Common Blue (Polyommatus icarus)
The Common Blue is found on rough, open ground, chalk downland and other high or low wild grassy places throughout the United Kingdom except, possibly, the Shetland Isles. In warmer years they spread their range into gardens and other areas that they do not normally occupy.
There are two broods except in the far north and parts of Wales. The butterflies are on the wing from May until late July and the, partial, second brood lasts until October. Those that don't feed up to produce the second brood develop slowly and join the latecomers in hibernation while in their third instar (icarus have five instars) in slight cocoon-
This is the most common and widespread British Blue. It lives in colonies and, although in sunny spells the males challenge each other, when the sun goes down the butterflies rest head down close together, sometimes several in a group. These butterflies used to be much more numerous and hundreds could be found roosting for the night within a square metre.
Males are bright blue with a distinct lilac tint. Females are brown, always with a little blue and sometimes with a lot. In this case there is a mixture of white streaking in with the blue that distinguishes a blue female from a male. Scottish and Irish forms are larger and brighter in the male, with bolder orange underside markings. Their females are also bigger and very blue, with bright orange lunules on the upper side.