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Common blue
Male
Female both in Oxfordshire
Scientific classification
Kingdom:	Animalia
Phylum:	Euarthropoda
Class:	Insecta
Order:	Lepidoptera
Family:	Lycaenidae
Genus:	Polyommatus
Species:	P. icarus
Binomial name
Polyommatus icarus (Rottemburg, 1775)

The common blue butterfly (Polyommatus icarus) is a butterfly in the family Lycaenidae and subfamily Polyommatinae. The butterfly can be found in Europe, North Africa, and the Canary Islands, but it is especially common throughout the British Isles. Recently, however, there has been an estimated 96% population loss for the common blue due to habitat loss.^[1] The butterfly's name comes from the coloring of the wings. The males usually have blue wings with a black-brown border and a white fringe at the edge. The females are usually brown with a little blue dusting and fewer orange spots on the lower parts of the wings.^[2]

Mating in Buckinghamshire
male left; female right

Taxonomy and phylogeny

This species was first defined in Rottemburg (1775). The subspecies was first defined in Kane (1893). Other vernacular names that have been given to P. Icarus include: Little Blew Argus, Blew Argus, Mixed Argus, Selvedg’d Argus, Utramarine Blue, Common Blue, Caerulean Butterfly, and Alexis.

Subspecies

Subspecies include:^[3]

• 
  P. i. icarus (Europe, Caucasus, Transcaucasia)


• 
  P. i. mariscolore (Kane, 1893) (Ireland)


• 
  P. i. fuchsi (Sheljuzhko, 1928) (South Siberia, Transbaikalia)


• 
  P. i. omelkoi Dubatolov & Korshunov, 1995 (Amur, Ussuri)


• 
  P. i. ammosovi (Kurenzov, 1970) (Central Yakutia, Far East, Kamchatka)


• 
  P. i. fugitiva (Butler, 1881) (Pakistan)


• 
  P. i. napaea (Grum-Grshimailo, 1891) (Tian-Shan)


• 
  P. i. zelleri Verity, 1919
  

Description

Polyommatus icarus has a wingspan of 28–36 millimetres (1.1–1.4 in).^[4] The dorsal side of the wings is an iridescent lilac blue with a thin black border. Females' wings are brown with a row of red spots along the edges of the wings and usually some blue at the base. The extent of blue and brown is extremely variable depending on location.^[2] The top of the wings may be mostly blue, especially in Ireland and Scotland, but it always has red spots. The ventral side has a grayish base color in the males and a more brownish hue in the females.^[2]

Both sexes have a row of red spots along the edge of the hindwings and extending onto the forewings, though they are generally fainter there, particularly in the males, where they are sometimes missing altogether. There are about a dozen black-centered white spots on the hindwings and nine on the forewings. These usually include one in the middle of the forewing cell, absent in Chapman's and Escher's blues. The fringes on the outer edge of the wings are uniform white, not crossed with black lines (lack checkering), as it is in the chalkhill blue and Adonis blues.^[2]

The larvae, or caterpillar is small, pale green with yellow stripes and, as usual with lycid larvae, and rather slug like.^[5]

Geographic range

The common blue butterfly is found in Europe, North Africa, the Canary Islands, and temperate Asia to Northern China. Recently it was discovered in Quebec, Canada. It is widespread in the British Isles, mainly in the countries of England, Ireland, Scotland, and Wales. Its distribution trend shows a 15% decline since the 1970s.^[1]

U.K. and Ireland

The common blue is Britain's (and probably Europe's) most common and most widespread blue. It is found as far north as Orkney and on most of the Outer Hebrides. A range of grassland habitats are used: meadows, coastal dunes, woodland clearings, and also many man-made habitats, anywhere their food plants are found.^[1]

North America

It is a recently introduced species in eastern Canada.^[6][7] The common blue butterfly was discovered in Mirabel, Quebec, Canada, by Ara Sarafian, an amateur entomologist who observed the butterfly from 2005 to 2008. He contacted the Canadian National Collection of Insects in Ottawa where the butterfly was identified as Polyommatus icarus, a new alien butterfly to Canada and to North America. The butterfly seems to be well established and is extending its range from year to year.^[8] In 2016, pictures were taken of a male butterfly in the forest near Kamloops, British Columbia.

Habitat

These butterflies inhabit flowery or grassy places, warm and cool, open or wooded areas and at all altitudes up to high alpine meadows at an elevation of 0–2,700 metres (0–8,858 ft) above sea level.^[4][2] It mostly resides on chalk or limestone grassland, but also in smaller numbers in woodland clearings, meadows, heathlands, sand dunes, along railway embankments, and under cliffs.^[1]

Source of decline

Previously, P. icarus was a very common species that occupied Europe and Asia, and was one of the most widely distributed butterflies in Britain. They are known to be tolerant of many habitats, including a wide range of grassland habitats. There has been an estimated 96% loss of the butterfly population, which could be due to the fact that 46% of the total land area it occupies has been lost since 1901. Host plant decline has also contributed to the loss of P.icarus population. The host plant Lotus corniculatus that provided habitat for P. icarus has lost 46% of the total area.^[1] This host plant is a favored plant for two reasons: it provides adult nutrition, as well as food for the larvae after it hatches.^[9]

Food resources

Larval food plants

The larvae feed on plants from the family Leguminosae (bean family). Recorded food plants are Lathyrus species, Vicia species, Vicia cracca, Oxytropis campestris, bird's foot trefoil (Lotus corniculatus), Oxytropis pyrenaica, Astragalus aristatus, Astragalus onobrychis, Astragalus pinetorum, black medick (Medicago lupulina), Medicago romanica, Medicago falcata, common restharrow (Ononis repens), wild thyme Thymus serpyllum, lesser trefoil (Trifolium dubium), Trifolium pratense and white clover (Trifolium repens).^[3][10]

Flavonoids

Common blues sequester flavonoids from their host plants and allocate these pigments that are UV-absorbing into their wings. These flavonoid pigments in females attract males to them more. Males who patrol areas of suitable habitats while searching for virgin females stop and inspect females who have flavonoid pigments in them. This may be due to the fact that flavonoid pigments that have UV absorption increase color saturation on females and allow females to be more conspicuous.^[11] There are also some other advantages of sequestering flavonoids, including the protection of eggs from adverse UV chemical reactions, as the butterflies will absorb the UV rays, and the flavonoids can offer a chemical defense against predators or pathogens.

Flavonoid sequestration is much more effective when coming from natural host plants than from experimentally offered diets. Females sequester about 60% more flavonoids than do males. This richness in females may increase visibility, but could also confer information about feeding history, and consequentially the quality of potential mate.^[12] Flavonoid sequestration is an important component of intraspecific visual communication and sexual signaling in polyommatus butterflies.

Parental care

Oviposition

During oviposition, females must locate a potential host plant and evaluate its suitability as the host plant for oviposition. P. icarus uses visual cues to conduct this task. Females use several plants in the family fabaceae as larval host plants, many which could also potentially function as nectar sources. P. icarus prefers plants with flowers over plants without, and prefer to oviposit near the flowers.^[9]

Life cycle

The common blue goes through two life cycles per year. The first brood usually emerges in May and lives until mid-June. The second brood emerges in late July or early August and lives until September.

• Diet as caterpillars: leaves of plants


• Diet as butterflies: wildflower nectar, excrement


• Lifespan: 3 weeks as a butterfly

Males are often very obvious as they defend territories against rivals and seek out the more reclusive females. In the south of Britain there are two broods a year, flying in May and June and again in August and September.^[4] Northern England has one brood, flying between June and September. In a year with a long warm season, there is sometimes a partial third brood in the south flying into October.

Eggs

The egg stage lasts for around eight days. The eggs are white and shaped like flattened spheres. The eggs are very small, about 0.60 millimetres (¹⁄₃₂ in).^[13] The ground color of the egg sac is pale-greenish grey, with the actual arrangement being white. Eggs are laid singly on young shoots of the food plant.

Larvae

The larvae emerge around a week or two after eggs are laid. The larvae of P. icarus feed on the underside of leaves, causing botching. Hibernation occurs as a half-grown larvae.^[14] They are attractive to ants of genera Myrmica, Lasius, Formica, Plagiolepiss ^[10], but not as much as some other species of blues. The chrysalis is olive green/brown and formed on the ground, where it is attended by ants of genera Myrmica, Lasius, Formica, Plagiolepiss, ^[10] which will often take it into their nests. The larva creates a substance called honeydew, which the ants eat while the butterfly lives in the ant hill. The relationship between these ants and blue common larvae is described to be facultatively mutualistic.^[15][14]

Instars of larvae

There are five instars of larvae. During the first instar, larva emerge and eat away the crown of the egg. The segments of the larvae are large and rounded, and the following segments are smaller. The body is a light green, and is whitish in light. This stage lasts about nine days.^[13] The second instar, after the first moult, adds a couple of body segments onto the larvae, and the body is more green. The third instar signifies more feeding and growing in size of the larvae. They are about 3.2 millimetres (0.13 in) long and the head and legs are black colored. In the fourth and fifth instar, the larvae becomes very green, has ten body segments, and measures about 13 millimetres (0.51 in) in length.

External effects on larval growth and development

Larval growth rates are thought to be determined mainly by temperature and food quality and availability. The larvae of P. icarus are oligophagous, meaning they utilize a range of host plants in the family fabacaceae, as well as have a mutualistic relationship with ants. For both male and female larvae, the total development time is longer with longer day lengths, corresponding to earlier times in the season. When larvae are born earlier in the season, they take a longer time to develop. When they are born later, they take a shorter time to develop. Thus the external effect of photoperiod affects the length of development for the larvae.^[14]

Pupae

Pupation, a stage that lasts about two weeks, occurs under silk strands at the base of the food plant. The chrysalis is olive green/brown and formed on the ground, where it is attended by ants. Ants may aid in protecting the pupa and may bury it to protect it from predators.

Adults

The male, with a bluer color, is more conspicuous than the female, which has brown upperwings. Males fly farther distances in search of territories that have fertile females. Females fly lower and search for nectar and places to lay her eggs. Wing span ranges from 29–36 millimetres (1.1–1.4 in). When the sexes meet, copulation occurs immediately, usually without any courtship ritual.

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  Mating
  

  
  

  
  


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  Egg
  

  
  

  
  


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  Male (underside)
  

  
  

  
  


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  Female (underside)
  

  
  

  
  

Physiology

Vision

Visual systems in butterflies are highly diverse and their color vision abilities have only begun to be explored. To see color, P. icarus uses a duplicated blue opsin in conjunction with its long-wavelength opsin LWRh. This enables the common blue to see color in the green part of the light spectrum extending up to 560 nm (2.2×10⁻⁵ in). There is also a difference between the dorsal and ventral eye-shine of P. icarus, with the dorsal retina dominated by yellow-reflecting ommatidia and the ventral exhibiting yellow and red-reflecting ommatidia. P. icarus is able to use color vision and distinguish between yellow of 590 nm (2.3×10⁻⁵ in) and blue of 430 nm (1.7×10⁻⁵ in), but are not able to distinguish between yellow and red of 640 nm.^[16]

See also

Wikispecies has information related to Common blue

• Butterflies of Asia


• Butterflies of Europe


• List of butterflies of Great Britain

References