The Chalkhill Blue is a butterfly

Butterfly

A butterfly is a mainly day-flying insect of the order Lepidoptera, which includes the butterflies and moths. Like other holometabolous insects, the butterfly's life cycle consists of four parts: egg, larva, pupa and adult. Most species are diurnal. Butterflies have large, often brightly coloured...

 in the family Lycaenidae

Lycaenidae

The Lycaenidae are the second-largest family of butterflies, with about 6000 species worldwide, whose members are also called gossamer-winged butterflies...

.

Males have pale silvery-blue wings with black and white border (see adjacent images). Females are dark brown, also with the black and white borders. As with many blue butterflies, separation from similar species in the field is on the underside markings.

In the UK

United Kingdom

The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

, this is a species favouring unimproved (and unfertilized) chalk downland. Its larval (caterpillar) foodplant is Horseshoe Vetch

Horseshoe Vetch

Hippocrepis comosa, known as Horseshoe Vetch, is a flowering plant. It is the sole food of the butterfly species Polyommatus coridon, or the Chalkhill Blue....

(Hippocrepis comosa). It is usually seen on the wing for about 6-8 weeks, July to September.

Photographs of a habitat of the Chalkhill Blue can be found on the following web page:

http://www.glaucus.org.uk/MillHill2005.html