The Common Blue is rather less common in Britain than it used to be, but it is certainly one of the most widespread of all the 'blues. You can expect see these pretty little insects on wasteland and roadsides as well as in flowery grassland and sand dune sites from April through until at least early September. The females hardly justify the name 'blue'; they have a distinctly violet tinge with orange markings near the edges of their wings.
Males of this species are paler and lack the orange upperwing markings; they do. however, vary in the depth of the blue colour of their wings.
Key identification features of the Common Blue are white outer edges of the wings that are not crossed by dark lines radiating to the edges; orange patches near the outer edges of the undersides of the wings; and an additional (compared with other 'blues') rounded dark spot on the forewing some 5mm or so away from the body of the insect.
In Britain the Common Blue occurs throughout England Wales and Scotland, and it is equally ubiquitous in Ireland. Elsewhere, this butterfly's range extends throughout Europe, including Scandinavia; down into North Africa, and across Asia into Japan.
The male Common Blue shown above and below is Polyommatus icarus f. celena and was photographed in the Algarve region of Portugal. (These two pictures are shown by courtesy of Rob Petley-Jones)
The larval foodplants used by the Common Blue are various members of the pea family, Fabaceae. They include various vetches (Vicia spp), Clovers (Trifolium spp), Medicks (Medicago spp) and most commonly Bird's-foot-trefoil (Lotus corniculatus).
Common Blues overwinter in their half-grown larval state. The pale green caterpillar have yellow stripes; they produce a honeydew-like liquid on which ants feed. To protect their food suppliers the ants in turn ward off predators that might otherwise eat the caterpillars.
The chrysalis of the Common Blue Butterfly, which is formed on the ground, is olive green or olive brown. The pupae are attended by ants that sometimes drag the chrysalises into their anthills, from where the adult butterflies eventually emerge.
In southern Britain there are usually two broods, with the the first brood of butterflies appearing in May and June.
The second brood flies during in August and September. In the extreme south of England occasionally the autumn weather is warm enough for a partial third brood to be seen flying in October.
Further north, in Scotland, there is usually just one brood, and the adults can be seen flying from June until early September.