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Living up to its name, this butterfly is the commonest blue found in the British Isles. While the male has bright blue uppersides, the female is primarily brown, with a highly variable amount of blue. This is the most widespread Lycaenid found in the British Isles and can be found almost anywhere, including Orkney. It is absent, however, from Shetland and the mountainous areas of Wales and Scotland. This butterfly forms reasonably discrete colonies measured in tens or hundreds, with individuals occasionally wandering some distance. Subspecies: Polyommatus icarus icarus The nominate subspecies was first defined in Rottemburg (1775) as shown here (type locality: Germany). In the British Isles it is found throughout England, Scotland, Wales, the Isle of Man and the Channel Islands. It may also be found in some parts Ireland, although there is some confusion regarding its status, as discussed under the subspecies mariscolore.
Subspecies: Polyommatus icarus mariscolore This subspecies was first defined in Kane (1893) as shown here (type locality: Ireland). The distribution of the subspecies mariscolore is a confusing one. Nash (2012) suggests that both mariscolore and icarus are found in Ireland, Riley (2007) suggests that mariscolore is the only subspecies found in Ireland and Thomas & Lewington (2010) suggests that mariscolore is found both in Ireland and in north-western Scotland. It is certainly true that not all Irish Common Blues conform to the description of mariscolore. Specifically, mariscolore is characterised by the amount of blue in the female, but many female Irish Common Blues are brown with a variable amount of blue. The subspecies mariscolore differs from the subspecies icarus as follows: 1. Generally larger in size, especially the female. 2. The upperside of the female has extensive patches of blue, with large and bright orange marginal spots.
This species has 2 broods in the southern counties of England, and 1 brood further north. There may be a 3rd brood in favourable years. Time of emergence is highly variable. In good years, adults may be seen as early as the middle of May on more southerly sites. These peak at the end of May, giving rise to a second generation that emerges in the second half of July, peaking in the middle of August. Colonies in northern England and Scotland typically have a single brood that emerges in June, reaching a peak in July. Subspecies: Polyommatus icarus icarus Subspecies: Polyommatus icarus mariscolore The chart(s) above have been correlated with the phenology plot below, taken from the UK Butterfly Monitoring Scheme. The blue line gives average counts over the full data set from 1976 to date, and the red line gives the average for the last year. 
This species is found in a wide variety of habitats, including unimproved grassland such as roadside verges and waste ground, downland, woodland clearings, heathland and even sand dunes.
The primary larval foodplant is Bird's-foot Trefoil (Lotus corniculatus). Black Medick (Medicago lupulina), Common Restharrow (Ononis repens), Greater Bird's-foot Trefoil (Lotus pedunculatus), Lesser Trefoil (Trifolium dubium) and White Clover (Trifolium repens) are also used.
Adults feed primarily on Bird's-foot Trefoil (Lotus corniculatus), Bugle (Ajuga reptans), Carline Thistle (Carlina vulgaris), Fleabane (Pulicaria dysenterica), Kidney Vetch (Anthyllis vulneraria), Knapweeds (Centaurea spp.), Marjoram (Origanum vulgare), Ragwort (Senecio jacobaea), Thistles (Cirsium spp. and Carduus spp.), Thyme (Thymus polytrichus), Vetches (Vicia spp.) and White Clover (Trifolium repens).
This species is most active in sunshine and is a frequent visitor to flowers. Males are the more active of the two sexes and set up territories which they patrol in search of females. The female is less conspicuous, spending most of her time nectaring, resting and egg-laying. When egg-laying, the female makes slow flights, low over the ground, searching out suitable foodplants on which to lay. When a suitable plant is located, a single egg is laid on the upperside of a young leaf. In dull weather this species roosts head down on a grass stem. As for similar species, such as the Brown Argus, this species roosts communally at night, with several individuals occasionally found roosting on the same grass stem. Subspecies: Polyommatus icarus icarus
Subspecies: Polyommatus icarus mariscolore
Description to be completed. Unclassified Aberrations
The white bun-shaped eggs are quite visible and can be easily found on good sites. This stage lasts just over a week.
The larva emerges after a week or two. On emerging from the egg the larva moves to the underside of the leaf, where it feeds, by day, on the lower surface without breaking through the upper leaf surface. This leaves characteristic blotches on the foodplant that can give away the presence of a larva. More mature larvae feed more extensively on the leaves. Those larvae that overwinter do so in leaf litter at the base of the foodplant, changing from green to olive, resuming their green colouring in the spring. Like many other species of blue, the larva is attractive to ants, although only in its last instar. There are 4 moults in total. If the larva does not overwinter, then this stage lasts around 6 weeks.
The pupa is formed on the ground or, occasionally, at the base of the foodplant, under a few silk strands. The pupa is attractive to ants which may bury it in earth. This stage lasts around 2 weeks.
Adonis BlueThe male Adonis Blue is often mistaken for a male Common Blue. However, the two can be distinguished by looking at the white fringes of the wings. Only on the Adonis Blue are the fringes intersected by black bands. This diagnostic can also be used to distinguish the two species based on their undersides. 
Adonis Blue male (left) and Common Blue male (right) Brown ArgusOf the two sexes, it is the female Common Blue that causes most confusion with the Brown Argus. The blue present in a female Common Blue is highly variable, with individuals ranging from almost completely blue through to completely brown. It is this latter colouring that causes the most confusion. Even so, the Brown Argus has no blue scales, but may give off a blue sheen from the wings and the hairs found on the thorax and abdomen. Another diagnostic is that the Brown Argus normally has a prominent dark spot in the centre of the forewings. 
Brown Argus (left) and female Common Blue (right) Differentiating Brown Argus and Common Blue from their undersides is even more problematic, and we need to resort to the pattern of spots. Here we have two distinguishing features. The first is that the Common Blue has a spot on the underside of the forewing that is absent in the Brown Argus. The second is that two of the spots on the leading edge of the hindwing are relatively-close in the Brown Argus, almost forming a "figure of eight", but are more spaced apart in the Common Blue. This diagnostic is particularly useful if the underside of the forewing isn't visible. 
Brown Argus (left) and Common Blue (right) Chalkhill BlueDescription to be completed. Holly BlueDescription to be completed. Northern Brown ArgusOf the two sexes, it is the female Common Blue that causes most confusion with the Northern Brown Argus. The blue present in a female Common Blue is highly variable, with individuals ranging from almost completely blue through to completely brown. It is this latter colouring that causes the most confusion. Even so, the Northern Brown Argus has no blue scales, but may give off a blue sheen from the wings and the hairs found on the thorax and abdomen. Another diagnostic is that the Northern Brown Argus normally has a prominent dark spot in the centre of the forewings and, in the case of the artaxercxes subspecies of Northern Brown Argus, it a distinctive white dot. Any identification challenges are usually, therefore, with respect to the salmacis subspecies of Northern Brown Argus that does not have this white spot. 
Common Blue female (left) and Northern Brown Argus ssp. salmacis (right) Differentiating Common Blue and Northern Brown Argus from their undersides is even more problematic, and we need to resort to the pattern of spots. Here we have two distinguishing features. The first is that the Common Blue has a spot on the underside of the forewing that is absent in the Northern Brown Argus. The second is that two of the spots on the leading edge of the hindwing are relatively-close in the Northern Brown Argus, almost forming a "figure of eight", but are more spaced apart in the Common Blue. This diagnostic is particularly useful if the underside of the forewing isn't visible. 
Common Blue (left) and Northern Brown Argus (right) Silver-studded BlueDescription to be completed.
Click here to see the distribution of this species overlaid with specific site information. Alternatively, select one of the sites listed below.
Despite a general decline in distribution, this butterfly remains widespread and is not currently a species of conservation concern.
From The State of Butterflies in Britain and Ireland and the UK Biodiversity Action Plan (UK BAP) (2007 review).
The following links provide additional information on this butterfly.
The species description provided here has been derived from the author's own observations and the information contained in the following works:
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![Common Blue - imago - Boston, Clare - Unknown date [Adrian Riley]](phpBB/gallery/images/upload/6557c53b4ef9a8047ebb9db3b039494f.jpg "Common Blue - imago - Boston, Clare - Unknown date [Adrian Riley]")
![Common Blue - aberration - Beenham Village - Jul-02 [Derek Brown]](phpBB/gallery/images/upload/cd01542aad41e9b919951fa7eb7f922d.jpg "Common Blue - aberration - Beenham Village - Jul-02 [Derek Brown]")
![Common Blue - aberration - Devils Dyke, Newmarket - 10-Jun-07 [Nick Ballard]](phpBB/gallery/images/upload/ec7a94cc2f3c90135b1a02898555968f.jpg "Common Blue - aberration - Devils Dyke, Newmarket - 10-Jun-07 [Nick Ballard]")
![Common Blue - imago - Sidmouth, Devon - 17-Jul-07 [Adrian Dexter]](phpBB/gallery/images/upload/29499221c35deca9bfb001cb8d155f9b.jpg "Common Blue - imago - Sidmouth, Devon - 17-Jul-07 [Adrian Dexter]")
![Common Blue - ovum - Godlingstone Hill - Sep-90 [Tim Norriss]](phpBB/gallery/images/upload/af5f967f8f58d1271539ff2e1b7aa809.jpg "Common Blue - ovum - Godlingstone Hill - Sep-90 [Tim Norriss]")
![Common Blue - ovum - Unknown location - 2004 (2) [REARED] [Reg Fry]](phpBB/gallery/images/upload/59b2fa4336c1102264244a944ffdf927.jpg "Common Blue - ovum - Unknown location - 2004 (2) [REARED] [Reg Fry]")
![Common Blue - ovum - Unknown location - 2004 [REARED] [Reg Fry]](phpBB/gallery/images/upload/470e9d1d02831b56d30365033f37fef4.jpg "Common Blue - ovum - Unknown location - 2004 [REARED] [Reg Fry]")
![Common Blue - larva - Kings Somborne - Jun-89 [Tim Norriss]](phpBB/gallery/images/upload/6a9a4a0b643d76b535e5e74f4d8500e6.jpg "Common Blue - larva - Kings Somborne - Jun-89 [Tim Norriss]")
![Common Blue - larva - Stockbridge Down - Jul-99 [Tim Norriss]](phpBB/gallery/images/upload/11098c4a2259f99b579a8de289e6e497.jpg "Common Blue - larva - Stockbridge Down - Jul-99 [Tim Norriss]")
![Common Blue - pupa - Stockbridge Down - Jul-99 [REARED] [Tim Norriss]](phpBB/gallery/images/upload/7c35a5d7f237c7adabdc7e93b7fe41b4.jpg "Common Blue - pupa - Stockbridge Down - Jul-99 [REARED] [Tim Norriss]")
