Butterflies of temperate Asia
Common Blue
Polyommatus icarus  ROTTEMBURG, 1775
Polyommatus icarus male, Adrian Hoskins

The taxonomy of the genus Polyommatus has recently been revised so that it now includes those species formerly placed in Lysandra, Neolysandra, Sublysandra, Plebicula, Elviria, Rimisia, Bryna, Meleageria, Agrodiaetus, Paragrodiaetus and Cyaniris. Defined thus Polyommatus comprises about 220 species, distributed variously across Europe, North Africa, temperate Asia and North America.

Males of icarus are very consistent in appearance, the uppersides being violet-blue with plain white fringes. Females however vary considerably. They always have orange submarginal lunules, but while some are heavily dusted with blue scales, others are almost devoid of blue.

Polyommatus icarus is found throughout Europe and across temperate Asia to northern China.

Polyommatus icarus female, Adrian Hoskins

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.
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 herbaceous Fabaceae 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 though: captive larvae prevented from making contact with ants survive well and produce healthy adult butterflies.
The pupa is pale green, with the wing cases tinged with buff. Ants are attracted to the newly formed pupa and quickly cover it with particles of soil and leaf litter. The pupal stage lasts about 2 weeks.
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, 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.
Polyommatus icarus, male at roost Adrian Hoskins


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