Silver-studded blue


The silver-studded blue is a butterfly in the family Lycaenidae. This eye-catching butterfly has bright blue wings rimmed in black with white edges and silver spots on its hindwings, lending it the name of the silver-studded blue. P. argus can be found across Europe and Asia, but is most often studied in the United Kingdom in which the species has experienced a severe decline in population due to habitat loss and fragmentation.
Importantly, P. argus engages in mutualism with ants that contribute to the butterflies’ reproductive fitness by providing protection from predation and parasitism from the point of egg laying to their emergence as adults. P. argus adults emerge in the end of June and beginning of July and engage in flight into the beginning of August.
The butterfly is adaptable to different habitats and is found in heathland, mossland, and limestone grassland. Tending towards a sedentary lifestyle and typically flying less than twenty meters a day, P. argus maintains a small radius home range. Their habitats lend themselves well to both foraging and egg laying as the host plants are ubiquitous in all three environments they occupy.

Appearance

Adult

Male P. argus have royal blue wings with a black border, white, wispy fringe, and metallic silver spots on the hindwings as well as spurs on their front legs. Females of this species are generally brown and more subdued in color, but also have the metallic spots on the hindwings. The undersides of the male and female butterflies are very similar. They are taupe in color, with rings of black spots along the edge of the wing.

Description in Seitz

L. argus L. . This and the following species have in the anal area of the hindwing beneath on the black submarginal dots situated between the median branches and the submedian vein some metallic glittering scales which are usually not represented in our figures. The upperside is deep dark blue, with a broad black border, argus is very similar to argyrognomon, but differs in quite a number of details, of which in the various individuals sometimes one sometimes another is more conspicuously developed. On the upperside the blue gloss is deeper, darker, duller, with a slight violet tint and not so far extended to the margm, the latter being broader and appearing blacker; the row of dark submarginal dots which occasionally appear on the hindwing above in both species, therefore stands always in the black border in argus. On the underside both sexes of argus have nearly always a strongly glittering blue-green dusting at the base of the wings, which is usually absent from argyrognomon; but we must add that also the inverse occurs, the blue basal dusting being absent from argus in exceptional cases and strongly marked in argyrognomon. As a rule argus L., moreover, is somewhat smaller than argyrognomon, but the size varies considerably according to the altitude of the locality and the various broods.
— ab. valesiana M.-Durr is frequently found among specimens from the Valais ; it is distinguished by a yellowish grey underside and very small ocelli. ab. corsica Bell., on the other hand, is a form which flies regularly in the mountains of Corsica and differs in the ocelli of the underside bearing pupils of the ground-colour and having larger borders, which almost form pale rings on a grey-brown ground. This form occurs as an exception everywhere, having been obtained by Standen even in Norway. Females which are nearly as blue above as the males are not very rare in certain localities, especially among the summer-broad, this is ab. coerulescens Peters. — A constant form is hypochiona Rbr., from the most southern very sunny districts of Europe ; underside rather pale, the black eye-dots small and placed in large pale halos, before the margin a row of bright red spots. Similar individuals are occasionally also found in other districts, e. g. at Digne and according to Jones even in
England. — A very large form is insularis Leech, from the north-island of Japan; its underside resembles hypochiona in its light pure ground-colour, but bears abundant and prominent black spots. Above the marginal spots of the hindwing are especially very distinct. — Also bella H.-Sch. is placed by Staudinger and Rebel as a variety of argus; it is a small form, which resembles hypochiona beneath, but is very dark above, with very thin red-yellow submarginal spots on the upperside of the hindwing, which are unfortunately not visible in our figure 78 d, the hindwing beneath bearing paler marginal spots; from the shores of the Black Sea, Armenia and Persia. — Perhaps also iburiensis Btlr. belongs here; it is unknown to me in nature and was described from one specimen from Hokkaido; it appears to be a kind of albino of insularis. — Very small specimens, as they are found especially in Valais, are alpina Berce; exceptionally large specimens were obtained by Chapman at Bejar in Spain and named by him bejarensis . — The form micrargus Btlr. from Tokio, which is quite unknown to me, is said to have the lilac colour of argia, with the black margin dentate and the fringes very bright white; beneath in colour like pylaon, the spots as in argus, but without the metallic scales on the anal spots. — pseudaegon Btlr. , from the north-island, is smaller than argus, with the black margin narrower, the white fringes thinner, and the black distal marginal spots of the hindwing above smaller, female very feebly dusted with steel-blue at the base, underside more extended green than in true argus and its ocelli smaller.Likewise unknown to me.— Sometimes there occurs a basal dot on the underside; this is ab. unipuncta Mousley. In ab. coeruleocuneata Ebert there appear blue wedge-shaped submarginal spots on the upperside of the hindwing. — Larva very variable, usually light green with dark dorsal spots, sometimes however grey or dark brown with pale spots. In June and the autumn on various plants, such as Coronilla, Trifolium, Colutea, and Genista, said to occur also on Erica, Ornithopus, Fragaria, Thymus and other plants. Near or upon the larva there is a guard of ants, usually specimens of Formica cinerea Mayr. The larva as well as the chrysalis are found in the nests of this ant. Pupa elongate, green with red dorsal line; abdomen above yellowish green. The butterflies are on the wing in May and again from July onwards, but are said to have only one brood in the north. They are common everywhere, especially on sandy soil, which also its ant prefers, but is said to noticeably decrease recently in numbers in some districts. The form alpina I found in large numbers in Valais below Zermatt towards evening in stony localities where they were at rest in small bushes ; I counted 38 sleeping specimens in a small thornbush, all having the small size of the form of that neighbourhood.

Larvae

The caterpillar of P. argus is green with a dark stripe along the length of its body and can reach 1.3 centimeters in length.

Sexual dimorphism

P. argus exhibit sexual dimorphism, as evidenced by the color of their wings. This eventually acts as an important visual cue when searching for suitable mates. Experiments have shown that species that have overlapping habitat distribution and are of similar color have distinct absorbance values within the UV range. This shows that the UV range colors are important for butterflies when recognizing members of its own species.

Geographic range

P. argus is found across the Palearctic. In the United Kingdom, the butterfly experienced a severe decline in population during the nineteenth and twentieth centuries. P. argus is generally considered to be endangered and extinct in the Northern United Kingdom and are primarily found in the Southern and Western portions of the United Kingdom.

Habitat

P. argus resides in heathland, mossland, and limestone grassland.
Heathlands are able to meet the needs of P. argus due to burning, cutting, and other disruptions of mature heaths. With these disruptions, the habitat becomes conducive to habitation by P. argus because of the high cover of E. cinerea and short C. vulgaris that is able to form a landscape with the patches of bare ground. This is characteristic of heathland in an early stage of development. This environment is suitable until the point at which the shrubs native to the environment mature and obscure the bare ground and vegetation margins that the butterflies use for oviposition.
Mossland, similar in nature to a wet heathland, has soil primarily composed of peat which supports one of the families of host plants of P. argus, Ericaceae. This host plant grows alongside other grasses, sedges, and rushes. While the main disturbance to heathland is quarrying, mossland faces peat digging which contributes to the transient and shifting nature of this particular habitat.
The hostplants of the first two environments, Ericaceae and Leguminosae, are less present in the third environment, limestone grassland. In this environment, the host plants of P. argus are primarily herbaceous Cistaceae as well as Leguminosae. In limestone grassland, the bare ground and vegetation margins instrumental to the life cycle of P. argus are created through grazing by other animals as well as by disruption of the habitat by natural disturbance of the stoney topography of this environment.
P. argus use shrubs for roosting, resting, basking, mate location, and shelter and for this reason, they tend to be found in higher numbers close to locations that are dense in shrubs. Most of the population gathers around these shrubs during weather that is colder, cloudy, and windy. When the weather is warm, sunny, and the breeze is still, P. argus spends less time in shrub dense habitats and more time in flight and finding host plants in areas rich in calcareous heath. These areas tend to be in exposed hillsides. For this reason, it often appears that the habitat of P. argus shifts with weather conditions.
In addition to choice of habitat due to host plants and topography, P. argus density correlates with the densities of nest of the butterflies’ mutualist ants, Lasius niger and Lasius alienus.

Home range and territoriality

Adult P. argus tend to be very sedentary, only moving around twenty meters every day. For this reason, the butterflies colonize on discrete territory and patches of land. Some butterflies, though, disperse and move over a kilometer between colonies. This is rare, however, as these butterflies tend to form metapopulations.

Food resources

Caterpillars and larvae

Different types of P. argus larvae choose different host plants. The limestone larvae, caernensis, preferentially select Helianthemum species over heathers as a host plant, while heathland larvae select heathers over Helianthemum.

Adult diet

Adult P. argus eat a combination of leguminous and ericaceous plants, depending on which environment they are in. On heathland, the butterflies tend to feast on Heather and Bell Heather, while on calcareous sites, they eat Lotus corniculatus, Helianthemum nummularium, and Hippocrepis comosa.

Parental care

Oviposition

P. argus lays its eggs differently based on its environment. In heathland, they lay their eggs at the base of Erica tetralix, Cailuna vulgaris, Erica cinerea, and Ulex europaeus. In mosslands, the butterflies tend to lay their eggs on the underside of the fronds of the Bracken Fern. This fern appears to be preferential for oviposition because they attract ants. In limestone grasslands, P. argus lays eggs at the stem of Lotus corniculatus and at the base of Helianthemum chamaecistus.

Egg guarding

Egg guarding primarily occurs through mutualism with the ants Lasius niger and Lasius alienus. Mothers lay their eggs in locations with strong pheromonal traces from these organisms. From the time of oviposition to hatching, L. niger and L. alienus protect the eggs from predation and parasitism by other organisms. In return, the ants feed on a saccharine secretion produced by glands on the larvae.

Life history

Life cycle

The life cycles of Plebejus argus are divided into four main stages: eggs, larvae, pupa, and adults.

Larva

Larvae usually spend the day in the nests of the ants. This helps them avoid predation and high temperatures, especially in the hot summer months. They come out at night and climb the host plant in order to feed.

Adult

P. argus adults emerge in the end of June and beginning of July and engage in flight into the beginning of August. In these butterflies, there is a sex difference in emergence, with males emerging four to nine days earlier than females. As colony size increases, P. argus are temporally recorded earlier in the year. Additionally, as colony size increases, female emergence happens over a greater range of time and there is a greater discrepancy between emergence of the males and emergence of the females.

Subspecies

When a male detects a female sitting quiescently, he flies towards the female. As he approaches the female, he flutters his wings in broad sweeping movements and she raises her abdomen while vibrating her half-open wings in a mate refusal posture. The female then flies away and is followed by the male. Once she lands, the male flutters around, either in the air before descending or after landing. The female displays a receptive posture by folding her wings after which the male positions himself parallel to the female before bending his abdomen, spreading the valves, and exposing the copulatory apparatus. He proceeds to attach to the copulatory apparatus and the butterflies reorient themselves into the copulatory position. The intermediate aspects of sexual chase depend on the sexual receptiveness of the female to the advances of the male.

Mutualism

P. argus lays eggs near nests of Lasius niger, the ant with which they form a mutualistic relationship. This mutualistic relationship benefits the adult butterfly by reducing the need for parental investment. Once the eggs hatch, the ants chaperone the larvae, averting the attacks of predatory organisms like wasps and spiders as well as parasites. In return, the ants receive a saccharine secretion fortified with amino acids from an eversible gland on the larvae's back. As first instar larvae prepare to pupate, the ants carry the larvae into their nests. Once the larvae become pupae, the ants continue to provide protection against predation and parasitism. The butterfly leaves the nest when it emerges in June.

Conservation

Habitat loss

P. argus has undergone habitat loss and fragmentation in the United Kingdom due to the development of industrial agriculture, new forestry practices, and landscape development. Additionally, the percentage of heathland has decreased by over fifty percent in the United Kingdom, greatly affecting the butterflies for which this was a primary habitat. The heathland that remained was reduced in quality due to shifting environmental influences. Rabbits helped keep vegetation short through grazing which was conducive to habitation by P. argus, but in the middle of the twentieth century, a virus, Myxomatosis, caused a significant decline in the population of rabbits and therefore the grass length grew to a point that was no longer conducive to the butterflies.

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