Butterflies of Britain
Family - LYCAENIDAE
Polyommatus icarus, male, Ballard
Down, Dorset ©
The taxonomy of the genus
Polyommatus has recently been revised so
that it now includes those species formerly placed in
Agrodiaetus, Paragrodiaetus and
Cyaniris. Defined thus
Polyommatus comprises about 220 species,
distributed variously across Europe, North Africa, temperate Asia and
Despite its name the
Common Blue can no longer be considered a common butterfly in the UK.
It still remains the most widespread 'blue', 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 bright violet-blue with
unchequered 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.
Polyommatus icarus female, Lardon
Chase, Berkshire ©
The undersides of both
sexes of Common Blue are marked with numerous white-ringed black spots
and orange crescents. Sometimes aberrant forms occur 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 are 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.
This species is found
right across Europe from northern Scandinavia to the smallest islands
of the Mediterranean. Beyond Europe, its range extends from the Middle
East across temperate Asia to northern China. It also occurs in north
Africa and the Canary Islands.
male, Hungerford, Berkshire ©
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.
at roost, Hungerford, Berkshire ©
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.
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
black medick Medicago lupulina
and other leguminous herbs are also used.
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 that are prevented from making contact with
ants survive well and produce healthy adult butterflies. Larvae of
the 1st brood feed up quickly and produce butterflies in late
summer. Those of the 2nd & 3rd broods ( where they occur ) hibernate
when small and reawaken in March to resume feeding.
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.
male in typical basking posture, Ballard Down, Dorset ©
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 bird's foot
trefoil, buttercup, daisy, black medick, hop trefoil, hoary
plantain, speedwell, milkwort, forget-me-not and comfrey. Summer
brood icarus 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 flowerheads.
Both sexes roost
overnight on grass heads, facing head-downwards, often in groups of
up to 4 or 5 individuals. Roosting at the top of the grasses is an
effective survival strategy, keeping them out of reach of mice and
other nocturnal predators.
at roost, Hungerford, Berkshire ©