The Chalkhill Blue, Polyommatus coridon

The Chalkhill Blue is limited to chalky areas where its food plant, Horseshoe Vetch, grows. My nearest substantial colony is on the Devil's Dyke on Newmarket Heath. There is a smaller colony at Fleam Dyke.

There is one brood per year with the adults in flight in July and August.

2011 had a hot dry spring, most butterflies were in flight about two weeks early. This year I saw about 8 on July 3rd. On July 15th there were several hundred dancing all over the south-facing bank opposite to the July Course stands on Newmarket Heath. They were active in a bright period in the middle of a warm, breezy day with alternating sunshine and cloud.
It looked as if there was about one female in flight for every 50 males.

On 3 Sept 2011 I saw 7 at Fleam Dyke and 20-30 at Devil's Dyke, and all looking very worn. I saw no blue males, just brown females and one that appears to be a a blue winged female (at Fleam Dyke).

Reference: Mike Gittos, Cambridgeshire and Essex Branch Newsletter, 55: 15-23 (Feb 2008). Detailed account of of the Devil's Dyke colony and its conservation. Also speculation on the possibilty that the current colony is descended from a re-introduction which may have wholly or partly replaced the previous population.

Distinguished from similar looking butterflies:

Male upperside:
differs from Common Blue by its paler shade of blue (not so different from a worn Common Blue), by its broader gray/black edging and its more distinct, though feint, spotting near the edges. Continuation of dark lines across the white borders.

Brown female upperside:
differs from brown female Common Blue by continuation of dark lines across the white borders.
differs from Brown Argus by continuation of dark lines across the white borders and by the much less bright orange fore-wing spots.

Blue female upperside:
differs from male Chalkhill Blues (usually) by a small discoidal mark.
differs from both male Common Blues and Holly Blues by the dark spots with paler surrounds along the fore-wing outer margins.

Male underside:
differs from Common Blue by continuation of dark lines across the white borders.
differs from Brown Argus by continuation of dark lines across the white borders, by lack of orange spots on the forewings and by spots between the middle and the base of the forewing.

Female underside:
differs from Common Blue by continuation of dark lines across the white borders.
differs from Brown Argus by continuation of dark lines across the white borders, by weaker, if any, orange spots on the forewings and by spots between the middle and the base of the forewing.