overnight
on grass
heads, facing head-downwards, often in groups of up to 5
individuals.
Roosting at the top of the grasses is probably an effective
survival strategy, keeping them out of reach of mice and other
nocturnal predators.
Common Blue
Polyommatus icarus, male at
roost on grass head, Alner's Gorse, Dorset
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