Pea Blue (Lampides boeticus, 亮灰蝶).
A small nice Old World butterfly: it can be found all across Asia, Europe and Africa. The blue colouring, which gives this butterfly its name, is hidden in the upper face of the wings. At a distance, they can be mistaken for more common plains Cupids (luthrodes pandava), but up close their they can be easily differentiated by their distinctive brown and white patterns and the lack of black spots on their wings.
