Chalk Hill Blue (Polyommatus coridon)
As its vernacular name implies, P. coridon is found on chalk downland, although limestone downland is also used. The adult butterfly is most-often seen in bright sunshine, where the ground may appear to shimmer with the activity of hundreds, if not thousands, of males actively searching for a female just a short distance above the ground. The distribution of this species follows the distribution of its larval foodplant, Horseshoe Vetch (Hippocrepis comosa), which, in turn, follows the distribution of chalk and limestone grassland. This species is therefore restricted to England, south east of a line running from Gloucestershire in the west and Cambridgeshire in the east. It is absent from most of central England, northern England, Scotland, Wales, Ireland, the Isle of Man and the Channel Islands.
It is univoltine with the adults emerging in mid July in typical years, a peak being reached at the end of July and early August. The 2012 emergence in Sussex was exceptional with numbers not recorded since the mid 20th Century. A detailed evaluation, undertaken on 3rd August 2012, estimated a population of 820,000 individuals at one site alone. This mass emergence is believed to have occurred due to the wet conditions continuously suffered throughout the spring and summer of 2012, which, in turn, would have led to an exceptionally lush growth of nitrogen-rich larval foodplant. The sexes are strongly dimorphic. The males - a single individual illustrated above - being a beautiful pale sky blue and the females primarily chocolate brown. The adults use a variety of nectar sources, and the males will also visit, often in some numbers, moist earth or animal droppings to gather salts and minerals. This is a highly-variable species and many named aberrations exist. Catch them in the right light and the males are simply stunning …
Hulme, N. (2013). The 2012 Butterfly Year. The Sussex Butterfly Report. Sussex Butterfly Conservation. Issue 5, Spring 2013, pp. 12-15.