Appearance''Polyommatus icarus'' has a wingspan of 28–36 millimetres . The dorsal side of the wings is an iridescent lilac blue, bright violet-blue,
or almost hyacinth-blue with a thin black border. Females' wings are brown or black-brown with a row of red reddish yellow spots along the edges of the wings and usually some blue at the base. The extent of blue and brown is extremely variable depending on location. The top of the wings in the female may be mostly blue, especially in Ireland and Scotland, but it always has red spots.
The ventral side has a greyish or dust-grey base colour in the males and a more brownish hue in the females. Both sexes have a row of red or orange spots along the edge of the hindwing and extending onto the forewing, though they are generally fainter there, particularly in the males, where they are sometimes missing altogether. There are about a dozen black-centered white spots on the hindwing and nine on the forewing. These usually include one in the middle of the forewing cell, absent in Chapman's and Escher's blues. The fringes on the outer edge of the wings are uniform white, not crossed with black lines as in the chalkhill and Adonis blues .
The caterpillar is small, pale green with yellow stripes and, as usual with lycid larvae, rather slug-like.
⤷ ''P. i. icarus''
⤷ ''P. i. mariscolore''
⤷ ''P. i. fuchsi''
⤷ ''P. i. omelkoi'' Dubatolov & Korshunov, 1995
⤷ ''P. i. ammosovi''
⤷ ''P. i. fugitiva''
⤷ ''P. i. napaea''
⤷ ''P. i. zelleri'' Verity, 1919
BehaviorAs a caterpillar, the common blue eats leaves. As an adult butterfly, it feeds on wildflower nectar and excrement. The adult lives 3 weeks.
Males are often very obvious as they defend territories against rivals and seek out the more reclusive females. In the south of Britain there are two broods a year, flying in May and June and again in August and September. Northern England has one brood, flying between June and September. In a year with a long warm season, there is sometimes a partial third brood in the south flying into October.
HabitatThese butterflies inhabit flowery or grassy places, warm and cool, open or wooded areas and at all altitudes up to high alpine meadows at an elevation of 0–2,700 metres above sea level. It mostly resides on chalk or limestone grassland, but also in smaller numbers in woodland clearings, meadows, heathlands, sand dunes, along railway embankments, and under cliffs.
ReproductionThe larvae feed on plants from the family Leguminosae . Recorded food plants are ''Lathyrus'' species, ''Vicia'' species, ''Vicia cracca'', ''Oxytropis campestris'', bird's foot trefoil , ''Oxytropis pyrenaica'', ''Astragalus aristatus'', ''Astragalus onobrychis'', ''Astragalus pinetorum'', black medick , ''Medicago romanica'', ''Medicago falcata'', common restharrow , wild thyme ''Thymus serpyllum'', lesser trefoil , ''Trifolium pratense'' and white clover .The egg stage lasts for around eight days. The eggs are white and shaped like flattened spheres. The eggs are very small, about 0.60 millimetres . The ground color of the egg sac is pale-greenish grey, with the actual arrangement being white. Eggs are laid singly on young shoots of the food plant.The larvae emerge around a week or two after eggs are laid. The larvae of ''P. icarus'' feed on the underside of leaves, causing blotching. Hibernation occurs as a half-grown larvae. They are attractive to ants of genera ''Myrmica'', ''Lasius'', ''Formica'', ''Plagiolepiss '', but not as much as some other species of blues. The chrysalis is olive green/brown and formed on the ground, where it is attended by ants of genera ''Myrmica'', ''Lasius'', ''Formica'', ''Plagiolepiss,'' which will often take it into their nests. The larva creates a substance called honeydew, which the ants eat while the butterfly lives in the ant hill. The relationship between these ants and blue common larvae is described to be facultatively mutualistic.There are five instars of larvae. During the first instar, larva emerge and eat away the crown of the egg. The segments of the larvae are large and rounded, and the following segments are smaller. The body is a light green, and is whitish in light. This stage lasts about nine days. The second instar, after the first moult, adds a couple of body segments onto the larvae, and the body is more green. The third instar signifies more feeding and growing in size of the larvae. They are about 3.2 millimetres long and the head and legs are black colored. In the fourth and fifth instar, the larvae becomes very green, has ten body segments, and measures about 13 millimetres in length.Larval growth rates are thought to be determined mainly by temperature and food quality and availability. The larvae of ''P. icarus'' are oligophagous, meaning they utilize a range of host plants in the family fabacaceae, as well as have a mutualistic relationship with ants. For both male and female larvae, the total development time is longer with longer day lengths, corresponding to earlier times in the season. When larvae are born earlier in the season, they take a longer time to develop. When they are born later, they take a shorter time to develop. Thus the external effect of photoperiod affects the length of development for the larvae.
FoodThe larvae feed on plants from the family Leguminosae . Recorded food plants are ''Lathyrus'' species, ''Vicia'' species, ''Vicia cracca'', ''Oxytropis campestris'', bird's foot trefoil , ''Oxytropis pyrenaica'', ''Astragalus aristatus'', ''Astragalus onobrychis'', ''Astragalus pinetorum'', black medick , ''Medicago romanica'', ''Medicago falcata'', common restharrow , wild thyme ''Thymus serpyllum'', lesser trefoil , ''Trifolium pratense'' and white clover .
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