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Common blue

The Common blue (Polyommatus icarus) is a small butterfly belonging to the family , characterized by its vibrant coloration: males display bright violet-blue uppersides with narrow black borders and white fringes on wings spanning 28–36 mm, while females exhibit more variable brown uppersides often dusted with blue near the base and featuring orange submarginal spots. This species is distinguished from similar blues, such as the or , by the absence of dark veins extending into the white wing fringes and its generally smaller size. Native to the , the Common blue is widespread across , from the to northern , and extends to and the , where it has been replaced by the related P. celina in some southern regions; it has also been introduced to , first recorded in , in 2005 and has since established populations spreading into the northeastern . In the , it is the most abundant and ubiquitous blue butterfly, occurring throughout , , , and but absent from the Shetland Islands, though populations have declined by 17% in abundance and 37% in distribution since the 1970s due to habitat loss and agricultural intensification, with recent monitoring showing low numbers in 2024 and average levels in 2025 amid ongoing long-term declines. Globally, it thrives in diverse habitats including sunny grassy meadows, heathlands, woodland clearings, coastal dunes, road verges, waste grounds, and even urban areas like parks and cemeteries, adapting to various soil types, altitudes up to 2,700 m, and climates from warm lowlands to high alpine meadows, provided there are flowery or grassy areas supporting its larval host plants. The life cycle of the Common blue typically includes two to three broods per year in southern regions, flying from May to October in the UK and across all warm months in southern Europe, with adults nectar-feeding on flowers or occasionally sap and excrement for about three weeks. Eggs are laid singly on the underside of leaves of leguminous host plants, primarily Common bird's-foot-trefoil (Lotus corniculatus), but also including black medick, common restharrow, and various clovers; the pale green caterpillars, marked with yellow stripes, feed on these plants and overwinter as pupae in leaf litter or soil. Despite local declines, the species holds a conservation status of Least Concern on the GB Red List (2022) and is not considered threatened at the European level, though habitat management is recommended to support its populations.

Taxonomy and nomenclature

Classification and phylogeny

The common blue butterfly is classified under the binomial nomenclature Polyommatus icarus (Rottemburg, 1775). It was originally described as Papilio icarus by the German entomologist Siegmund Adrian von Rottemburg in 1775, based on specimens from . Over time, the species underwent several taxonomic reclassifications, moving from the genus to Lycaena (as Lycaena icarus) in the early 19th century, before being placed in the modern genus Polyommatus established by in 1804. Other historical synonyms include Polyommatus icarus alexis (Scopoli, 1761), reflecting early variations in species delimitation. The full taxonomic hierarchy of P. icarus is as follows: Domain Eukaryota, Kingdom Animalia, Phylum Arthropoda, Class Insecta, Order Lepidoptera, Superfamily Papilionoidea, Family Lycaenidae, Subfamily Polyommatinae, Tribe Polyommatini, Genus Polyommatus Latreille, 1804, Species P. icarus. This placement situates it within the diverse family Lycaenidae, known as gossamer-winged butterflies, which comprises over 6,000 species worldwide. Phylogenetically, P. icarus resides in the Polyommatus section of the tribe Polyommatini, which molecular analyses have confirmed as based on multi-locus datasets including mitochondrial and nuclear ITS2 sequences. The Polyommatus originated in during the , approximately 3.3–3.9 million years ago, with the Polyommatini tribe tracing back to a broader Asian cradle around 10 million years ago in the . Within the , P. icarus belongs to the subgenus Polyommatus s. str., forming a with close relatives such as P. buzulmavi as its sister species and showing deep from P. celina (over 5% at ), which occupies North African ranges. Recent molecular studies, including 2023 mtDNA analyses, reinforce the of Polyommatus s. str. and highlight the Palaearctic diversification of these lineages from Asian congeners during the to transition. The Polyommatus section is sister to the Everes section, underscoring a shared evolutionary history within Polyommatinae that involved multiple dispersals across .

Subspecies

The common blue butterfly, Polyommatus icarus, exhibits considerable intraspecific variation, leading to the recognition of numerous across its extensive Palearctic range. These differ primarily in wing coloration intensity, size, spotting patterns on the undersides, and the extent of orange lunules on the hindwings, adaptations that may reflect local ecological pressures such as and type. While the exact number varies by taxonomic authority, over a dozen have been described, with ongoing descriptions in peripheral regions like . The nominate subspecies, P. i. icarus, is the typical form distributed throughout central and northern Europe, including much of the and . Males display a vibrant violet- upperside with narrow black marginal borders and white fringes, while females are predominantly on the upperside with scaling at the wing bases and distinct orange submarginal lunules bordered by black spots on the hindwings; the underside is grayish with black spots and orange lunules in both sexes. This is highly , with females showing a range from largely to extensively forms depending on locality and generation. In Mediterranean regions, such as and adjacent areas, P. i. zelleri represents a common form occurring from to about 1500 m . It closely resembles the nominate but features neater undersides in summer generations, with reduced blue suffusion on the uppersides and prominent marginal lunules. At higher s in the same region, from 1500 to 3000 m in the and Antilebanon mountain ranges, P. i. is found, distinguished by its slightly larger size, more pointed wings, reduced or absent lunules, and partial black spotting on the undersides; this shows no differences in genitalia or number (n=23) from P. i. zelleri, though interbreeding is rare. Recent taxonomic work has added to the known in eastern parts of the . In southwestern , P. i. lacuina was described from the Sutai Uul mountains at 1750–2400 m, where males exhibit a pale violet-blue upperside and reduced submarginal spotting on the grayish undersides, while females are darker brown with well-developed orange spots. Similarly, P. i. incoronatus occurs on the southern slopes of the Mongolian at 2000–2400 m along the Mogoin-gol river, characterized by denser violet-blue uppersides in males, larger overall size (forewing length up to 16.5 mm), and more pronounced underside patterns in females, which from violet to brownish. These additions highlight ongoing refinements in the since 2020, with no major overhauls to the core subspecies but continued exploration of Asian variants. A notable taxonomic clarification involves the separation of Polyommatus celina as a distinct from P. icarus in 2011, based on genetic and morphometric analyses showing consistent differences in North African and Island populations, including the presence of marginal spots on the upperside forewings; this revision resolved prior confusion where some North African forms were treated as of P. icarus. In introduced North American populations, first recorded in in 2005 and spreading southward, no distinct status has been proposed, though monitoring continues for potential local adaptations.

Etymology

The scientific name of the common blue butterfly, Polyommatus icarus, originates from classical Greek roots. The genus name Polyommatus combines "poly-" (many) and "omma" (eye), referring to the numerous eyespot markings on the undersides of the wings typical of in this . The species epithet derives from , the figure in who, with wings fashioned by his father , flew toward the sun until the wax melted, causing his fall; this alludes to the vivid sky-blue coloration of the male's upperwings, evoking the heavens. The name was first applied by Samuel Albrecht von Rottemburg in his 1775 description as Papilio icarus. The common English name "common blue" is descriptive, highlighting the species' abundant distribution across and the prominent blue hue of the male's wings, distinguishing it from less prevalent blue butterflies. The species was illustrated in James Petiver's 1717 work Papilionum Britanniae as "Blue Argus", with the name "common blue" first documented later in the . In entomological texts, the binomial name often invokes the to poetically link the butterfly's flight to the legendary aspiration for the sky. Beyond these, the species lacks prominent folk names in English traditions but has regional variants in other languages, such as "azuré commun" in , emphasizing its azure appearance and commonality.

Morphology

Adult characteristics

The adult common blue , Polyommatus icarus, exhibits a ranging from 28 to 36 mm. This small size contributes to its agile, low flight over vegetation. Males display striking on the upperside of their wings, featuring an iridescent lilac-blue coloration with narrow black borders and white fringes on the wings. This blue hue arises primarily from in the wing scales, enhanced by androconia—specialized scent-disseminating scales arranged in rows that contribute to the overall vibrancy. The underside of male wings is typically light grayish with a series of black spots outlined in white and orange-red lunules near the margins, providing subtle when at rest. In contrast, females show a more subdued upperside, generally brown with variable dusting at the base and a row of marginal spots, though the extent of blue scaling can range from minimal to extensive depending on local populations. Their underside mirrors the male's pattern but on a browner ground color with more pronounced black spotting, aiding in blending with earthy substrates. This dimorphism—brighter in males for mate attraction and duller tones in females for —reflects adaptive differences in reproductive strategies within the species. Seasonal variations occur, with summer brood adults often smaller and displaying more vibrant coloration compared to spring emergents, influenced by environmental conditions during development. Subspecies exhibit subtle differences, such as bluer females in northern populations like P. i. mariscolore.

Immature stages

The eggs of the Common blue (Polyommatus icarus) are semi-spherical in shape, measuring 0.61–0.74 mm in diameter, and appear white to pale green upon deposition. They feature a flattened appearance with an intricate microsculpture on the surface, including spiny tubercles and a smooth annular zone around the micropyle. These eggs are laid singly, typically on the upper surface of host plant leaves such as Bird’s-foot Trefoil (Lotus corniculatus). The larvae exhibit a slug-like body form characteristic of many lycaenids, progressing through five instars. Early instars are pale green with a dark dorsal line and sparse hairs, reaching up to 12 mm in length when fully grown. Later instars shift to a yellowish-green hue accented by yellow stripes and black spots, maintaining a compact, tapered shape. Locomotion is facilitated by short antennae for sensory detection and three pairs of thoracic legs paired with five pairs of abdominal prolegs equipped with crochets for gripping foliage. Specialized structures include tentacle organs and pore cupola organs on the integument, along with a Newcomer's gland, which are typical for lycaenid larvae adapted to plant feeding and potential ant associations. The measures 9–11.9 mm in length and adopts a chrysalis shape, secured by strands and a cremaster hook to the . It displays an green to brown coloration with a speckled and subtle metallic sheen, often formed at the base of host plants. A stridulatory is present on the pupal , enabling sound production potentially for communication.

Distribution and habitat

Native and introduced range

The common blue (Polyommatus icarus) is native throughout much of the Palearctic region, occurring widely across from the in the west to in the east, as well as in , the , and extending eastward to northern and ; it is replaced by the related P. celina in some southern regions such as parts of the Mediterranean. This butterfly inhabits elevations ranging from to 2,700 m, including high meadows. In the , it is particularly common in , , and the , while remaining widespread but less abundant in Ireland. Introduced populations were first recorded in , , in 2005 near Mirabel Airport, from where they have spread to the area; sightings in in 2021 suggest ongoing expansion, with potential establishment at additional North American sites. Range trends indicate a contraction of approximately 15% across since the 1970s, though 2023 analyses of garden monitoring data reveal stable or increasing abundances in urban green spaces.

Habitat preferences

The common blue butterfly, Polyommatus icarus, primarily occupies open, sunny habitats that support its thermoregulatory needs and reproductive cycle. It thrives in grasslands, meadows, chalk downlands, coastal dunes, woodland edges, road verges, and disturbed areas such as waste ground, disused quarries, golf courses, and urban cemeteries. These sites typically feature short turf and sheltered microhabitats that allow for basking and efficient flight, with the species favoring flower-rich swards in warm, exposed conditions. Microhabitat preferences include a broad tolerance for soil types, ranging from acidic grasslands to neutral and substrates, provided there is availability of suitable for oviposition and larval development. The butterfly exhibits notable adaptability to human-modified environments, including gardens and roadside verges, where it can persist in fragmented or low-diversity patches as long as sunny exposures are maintained. In terms of altitudinal and climatic conditions, P. icarus ranges from to high meadows in temperate zones across and beyond, reflecting its versatility in elevation. Optimal activity occurs at air temperatures of 20–30°C, enabling thoracic temperatures around 30°C for sustained flight and , though it can endure cooler or warmer fluctuations in diverse microclimates. Recent monitoring in the demonstrates the species' increasing adaptation to urban gardens, with occupancy rising to 29% of surveyed sites in 2023 from 27% in 2022, underscoring its resilience in habitats amid broader landscape changes.

Threats and conservation

The common blue (Polyommatus icarus) has undergone substantial historical declines across its range. In the , populations have decreased by an estimated 74% since 1901, largely attributable to and loss in areas. Europe-wide, the species has experienced a 15% contraction in distribution since the , reflecting broader trends in assemblages. Recent EU studies (as of 2023) indicate ongoing declines of approximately 50% in abundances between 1991 and 2023, though more resilient species like the common blue exhibit stable or positive trends. Key threats driving these declines include habitat loss from agricultural intensification, which eliminates or degrades the herb-rich grasslands essential for larval host plants. exposure, particularly to neonicotinoids such as at field-relevant concentrations, impairs larval development and survival, exacerbating vulnerability in agricultural margins. further compounds these risks by altering temperature regimes and prompting habitat shifts, with a 2025 Yale-led study highlighting severe erosion of up to 64% of butterfly temperature niches in tropical montane regions by 2070, posing risks to montane species including temperate ones like the common blue through upward shifts in suitable elevations. In introduced ranges, such as , 2021 Canadian research documents the species' to disturbed and roadside habitats, signaling potential invasive spread facilitated by widespread host plants. Conservation initiatives aim to mitigate these pressures through targeted management and propagation. Although not explicitly listed under the EU , the common blue benefits indirectly from protections for priority habitats (e.g., I types 6510 and 6210), which support its populations via networks. In the UK, Butterfly Conservation runs programs focused on meadow restoration, creating flower-rich to bolster breeding sites for the and reverse local extinctions. Captive breeding protocols have advanced significantly, with 2023 research outlining scalable, environmentally friendly methods for laboratory rearing and reintroduction, enabling production of thousands of individuals annually for supplementation. Current assessments reveal a mixed status, with stability in certain adapted populations but ongoing vulnerability elsewhere. In the UK, garden and urban settings show relative stability or increases for common blue abundances from 2007 to 2020, driven by nectar provision and reduced mowing. Conversely, core grassland habitats remain precarious, as evidenced by the near-stable but marginally declining trends (-0.7% annually) in the 2025 German Grassland Butterfly Index, underscoring the need for intensified restoration in agricultural landscapes. These efforts particularly target affected grassland types, such as unimproved meadows, to enhance and .

Ecology

Food resources

The larvae of Polyommatus icarus primarily feed on plants in the Fabaceae family, utilizing over 20 recorded species as hosts, including Lotus corniculatus (bird's-foot trefoil), Trifolium repens (white clover), Trifolium pratense (red clover), Medicago sativa (alfalfa), Coronilla varia, and Vicia villosa. They show a preference for inflorescences and tender young shoots of these plants, which provide optimal nutrition for development. Adults are nectar feeders, drawing from a broad array of flowering plants across multiple families, such as thistles ( spp.), knapweeds ( spp.), clovers ( spp.), and occasionally heathers ( spp.). Males often engage in puddling behavior, congregating on damp or to extract essential minerals like sodium, which they transfer to females during via spermatophores. Flavonoids from larval host plants serve as chemical cues influencing host selection and are actively sequestered by larvae for defensive purposes, with isoflavones (common in Fabaceae) incorporated into tissues to deter predators. These compounds, such as quercetin and kaempferol derivatives, are metabolized and allocated primarily to the wings in adults, where they absorb ultraviolet light and contribute to coloration patterns that enhance mate attraction. Studies from the early 2000s demonstrate that sequestration efficiency varies by host plant quality, with higher uptake from inflorescences of Medicago sativa and Trifolium spp. compared to foliage. The exhibits polyphagous tendencies, accepting a wide range of hosts, though specificity can vary regionally—for instance, fewer species are utilized in cooler, damper areas like compared to [continental Europe](/page/continental Europe). This flexibility allows to local floral availability while prioritizing nutrient-rich options for optimal larval performance.

Reproductive behavior

The common blue butterfly, Polyommatus icarus, employs a in which males defend territories and actively patrol habitats to locate receptive females. Males exhibit territorial behavior, perching in prominent positions or engaging in patrolling flights to intercept females, often leading to aggressive interactions with rival males. Pheromones play a key role in mate location, with unfertilized females releasing scents detectable by males' olfactory organs from considerable distances, while males may also secrete pheromones during encounters to facilitate . Courtship displays are minimal or absent in many observations, with mating often occurring immediately upon encounter without elaborate rituals; however, males may approach females with rapid wing fluttering to signal interest. Copulation is prolonged, typically lasting up to an hour or more, during which the male transfers a spermatophore to the female. Post-copulation, females become unreceptive to further matings, while males continue to seek additional partners. Following mating, females engage in oviposition by laying eggs singly on the leaves or stems of host plants, primarily species in the family such as . Each female produces approximately 100-150 eggs over her lifespan, with site selection influenced by plant quality, including the presence of flowers for nectar and chemical cues like cyanogenic glycosides that signal suitability for larval development. Eggs are preferentially placed in sunny, exposed microhabitats to optimize conditions for hatching and early larval . Parental care is absent after oviposition, though some larvae later form mutualistic associations with for protection.

Life cycle

The common blue butterfly, Polyommatus icarus, exhibits a multivoltine life cycle that varies by latitude and climate, typically producing two to three generations per year in southern and central Europe, while northern populations, such as those in the UK and Scandinavia, often complete only one or two broods annually. In bivoltine regions, the first generation emerges in spring following overwintering, and the second develops directly through the summer; a third partial brood may occur in warmer southern areas under favorable conditions. The total developmental cycle for each non-overwintering generation spans approximately 4-6 weeks, synchronized with the availability of host plants in legume-rich habitats. The cycle begins with the egg stage, where females lay small, white, flattened eggs singly on the upper surfaces of host plant leaves, such as those of Lotus or Trifolium species. Eggs incubate for 5-10 days before hatching, with the exact duration influenced by temperature and photoperiod. Upon hatching, larvae emerge as pale green, slug-like caterpillars that initially feed on leaf tissue, creating characteristic windows by consuming the mesophyll while leaving the epidermis intact. The larval stage consists of four instars and lasts 2-4 weeks of active feeding per generation, during which the caterpillars grow rapidly and may interact with tending ants for protection. External factors, such as parasitoids and predators, can significantly impact larval growth and survival rates. Overwintering occurs as half-grown larvae in the second or third , typically from late autumn until , when the is broken by increasing temperatures and day length. These hibernating larvae remain dormant at the base of host plants or in leaf litter, resuming feeding in early to complete . Following the final larval , pupation takes place in a silk-lined chamber on or near the ground, lasting 9-14 days; this non-feeding stage is sensitive to temperature, with triggered by warmer conditions. Adults emerge synchronized with peak host plant flowering, exhibiting a lifespan of 2-3 weeks focused primarily on and oviposition. Generational patterns show variation in adult size, with the spring brood (overwintering generation) producing larger individuals due to extended growth periods across seasons, while the autumn brood (direct-developing) results in smaller adults owing to compressed development time in late summer. This allows P. icarus to adapt to regional climate differences, ensuring across its range.

Physiology and behavior

Sensory systems

The common blue butterfly, Polyommatus icarus, exhibits trichromatic with sensitivity to (UV), , and wavelengths, mediated by distinct proteins in its compound eyes. The photoreceptors include UV-sensitive (UVRh, λ_max 360 nm), -sensitive (BRh1, λ_max 437 nm), and a duplicated (BRh2, λ_max 500 nm) that functions alongside the long-wavelength receptor (LWRh, λ_max 568 nm) to extend sensitivity into the spectrum up to approximately 560 nm. This duplication allows for effective , which is absent in many other that lack such an adaptation, and supports behavioral tasks such as identifying suitable flowers for feeding and detecting conspecific mates through coloration contrasts. The compound eyes of P. icarus are composed of ommatidia organized into heterogeneous classes, with ventral regions featuring red-reflecting ommatidia and filtering pigments that enhance contrast sensitivity without enabling red vision beyond 570 nm. Recent analyses of lepidopteran visual systems highlight how this duplication improves detection of green contrasts in habitats, where the butterfly's primary and mating occur amid vegetation with varying green hues.

Movement and flight

The common blue butterfly, Polyommatus icarus, exhibits a weak, fluttering flight style characterized by low-altitude movements typically below 2 meters above the ground, which facilitates its interactions with low-growing vegetation and host plants. This pattern reflects an energy-conserving strategy suited to its habitats, where prolonged flights are minimized. For , adults adopt basking postures with wings partially open or closed, absorbing solar radiation to maintain thoracic temperatures necessary for activity, particularly in cooler conditions. Dispersal in P. icarus is generally limited, with average lifetime movements ranging from 100 to 500 meters, though males may travel farther, up to 1 km in some cases, driven by mate-searching behaviors. Studies indicate mean displacements of 165–191 meters in native populations. In introduced North populations, such as those in urban , , dispersal patterns are similar but influenced by features; a 2024 mark-recapture study recorded mean displacements of 140-150 meters for both sexes, with maximums exceeding 1 km, and rare long-distance events over 3 km potentially aiding range expansion. No significant sex-based differences in overall dispersal likelihood were observed. Behaviorally, P. icarus engages in patrolling flights by males, who actively search for females near the ground in suitable habitats, while hill-topping is rare and not a primary strategy. Local movements are closely tied to resource availability, such as nectar sources and oviposition sites, rather than true , resulting in sedentary populations with discrete colonies. In introduced ranges, wind—both natural and vehicle-induced—assists spread by carrying individuals across barriers like roads, contributing to rapid colonization of urban areas.

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