UK Butterflies

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Silver-studded Blue

Plebejus argus

ple-BEE-uss AR-guss

Wingspan
26 - 32 mm

Checklist Number
61.014

Silver-studded Blue

Plebejus argus

ple-BEE-uss AR-guss

Wingspan
26 - 32 mm

Checklist Number
61.014


The Silver-studded Blue gets its name from the light blue reflective scales found on the underside of most adults and which are quite visible when light reflects off them. As with many other species of blue, the males are blue while the female is a less-conspicuous brown. However, this butterfly is highly variable in appearance and, as stated in Dennis (1977), variation extends to "differences in the male upperside coloration, the width of the marginal border (wide in the New Forest, narrow in Devonshire) and the underside grey coloration (dull grey in the New Forest, silvery grey in east Suffolk); in the female to the development of the blue coloration and the orange lunules on the upperside, and on the underside to the coloration, the width and prominence of the medial white band".

Both Worms (1949) and Thomas (1999) provide an analysis of many populations found in the British Isles, all of which are considered to belong to the nominate subspecies by default, despite apparent differences in appearance. This includes populations found on mossland at Llyn Hafod-y-llyn in north Wales, on limestone at Portland Bill in Dorset, at Holy Island off Anglesey, at Prees Heath in Shropshire, and on the sand dunes of south-west England.

This delightful butterfly is found in close-knit colonies, with individuals rarely flying any distance. Most colonies contain less than a thousand adults. However, a few colonies are huge, with the number of adults measured in tens of thousands, providing an amazing spectacle. Most colonies of this species are found in southern England, although there are several other notable colonies in the east of England and Wales. With the exception of a single site at Prees Heath in Shropshire, this butterfly is absent from central and northern England, Scotland, Ireland and the Isle of Man.

This species was first defined in Linnaeus (1758) as shown here (type locality: Europe, Africa).

The British population is now represented by the nominate subspecies, with the exception of the population found on the Great Orme and nearby areas in north Wales, where it is replaced by the subspecies caernensis. Former populations of subspecies masseyi and cretaceus are now extinct.

This subspecies was first defined in Tutt (1909a) as shown here (type locality: Dover, Kent, England).

This reference merely names the subspecies, but defers the description to Tutt (1909b), which itself refers back to Tutt (1909a) and also mentions Barrett (1893), Jones (1905a) and Jones (1905b). This subspecies is extinct and was formerly found on chalk downland in Kent, Surrey, Hampshire and Dorset. It differs from the subspecies argus as follows:

  • 1. It is slightly larger in size, on average.
  • 2. The male upperside is of a brighter blue.
  • 3. The male upperside has narrower dark borders.
  • 4. The male upperside hindwing has distinct marginal spots, rather than a dark band.

The colony found in the quarries of Portland Bill exhibits characteristics that are similar to this subspecies.

Plebejus argus ssp. cretaceus (Tutt, 1909)

var. cretaceus, Tutt, "Ent. Rec.," xxi., pp.58-59 (1909) [Tutt (1909a)]. Aegon, Barr., "Lep. Br. Isl.," pl.xi., fig.1 (1893) [Barrett (1893)]. Hypochiona, Jones, "Ent. Mo. Mag.," xli., p.254 (1905) [Jones (1905a)]; "Proc. Ent. Soc. Lond.," p.xlvi (1905) [Jones (1905b)].

♂. Bright blue, dark margin of forewings narrow or absent (sometimes quite pallid), discoidal lunules obsolete, on hindwings; interneural marginal spots replace usual band; underside bluish-grey, spots clearly defined.

♀. Dark fuscous, often tinged with blue, orange marginal lunules on hindwings poorly developed, on forewings usually obsolete; underside pale brownish, with well-defined spots, and whitish submarginal border.

This is the form that we noted (British Butterflies, p.183) [Tutt (1896a)] as "occurring near Dover and in other chalky districts, the ground colour of the males much paler in tint that those from the heathy localities, and the dark margin practically obsolete." It is the common form found on the open downs of southern England, and has a wide distribution on the Continent. It is most closely allied to aegiades, Gerh. (anteà p.192), but has well-developed metallic kernels on the underside of the hindwing, and is also very similar in its general characters to the specimens of var. orientalis in the British Museum coll. (anteà pp. 192-193).

This subspecies was first defined in Tutt (1909a) as shown here (type locality: Witherslack Mosses, Westmorland, England).

In the original definition of masseyi, the authors refer to Massey (1895), saying "The third form above [ssp. masseyi], the moorland form, was largely on our authority referred (Ent. Rec., vii., p.128) to the var. corsica, Bellier. This conclusion was based on a male and female received from Mr. W.E. Nicholson from Lewes, as var. corsica, the two forms agreeing fairly well on the upperside, especially in the female, but it is, we have since discovered, on the underside that Bellier de la Chavignerie found the chief character of the Corsican form, and, in this, our "moorland" form disagrees entirely. This form, like our "down" form, therefore, appears to required a distinctive cognomen, and we shall deal with it in our revision of the species as var. masseyi, as we are indebted entirely to Mr. Massey for our examples of the species, from which our studies have been made, and we owe him some apologies for misleading him, in our ignorance, into applying the name corsica, to our insect, on the first introduction of the race to the notice of British lepidopterists".

The corresponding description of masseyi, given below, is found in Tutt (1909b) and also refers to Massey (1895), Murray (1897) and Tutt (1909a). This subspecies was formerly found on the mosses of Westmorland and Lancashire, but has been extinct since approximately 1942. It differs from the subspecies argus as follows:

  • 1. The male upperside is often of a brighter blue.
  • 2. The male upperside has a reduced dark border, revealing dark nervures on the outside of the forewings, and a row of black spots on the hindwings.
  • 3. The male underside is paler, with reduced orange markings at the margins.
  • 4. The female upperside, which characterises this subspecies, has a blue flush that extends over most of the hindwings and the base of the forewings, leaving a broad brown outer margin.

The colony at Prees Heath exhibits some characteristics of this subspecies. Another site, at Hafod Garregog National Nature Reserve in north Wales, represents the last mossland site for Silver-studded Blue, and also resembles this subspecies to a degree, with some females exhibiting an amount of blue scaling.

Plebejus argus ssp. masseyi (Tutt, 1909)

var. masseyi, Tutt, "Ent. Rec.," xxi., p.58 (1909) [Tutt (1909a)]. Corsica, Massey, "Ent. Rec.," vii., pp. 127 et seq. (1895) [Massey (1895)]; Murray, "Ent. Rec.," ix., p.294 (1897) [Murray (1897)].

♂s varying in colour from those having a distinct argus (aegon) tint (with just a trace of red in it), to the blue which characterises argyrognomon. The dark marginal border is narrow, and, in some specimens, restricted to a fine line, with dark nervures showing on the outside of the wing. The hindwings in all the specimens present a more or less complete row of marginal black dots, separated by the fine black nervures, and, in some specimens, edged externally with paler - one specimen shows a pale coloration along the costa, and at the upper part of the outer margin of the hindwing. The undersides of the forewings of the ♂s may be described as pale grey with blue bases, with but little orange on the marginal border, and with slight variation in the size of the spots forming the transverse angulated row crossing the forewings. The hindwings are characterised by a white band between the marginal row of orange spots and the transverse row of black spots, the latter of which in the antepenultimate spot shows some tendency to vary, in some specimens being much higher than, in others more in a line with, the remainder.

The upperside of the ♀s is most remarkable, the forewings being blue at the base, the colour extending to the centre of the wing, and reappearing again as a transverse shade reaching from the costa to the inner margin, leaving the broad brown outer marginal band, which is so characteristic of the ♂ P. argus (aegon). Faint traces of orange spots are sometimes present in this outer marginal band. The hindwings are shaded entirely with blue, which is of two tints, as in the ♂s, the more purple argus (aegon) tint and the bluer argyrognomon tint, the latter very bright and characteristic; a series of marginal spots is present in all the specimens, but the quantity of orange present is distinctly limited. The undersides of the ♀s are characterised by the distinct white band which traverses all the wings between the orange marginal band and the transverse row of black spots, and also by the strong development of the silver studs in the orange bands. There is also some variation in the transverse band of spots on the hindwings, the antepenultimate spot being well raised out of the level of the others (an argus character) in some specimens, whilst it is almost in a line with the other spots (an argyrognamon character) in other specimens. In colour the undersides are pale grey with a brownish tinge, but with scarcely a trace of greenish-blue at the base (completely absent in some).

This subspecies was first defined in Thompson (1937) (type locality: Great Ormes Head, North Wales).

This subspecies was formerly restricted to the Great Ormes Head in Caernarvonshire, North Wales. However, in 1942, 90 adults were introduced to the Dulas Valley, some 13km to the east, and the colony has gradually spread along the valley at a rate of 1km per decade, colonising 15 new sites. It is estimated that the total population now numbers 100,000. This subspecies differs from the subspecies argus as follows:

  • 1. It is smaller in size.
  • 2. The male upperside has narrower, sometimes absent, dark borders.
  • 3. The male underside is paler.
  • 4. The female upperside has a blue flush that extends over most of the hindwings and base of the forewings.

Plebejus argus ssp. caernensis (Thompson, 1937)

Male. Very much smaller than type argus, those in my series varying between 16 mm and 22 mm when fully expanded. In colour they are much paler than the type, approaching the shade of argiolus; the black border is greatly reduced, and in some specimens almost absent.

The underside varies from chinese white to a beautiful steely grey, but all the markings are smaller than usual and the lunules dull and inconspicuous.

Female. Smaller than the male, varying from 15 mm to 20 mm. The majority are very strongly suffused with purple blue, and the upperside variation is so great that I must divide them into four sections. I allot a varietal name to each.

var. splendida. Specimens entirely suffused with blue, the brown scaling being obliterated.

var. pseudomasseyi. Specimens suffused with blue on all parts except a circular patch on the discal area of the forewings. (Almost the same as Tutt's description of var. masseyi).

var. semisuffusa. The blue suffusion confined, or almost so, to the hindwings.

var. pulchrina. Forewings without blue markings but the hindwings brilliantly rayed with bright blue.

In each of the above varieties the blue suffusion may be any shade from a deep violet-purple to a brilliant greenish-blue, almost the tint of bellargus. On the upperside of the female the orange lunules are usually almost inconspicuous, and often entirely absent.

Caernensis occurs at a considerable elevation on limestone cliffs near the sea; and feeds, so far as I have been able to ascertain, exclusively on Rock Rose (Helianthemum). The time of its appearance is much earlier than type argus, at any rate in Wales, often emerging as early as the second week of June. This form never flies with the type, but is confined to totally different surroundings and conditions. There are many localities for type argus in the district, but all these are situate on sandy heaths and low lying waste lands; whenever a colony is found on the cliffs it is composed entirely of caernensis.

 
Family:LycaenidaeLeach, 1815
Subfamily:PolyommatinaeSwainson, 1827
Tribe:PolyommatiniSwainson, 1827
Genus:PlebejusKluk, 1780
Subgenus:  
Species:argus(Linnaeus, 1758)
Subspecies:argus (Linnaeus, 1758)
 cretaceus Tutt, 1909
 masseyi Tutt, 1909
 caernensis Thompson, [1937]

Description to be completed.

The distribution data (2000-2009) has been made available through the generosity of Butterfly Conservation and any subspecies distribution is taken from Riley (2007). Click here to see the distribution of this species together with site information overlaid.

This species is typically found on heathland that has not become too overgrown. It is also found on both limestone and chalk grassland and was formerly found in upland mosses. This species is a warmth-loving butterfly and, as such, is often found in sheltered areas, or those that are south-facing.

Although long-term declines in distribution seem to have stabilised, there are no signs that this decline is reversing. This species has suffered from increasingly-fragmented habitat as heathland is lost to development and agriculture. This species rarely flies any distance, often moving less than 20 metres in its life, making it difficult for this species to colonise new sites that may be close by. This species is therefore a priority species for conservation efforts.

The table below 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).

UK BAP Status
Occurrence Change
1976-2014 (%)
Abundance Change
1976-2014 (%)
Occurrence Change
2005-2014 (%)
Abundance Change
2005-2014 (%)
Priority Species
Click here to access the Biodiversity Action Plan (BAP).
Large Decrease -64Increase +19Increase +19Stable -9

Most colonies start to emerge in the middle of June flying into the first part of July and even early August, although the caernensis subspecies found on the Great Orme emerge earlier, typically at the end of May and start of June, flying throughout June and into the start of July. There is one brood each year.

Plebejus argus ssp. argus

Plebejus argus ssp. caernensis

As for the larva and pupa, newly-emerged adults are also often attended by ants which offer them some protection from predators. Unlike many other species, this blue will fly in overcast weather, so long as the temperature is sufficiently high. The adults roost communally and this can provide a wonderful sight first thing in the morning, as the adults open their wings to catch the heat from the sun's rays. Males can be observed flying low over vegetation, searching out females, which are mated very soon after emergence. Neither sex spends much time visiting flowers to nectar but, when they do, Bell Heather is a favourite nectar source.

Adults feed primarily on Bell Heather (Erica cinerea), brambles (Rubus spp.) and Common Bird's-foot-trefoil (Lotus corniculatus).

The egg is laid singly either on the foodplant, or on surrounding vegetation or debris. Eggs are often laid close to the ground, where it is believed the higher temperature favours development. This species overwinters in this stage, the larva being fully-developed inside the egg.

"The eggs are laid singly and adhere firmly to the plants on which they are deposited. The egg state lasts from July or August until the end of the following March, or early in April, when they hatch. The egg is of a compressed spherical shape; it measures 0.60 mm. wide and 0.30 mm. high. The base is slightly concave and the crown more so; the micropyle is deeply sunken and very finely punctured; the punctures are smallest in the centre; they increase in size on nearing the side, where they develop into a beautifully formed pattern resembling fine lacework, composed of a number of prominences placed at somewhat regular intervals, and connected one to the other by six keels or spokes, the interstices between each being very deep. The reticulations again decrease in size on nearing the base, which is likewise deeply punctured and of a spongy-like pattern. Both the colour and texture greatly resemble white porcelain; all the depths produce a deep purplish-grey shade." - Frohawk (1924)

The newly-emerged larva feeds on the tenderest parts of the foodplant, including young shoots, buds and flowers. The larva is often found in the presence of ants, especially the black ant Lasius niger, which are believed to offer the larva some form of protection against parasites and other predators. Ants are attracted from secretions exuded from a Newcomer's gland at the tail end of the larva. There are 4 instars in total.

The primary larval foodplants are Black Medick (Medicago lupulina), Common Bird's-foot-trefoil (Lotus corniculatus), Common Rock-rose (Helianthemum nummularium), Gorse (Ulex europeaus), heathers (Calluna and Erica spp.), Hoary Rock-rose (Helianthemum oelandicum) and Horseshoe Vetch (Hippocrepis comosa).

1st Instar

"The larva makes its exit by eating out a small round hole in the crown of the egg, just large enough to allow it to creep out. Directly after emergence the larva measures 1.1 mm. long. It is of the usual Lycaena shape, but rather stout in proportion. The segments are humped dorsally; the sides flattened with a projecting lateral ridge. The head is deep shining olive-black with a few extremely small whitish hairs round the mouth. The first segment is large, flattened and projects hood-like over the head, and has a large central dorsal shining olive-brown disc; there is also a similar but much smaller one on the anal segment. The surface of the first segment and the anterior half of the second segment are studded with dark shining warts, each emitting a long white slightly serrated hair, curving forwards. The posterior half of the second and each of the following segments have two hairs curving backwards on each side of the dorsal furrow. The anterior hair on each segment is very long, the posterior one short; both are finely serrated and have dark shining bases. On each segment below the hairs are two dark brown sub-dorsal lenticles; the first is very large, and a pair of much smaller ones just above the spiracle; the posterior one emits a minute glistening white hair-like point. The spiracles are brown. On the lateral ridge are three long projecting hairs with brown bulbous bases (similar to those on the dorsal surface); these form a projecting lateral fringe continued round both ends of the larva. On the claspers and ventral surface are a few dark warts emitting simple hairs. The legs are olive-brown, the claspers the same colour as the body. Upon emergence the young larvae were placed inside the expanding furze blossoms, and shortly afterwards they started feeding on the tender portions of the stamens and petals." - Frohawk (1924)

2nd Instar

"The first moult took place at the end of April; the first stage lasting nearly a month. After the first moult, when thirty days old, it measures 2.1 mm. long. The back is arched, the sides very sloping and the ventral surface much flattened. The head is shining black, and while at rest it is completely withdrawn under the large overlapping anterior segment. It is only protruded while feeding and partially so when crawling. During its progress the head is kept in incessant motion, waving from side to side. The colour of the body is pale ochreous, with medio-dorsal, sub-dorsal and lateral dark chocolate-brown longitudinal lines, each being bordered by a whitish buff line. The whole surface is sprinkled with dark brown lenticles; there are also longitudinal rows of short greyish-green tubercles, each emitting a curved whitish hair. The legs are black and the claspers ochreous. On the anterior segment is a dark purplish-brown dorsal disc. The honey gland on the tenth segment is not yet visible. The stamens and petals of the furze blossoms still form their only food, some of the petals being thickly perforated by them. The larva is extremely sluggish in its movements. On June 1st the larvae were in various stages, varying from 2.5 mm. to 6 mm. in length. Two were in the second stage; about half had moulted twice, and the remainder had moulted the third time. They all very closely resembled each other in their different stages, but after the third moult the colouring is more vivid." - Frohawk (1924)

It is thought that Frohawk mistakenly added an additional instar: "After the second moult it is precisely similar to the previous stage (but the honey gland shows as a slight incision), excepting its advanced growth, measuring 4.2 mm. long." - Frohawk (1924)

3rd Instar

"After the third [second] moult it is 6.3 mm. long. The colour is pale greyish-green, with a dark chocolate-brown medio-dorsal stripe bordered on either side by a white line and a double grey-green sub-dorsal stripe composed of two slightly oblique marks on each segment, each bordered above by whitish; and a lateral dilated white stripe bordered below by an olive band. The under surface is grey-green; the head is small, intensely black and shining. The large compressed anterior segment has a conspicuous chocolate-brown patch in the centre. The body is clothed in exceedingly fine short hairs; those on the dorsal area are the longest and curve backwards. They still continue feeding on the blossoms." - Frohawk (1924)

4th Instar

"During the first week of June a few moulted for the fourth [third] and last time, and the first became fully grown about June 20th. When fully grown it measures while at rest 12.7 mm. in length. The anterior segment is projected, flattened and rounded in front, completely concealing the head; from the second to ninth segments (both inclusive) the back is considerably elevated, and the segments of uniform size; their sides are flat and sloping to the lateral dilated ridge, the second segment rising abruptly above the first; the last three posterior segments are flattened, especially the last, which terminates in a broad rounded flap much compressed, similar to the first segment; the eleventh segment is furnished with two retractile tubercles, which are occasionally thrust out when the caterpillar is suddenly alarmed; otherwise they remain withdrawn, forming short blunt processes; the tubercle is pale straw-yellow in colour, and surrounding the apex of the sheath is a series of black warts, each terminating in a short spine resembling a thorn. On the tenth segment is a well-developed honey gland. The ventral surface of the larva is much flattened, and while resting it lies so flat that the legs and claspers are almost hidden; the head is very black and shining. The ground colour of the body is very pale green; a dark purplish medio-dorsal stripe runs the entire length; it is broadest on the first segment, and is bordered on each side by a white line, then a pinkish and green band blending into the pale green of the ground colour, followed by a conspicuous sub-dorsal dark green stripe, composed of a series of rather oblique marks, one on each segment, and a somewhat similar stripe, but narrower, runs parallel to it immediately above the spiracles; a sub-spiracular dark green stripe, bordered below by a white stripe, passes along the lateral ridge, which borders on the green of the ventral surface; the spiracles are white; the entire surface is sprinkled thickly with black and white granulations, the white ones predominating. Each granule emits a very fine minute ochreous hair. Those along the dorsal surface are the longest and stoutest. The legs are greenish, and the claspers green and brown-pink; the segments are strongly defined and somewhat overlapping. Another form of the larva, which appears of almost equal occurrence, has the first sub-dorsal and sub-spiracular stripes lilac-red, and generally the ground colour is more tinged with olive, giving the whole a decided reddish hue. The larva in all its stages is very sluggish in its movements; it has a slow gliding motion, continually protruding and withdrawing its head and waving it to and fro during progression. It feeds upon the gorse blossoms as long as it remains in bloom, and when they are over it readily takes to the young tender spines, which form the sole food of the larvae during their last stage. The larvae in their earlier stages closely resemble in form and colour the small brown bracts of the bloom." - Frohawk (1924)

Pupation takes place in a silk-lined chamber just below ground, often in an ants' nest. As for the larva, the pupa is often attended by ants, and it has been known for ants to form nests around the pupation site. This stage lasts between 2 and 3 weeks.

"The first pupated on June 22nd. The pupa measures 8.5 mm. in length. Dorsal view: It is widest across the middle of the abdomen; the head is rounded; base of wing is slightly angular and prominent, the wing curving gently over the side of the abdomen, and rather swollen; the abdomen is attenuated to the anal extremity. Lateral view: The head is rounded; thorax rounded and swollen and nipped in behind; the abdomen is swollen about the middle and curves to the anal segment, which is blunt, rounded and furnished with hooks; the wing is a little swollen near the apex. The whole surface is smooth, but not very shining. The colour is pale ochreous-green; the abdomen is darker olive-green; head and anal segment only very slightly tinged with green. The wing shades into whitish at the apex and is semi-transparent; the spiracles are brown; a dull brown medio-dorsal streak traverses the abdomen and terminates in a dark spot on the seventh abdominal segment. Such is the description when twelve days old. About three days before the emergence of the imago, the pupa begins to deepen in colour, gradually changing to a dark leaden-grey, and finally the wings assume the colouring of the imago, but of course of a paler hue; the neuration and dark marginal band of the wing in male specimens show very clearly. The pupa is apparently not attached by the anal hooks, merely being suspended by a few strands of silk spun around it to a stem of the plant close to the earth. The first imago, a male, emerged on July 10th." - Frohawk (1924)