Chalk Hill Blue

Polyommatus coridon (po-lee-oh-MAY-tuss KO-ri-don)

Chalk Hill Blue male - Springhead, Sussex 15-July-2014
Photo © Neil Hulme
 

Wingspan
33 - 40mm

Checklist Number
61.020

Family:LycaenidaeLeach, 1815
Subfamily:PolyommatinaeSwainson, 1827
Tribe:PolyommatiniSwainson, 1827
Genus:PolyommatusLatreille, 1804
Subgenus:LysandraHemming, 1933
Species:coridon(Poda, 1761)

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Introduction

As its name suggests, the Chalk Hill Blue 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 searching for a mate just a few inches above the ground. The distribution of this species follows the distribution of Horseshoe Vetch 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 West Gloucestershire in the west and Cambridgeshire in the east. This species is absent from most of central England, northern England, Scotland, Wales, Ireland, the Isle of Man and the Channel Islands.

Polyommatus coridon

This species was first defined in Poda (1761) as shown here (type locality: Graz, Austria).

Male Chalkhill Blue, Oxenbourne Down, 20 July 2012

Male
Photo © Pauline

Chalkhill Blue - Hatch Hill - Somerset - 31/07/14

Male Underside
Photo © William

Chalkhill Blue (female), Springhead Hill (20 July 2012)

Female
Photo © Mark Colvin

Chalkhill Blue, female, Stockbridge Down, 06/08/2013

Female Underside
Photo © Pauline

Photo Album ...


History

The table below shows a chronology of vernacular names attributed to this species. Any qualification of the name (e.g. male, female) is shown in brackets after the name.

YearNameReference
1704Pale Blue ArgusPetiver (1702-1706)
1775Chalk Hill BlueHarris (1775b)

Conservation Status

Weather patterns over the last decade or so have not been kind to the Chalkhill Blue and, even though the population trend is one of increase, there is a worrying decline in distribution. This is therefore a butterfly of conservation concern.

UK BAP StatusOccurrence Change
1976-2014 (%)
Abundance Change
1976-2014 (%)
Occurrence Change
2005-2014 (%)
Abundance Change
2005-2014 (%)
Species of Conservation Concern
Large Decrease-50
Increase+20
Stable+8
Large Increase+55

The table above shows the occurrence (distribution) and abundance (population) trends, using information from The State of the UK's Butterflies 2015 (Fox, 2015). Any UK BAP status is taken from the UK Biodiversity Action Plan (UK BAP) (2007 review).

Habitat

This species lives in discrete colonies where its foodplant, Horseshoe Vetch, is found in abundance. It is also a warmth-loving butterfly, and is typically found on sheltered, south-facing hillsides.

Distribution

 

Click here to see the distribution of this species or here to see the distribution of this species together with specific site information overlaid.

Life Cycle

There is one brood each year with the adults emerging in mid-July in typical years, a peak being reached at the end of July and early August.

The chart(s) above have been correlated with the phenology plot below, taken from the UK Butterfly Monitoring Scheme. The blue line gives average counts over the full data set from 1976 to date, and the red line gives the average for the last year.

 

Imago

The sexes are strongly dimorphic; the males being a pale sky blue, and the females being a 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.

At good sites, this species can be found roosting communally on grass stems at the lower slopes of a hillside, occasionally with several individuals on the same stem. This is a highly-variable butterfly and many named aberrations of this species exist.

Adults feed primarily on Bird's-foot Trefoil (Lotus corniculatus), Carline Thistle (Carlina vulgaris), Field Scabious (Knautia arvensis), Selfheal (Prunella vulgaris), Thistles (Cirsium spp. and Carduus spp.) and Thyme (Thymus polytrichus).

Polyommatus coridon

Chalkhill-Blues, Draycott 27.08.2015

Photo © Chris Gladman

Chalkhill-Blue-Newmarket 24 July 2011 03C1525

Photo © IainLeach

Chalkhill Blue - male - Denbies Hillside, Surrey - 07-Aug-13

Photo © Pete Eeles
07-Aug-2013

Chalkhill-Blue-Newmarket 24 July 2011 03C1794

Photo © IainLeach

Chalkhill Blue - Stockbridge Down - 11 Aug 2010 (1)

Photo © Clive
11-Aug-2010

Chalkhill Blue - imago - Stockbridge Down - 20-Jul-10 (1)

Photo © Pete Eeles
20-Jul-2010

Chalkhill Blue - female - Stockbridge Down - 06-Aug-13-5

Photo © Pete Eeles
06-Aug-2013

Chalkhill Blue Pair - Colley Hill, Reigate, Surrey 9-Aug-11

Photo © Vince Massimo
09-Aug-2011

Chalkhill Blue - imago - Stockbridge Down - 23-Jul-08 (4)

Photo © Pete Eeles
23-Jul-2008

Chalkhill-Blue- 5D37837 Newmarket Aug 2013

Photo © IainLeach

Chalkhill Blue - imago - Stockbridge Down - 24-Jul-14

Photo © Pete Eeles
24-Jul-2014

Chalk-Hill-Blue- 5D39975. Surrey, July 2015.

Photo © IainLeach

Chalk Hill Blue male - Springhead, Sussex 15-July-2014

Photo © Neil Hulme
15-Jul-2014

Chalkhill-Blue-Newmarket 19 July 2010 03C0770

Photo © IainLeach

Chalkhill Blue - Barnack Hills and Holes NR. 04/08/2013

Photo © DerekLees

Chalkhill Blue - imago - Stockbridge Down - 23-Jul-08 (3)

Photo © Pete Eeles
23-Jul-2008

Chalkhill Blue Pair - Colley Hill, Reigate, Surrey 28-July-11

Photo © Vince Massimo
28-Jul-2011

Chalkhill Blue - imago - Stockbridge Down - 23-Jul-08 (7)

Photo © Pete Eeles
23-Jul-2008

Chalkhill Blue Female - Colley Hill, Reigate, Surrey 28-July-11

Photo © Vince Massimo
28-Jul-2011

Chalkhill Blue - imago - Stockbridge Down - 06-Aug-09 (3)

Photo © Pete Eeles
06-Aug-2009

Photo Album (74 photos) ...


Ovum

Eggs are laid singly either directly on a stem of the foodplant or in nearby vegetation. Like many other Lycaenids, the female butterfly will often crawl among the vegetation, searching for suitable egg-laying sites, before finally laying a single egg. This species hibernates as a fully-formed larva within the egg, the egg stage lasting up to 8 months.

"On August 13th, 1900, between two and three p.m., the author watched several females depositing on the slopes of a chalk down in Surrey. They fluttered slowly over the low-growing herbage, which consisted of rock-rose (Helianthemum vulgare), thyme (Thymus serpyllurn), horse-shoe vetch (H. comosa), and other small chalk plants and grasses, among which they settled and crept about, sometimes for a distance of two feet or more, and now and again they lowered and curved the abdomen amongst the intermingled small stems, and deposited an egg here and there. They were laid on either the stems of H. comosa, or any other plant stem near, often that of H. vulgare ... In all cases the eggs are laid singly. The egg is 0.50 mm. in diameter, and 0.30 mm. high. It differs in shape from the ova of all other British Lycaenidae. Instead of the usual concave surface above, it is flat, with merely the micropyle sunken; it is also higher in proportion, and the sides are almost perpendicular. The base is flattened; the micropyle is very finely punctured; an irregular network pattern covers the upper surface, and gradually increases in size to the outer rim, where it develops into a beautiful lace-like pattern, which is considerably bolder than that of the other "blues'" eggs. The reticulations are united by very large prominent projecting knobs, and vary in number from five to seven. The sunken spaces between are very finely granulated. The colour when first laid is almost white, mainly caused by the whiteness of the glass-like reticulations and knobs, the colouring gradually changing to a slightly greenish-grey hue. As the egg does not hatch until the following spring, the reason for no particular plant being selected as a site for the egg is at once apparent, as necessarily all the plants die off in the winter and become an entangled mass of withered stems. The young larva, upon emerging in the spring, must then search for the fresh growth of its proper food plant, the horse-shoe vetch (H. comosa). (In captivity the larva readily feed on green peas.) Before hatching, the egg assumes a rather lilac tinge. The eggs began hatching April 1st, 1901, remaining in the egg state 228 days." - Frohawk (1924)

Chalkhill Blue - ovum - Stockbridge Down - 14-Aug-88 [Tim Norriss]

Photo © Tim Norriss

Chalkhill Blue - ovum - Thatcham - 26-Mar-11 (1) {REARED}

Photo © Pete Eeles

Chalkhill Blue - ovum - Thatcham - 26-Mar-11 (2) {REARED}

Photo © Pete Eeles

Chalkhill Blue ovum

Photo © Tony Moore
08-Sep-2013

Chalkhill Blue Ovum - Hatch Hill - 31/07/13

Photo © William
31-Jul-2014

Chalk Hill Blue - ovum - Thatcham - 21-Jul-17 [REARED]-2

Photo © Pete Eeles
21-Jul-2017

Chalk Hill Blue - ovum - Thatcham - 07-Aug-17 [REARED]

Photo © Pete Eeles
07-Aug-2017

Chalk Hill Blue - ovum - Thatcham - 21-Jul-17 [REARED]

Photo © Pete Eeles
21-Jul-2017

Photo Album (8 photos) ...


Larva

On emerging from the egg, the nocturnal larva initially feeds by grazing the surface of the leaf, often from the underside, but leaves the epidermis of the opposite side of the leaf intact. During the day the larva is concealed at the base of the foodplant. Like many other Lycaenid larvae, the Chalk Hill Blue has a Newcomer's gland on the 7th segment, the secretions from which are highly attractive to ants which then afford the larva some protection against predators. The larval stage lasts between 9 and 10 weeks.

The primary larval foodplant is Horseshoe Vetch (Hippocrepis comosa).

1st Instar

"The larva emerges from the egg by eating a large circular hole in the crown. Directly after emerging the larva measures 1 mm. long. It is of the usual Lycaena form, having the sides much flattened, the lateral dilated ridge strongly pronounced, the ventral surface flat, and a decided longitudinal medio-dorsal furrow, the segments humped sub-dorsally. The first segment, which is sloping and projects over the head, bears a number of dark shining warts emitting whitish bristles directed forwards; similar ones also cover the anterior half of the second segment. The posterior half and each of the following segments have each two hairs curving backwards on either side of the furrow; the first is long, with a large dark shining bulbous base; the second is about one-third the size, with a proportionally small base, and is situated a little below on the posterior half. The eleventh segment has the smaller one missing; the twelfth segment is similar to the first, the hairs projecting over the edge. It is likewise sloping and somewhat compressed. All these sub-dorsal hairs form longitudinal series. On the third to eighth segments inclusive, immediately below the large anterior hair, is a large, conspicuous black shining lenticle, exactly resembling the spiracle. This is duplicated by a smaller one placed a little lower down on all the segments except the last two, and between these and the spiracles is another series of much smaller ones. The spiracles are large, black and distinct. Along the lateral ridge is another series of hairs, three on each segment, with black shining bases, and all directed laterally. These hairs, as well as those on the dorsal surface, are all strongly serrated. On the ventral surface are a few dark shining warts and simple spine-like hairs; the head is black and shining. The colouring of the entire body, including the legs and claspers, is a pale yellow. They feed on both the upper and under surface of the leaves, but chiefly on the cuticle of the latter, and mostly during the night, resting on the base of the stems at day. Just before first moult, when seventeen days old, it measures 1.3 mm. long and is similar in colouring to when it first emerged." - Frohawk (1924)

Chalkhill Blue - larva (1st instar) - Thatcham - 20-Mar-14 [Reared]

Photo © Pete Eeles
20-Mar-2014

Chalkhill Blue - larva (1st instar) - Thatcham - 22-Mar-14 [Reared]-5

Photo © Pete Eeles
22-Mar-2014

Chalkhill Blue - larva (1st instar) - Thatcham - 22-Mar-14 [Reared]-8

Photo © Pete Eeles
22-Mar-2014

Photo Album (3 photos) ...


2nd Instar

"Shortly before the second moult, about twenty-six days, it is 2.12 mm. long and very similar to the previous stage, but has four additional very short hairs on each side of the segments between the dorsal pair of hairs and the spiracle, and other additional ones along the lateral region. It is sprinkled with dark spots, but not nearly so numerous or so evenly distributed as in first stage. The surface is also studded with lenticles. The spiracles are shining black; the colour is pale yellow-ochreous with a darker medio-dorsal stripe bordered on either side by pale yellowish. There is also a pale lateral line. The head is shining black." - Frohawk (1924)

3rd Instar

"Six days after second moult, about thirty-eight days old, it is 4 mm. long. The entire surface is sprinkled with white serrated hairs, with black bases, each mounted on a dusky olive pedestal. The spiracles are now conspicuously large, prominent and shining black; the lenticles remain quite small, as in previous stage. The longest hairs are those on the dorsal surface. The colour is bluish-green, with distinct longitudinal sub-dorsal and lateral lines of light gamboge-yellow. Head black, legs dusky, claspers pale primrose-yellow. Just before third moult it measures 5 mm. long. One fixed for third moult, early morning May 22nd, 1901, and moulted on the 25th, by which time all had moulted the third time." - Frohawk (1924)

4th Instar

"After third moult, about fifty-five days old, it is 8.5 mm. long. It is similar to the previous stage, excepting the colouring is richer and more clearly defined; the green being more intense, shows up with contrast the bright yellow sub-dorsal and lateral lines; the latter is continuous along the entire length encircling the first and last segments, but palest on the former. The sub-dorsal line is composed of a series of longitudinal oblong marks commencing on the second and ending on the ninth segments, one on each segment, which are humped dorsally; the markings diverge on the second segment and gradually become closer on each successive segment, until they nearly unite on the ninth. If disturbed it falls from the plant and remains partly rolled up in a crescent form for many minutes. It crawls with a slow slug-like motion." - Frohawk (1924)

5th Instar

"After fourth and last moult, sixty-eight days old, it measures when crawling 16 mm. in length. Both the first and last segments are flattened and rounded; the second to ninth segments inclusive are humped dorsally with a medio-dorsal furrow, the sides rather flattened and sloping to the lateral ridge. Between this and the claspers the segments are slightly concave and bordered above the claspers with a longitudinal series of crescentic lemon-yellow markings. The sub-dorsal and lateral yellow lines are, as in the previous stage, very conspicuous, of a bright gamboge-yellow; the spiracles are also large, black and prominent. The ground colour is a clear pure green. In the centre of the first segment is a diamond-shaped disc of a glaucous-green, studded with dark serrated spines with blackish bases; in the side corners of the disc these are clustered together forming a black spot. The whole surface of the body is finely granulated and densely sprinkled with tiny white curved serrated hairs with darkish bases. On the dorsal surface they are developed into rather long serrated spines with rust-brown tips, which gradually fade into whitish. The base of each is bell-shaped, of a shining pale green with a brown summit; the spines on the ventral surface are paler and smaller. There are also scattered over the body numerous black shining lenticles as before described. The legs are brownish and the clasper feet whitish. The head is shining black and set on a retractile neck. While at rest it is withdrawn and hidden under the first segment. On the tenth segment is a well-developed honey-gland surrounded with black-rimmed lenticles and amber-coloured pyriform stud-like processes with black bases; and on the eleventh segment a retractile white tubercle on each side below and behind the spiracle. The larvae feed chiefly at night, and eat all parts of the plant, including the blossoms and stems; during the day they rest low down among the base of the stems of their only food plant, H. comosa. The majority of the larvae were fully grown by middle of June, when they concealed themselves at the base of the plant and pupated without any attachment or web of any kind, simply pupating on the ground, but well hidden among the roots and stems. Three crept into small holes in the surface of the earth for pupation." - Frohawk (1924)

Chalkhill Blue - larva - Stockbridge Down - 12-Jun-04 (2)

Photo © Pete Eeles
12-Jun-2005

Chalkhill Blue - larva - Stockbridge Down - 12-Jun-04 (3)

Photo © Pete Eeles
12-Jun-2005

Chalkhill Blue - larva - Stockbridge Down - 12-Jun-04 (4)

Photo © Pete Eeles
12-Jun-2005

Chalkhill Blue - larva - Stockbridge Down - Jun-88 (2) [Tim Norriss]

Photo © Tim Norriss

Chalkhill Blue - larva - Stockbridge Down - Jun-88 (3) [Tim Norriss]

Photo © Tim Norriss

Chalkhill Blue - larva - Stockbridge Down - Jun-88 [Tim Norriss]

Photo © Tim Norriss

Chalkhill Blue - larva - Unknown location - Unknown date [Brian Clegg]

Photo © Brian Clegg

Chalkhill Blue - larva - Thatcham - 18-Jun-13 (1) [REARED]

Photo © Pete Eeles
18-Jun-2013

Chalkhill Blue - larva - Thatcham - 18-Jun-13 (2) [REARED]

Photo © Pete Eeles
18-Jun-2013

Chalkhill Blue - larva - Thatcham - 18-Jun-13 (3) [REARED]

Photo © Pete Eeles
18-Jun-2013

Chalkhill Blue - larva - Thatcham - 18-Jun-13 (4) [REARED]

Photo © Pete Eeles
18-Jun-2013

Chalkhill Blue - larva - Thatcham - 30-Jun-13 (1) [REARED]

Photo © Pete Eeles
30-Jun-2013

Chalkhill Blue - larva - Thatcham - 30-Jun-13 (2) [REARED]

Photo © Pete Eeles
30-Jun-2013

Chalkhill Blue - larva - Thatcham - 30-Jun-13 (3) [REARED]

Photo © Pete Eeles
30-Jun-2013

Photo Album (14 photos) ...


Pupa

The pupa is formed on the ground below the foodplant. Like the larva, the pupa produces secretions that ants find attractive. It is believed that ants will carry pupae away and bury them in the earth. This stage lasts approximately 4 weeks.

"The pupa measures 12 mm. long. It is rounded with obtuse extremities, the thorax swollen and very slightly keeled dorsally, indented at the first abdominal segment, the abdomen gradually swelling to the middle, then attenuating and curving dorsally. The anal segment has no cremastral hooks; the wings are ample and swollen in the middle. The general ground colouring is ochreous-yellow, inclining to olive-yellow on the abdomen and greenish on the thorax and amber on the head and pro-thorax. The entire surface is covered with fine raised rust-brown reticulations, and excepting the wings, it is sprinkled with numerous shining brown warts, each emitting a short fine whitish hair, and along the spiracular region and pro-thorax the surface is densely studded with minute truncated tubercles, some bearing minute hairs of varying formation, some being club-shaped and finely serrated, others being more slender and amply ciliated; many of those on the spiracular line are black and whitish; the spiracles are buff. There is a medio-dorsal abdominal longitudinal brownish line. Shortly before emerging the eye is the first indication of maturing of the imago, showing as a blackish spot; then the whole colouring becomes an opaque ochre-yellow and the thorax amber-brown; finally the abdomen turns blackish. In the male pupa the wings show a pale bronze, and in the female black. The pupal state lasts about thirty days. The first imago (a female) emerged on July 18th." - Frohawk (1924)

Chalkhill Blue - pupa - Thatcham - 04-Aug-13 (1) [REARED]

Photo © Pete Eeles
04-Aug-2013

Chalkhill Blue - pupa - Thatcham - 04-Aug-13 (2) [REARED]

Photo © Pete Eeles
04-Aug-2013

Chalkhill Blue - pupa - Thatcham - 13-Jul-13 (1) [REARED]

Photo © Pete Eeles
13-Jul-2013

Chalkhill Blue - pupa - Thatcham - 13-Jul-13 (2) [REARED]

Photo © Pete Eeles
13-Jul-2013

Photo Album (4 photos) ...


Aberrations

Description to be completed.

Click here to see the aberration descriptions and images for this species.

Similar Species

Adonis Blue

The female Adonis Blue is easily mistaken for a female Chalk Hill Blue and the two species occasionally fly together toward the second half of August on some sites. Distinguishing the two is not at all easy. One guideline is that the pale scales on the hindwings, between the red dots and the white fringe, are blue in a female Adonis Blue, and white in a female Chalk Hill Blue.


Adonis Blue female (left) and Chalk Hill Blue female (right)

Brown Argus

Description to be completed.

Common Blue

Description to be completed.

Northern Brown Argus

Description to be completed.

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The following links provide additional information on this butterfly.

References

The species description provided here references the following publications:

ReferenceDetails
Frohawk (1924) Frohawk, F.W. (1924) The Natural History of British Butterflies.
Harris (1775b) Harris, M. (1775) The English Lepidoptera: or, The Aurelian's Pocket Companion.
Hemming (1933) Hemming, F. (1933) Holarctic Butterflies: Miscellaneous Notes on Nomenclature. The Entomologist.
Latreille (1804) Latreille, P.A. (1804) Nouveau Dictionnaire d'Histoire Naturelle.
Leach (1815) Leach (1815) In Brewster: The Edinburgh Encyclopaedia.
Petiver (1702-1706) Petiver, J. (1702-1706) Gazophylacii naturae et artis decas prima.
Poda (1761) Poda von Neuhaus, N. (1761) Insecta musei Graecensis.
Swainson (1827) Swainson, W. (1827) A Sketch of the Natural Affinities of the Lepidoptera Diurna of Latreille. The Philosophical magazine : or Annals of chemistry, mathematics, astronomy, natural history and general science.