Overview
The Common Blue (Polyommatus icarus) is the most widespread, abundant, and cherished gossamer-winged butterfly (family Lycaenidae) across Europe and temperate Asia. Flitting gracefully over wildflower-rich meadows, chalk downlands, and coastal dunes throughout the summer months, this radiant miniature lepidopteran brings flashes of dazzling, sky-blue brilliance to sunny grasslands from the Atlantic to the Pacific.
Ecologically, the common blue is an indispensable grassland pollinator and an evolutionary marvel of insect mutualism. Its specialized caterpillars engage in an obligate, protective symbiotic partnership with meadow ants (genus Lasius and Myrmica), producing sweet, sugary secretions in exchange for round-the-clock armed defense against parasitic wasps. Widely celebrated as the quintessential British and European meadow butterfly, it serves as a sensitive indicator of unimproved grassland health.
AT A GLANCE
- Common Name: Common Blue
- Scientific Name: Polyommatus icarus
- Category: Invertebrates
- Subcategory: Insects
- Size: Wingspan 28 to 36 mm (1.1 to 1.4 in); body length 12 to 16 mm (0.5 to 0.6 in)
- Weight: 0.08 to 0.15 g (0.003 to 0.005 oz)
- Lifespan: 8 to 10 months from egg through overwintering caterpillar; adult butterfly lives 2 to 3 weeks
- Diet: Caterpillars feed strictly on bird’s-foot trefoil (Lotus corniculatus) and other fabaceous legumes; adults drink nectar from trefoils, clovers, marjoram, and thistles
- Habitat: Chalk downlands, sunny coastal dunes, rough pastures, roadside verges, wildflower meadows, and railway embankments
- Range: Widespread across the Palearctic realm, from the British Isles and western Europe across North Africa, temperate Asia, Russia, and northern China; recently established in Quebec, Canada
- Conservation Status: Least Concern (IUCN Red List)
Biology and Physical Characteristics
Polyommatus icarus displays spectacular, high-contrast sexual dimorphism. The male is breathtaking: the upper surface of his wings is an intense, radiant, shimmering violet-blue or iridescent sky-blue, bordered by a razor-thin black margin and a fringe of pure, snowy-white hair scales. The female is remarkably variable but typically dark chocolate-brown above, dusted with variable amounts of blue scales near the wing bases and decorated with a neat, festive border of bright orange submarginal crescents along both fore- and hindwings. The undersides of the wings in both sexes are a delicate masterpiece of camouflage: a warm greyish-fawn ground color adorned with an intricate pattern of black dots encircled by crisp white halos, and a brilliant band of orange spots along the margins. A key taxonomic diagnostic separating it from other blue butterflies is the presence of one or two small black dots in the cell on the underside of the forewing.
Behavior and Ecology
Common blues are sun-worshipping, diurnal fliers that become active only when meadow temperatures rise. Males are territorial patrollers, darting across the sward to intercept passing females and chasing rival males in dizzying, vertical spirals. Feeding adults favor purple and yellow meadow flowers, perching with wings closed to display their cryptic underwings. Courtship is gentle: the male lands beside a female, vibrating his wings to waft sweet, chocolate-scented courtship pheromones released from specialized androconial wing scales. The female lays delicate, white, disc-shaped eggs resembling tiny sea urchins on the leaves of bird’s-foot trefoil (*Lotus corniculatus*). The slug-like green caterpillar possesses specialized dorsal glands that secrete a nutritious, amino-acid-rich nectar that ants eagerly harvest; in return, the ants fiercely guard the caterpillar from spiders and parasitic ichneumon wasps, even burying the pupa in subterranean ant nests for winter protection.
Conservation and Human Impact
The Common Blue is evaluated as Least Concern on the IUCN Red List, boasting an enormous, stable, and highly adaptable population across its vast Palearctic range. In the 2000s, it successfully established an exotic wild population in the Montreal region of Quebec, Canada, marking its first wild footprint in North America. While globally secure, localized populations in Western Europe suffer when wildflower meadows are converted to monoculture agricultural turf or subjected to intensive mowing during peak summer flight periods.
Interesting Factoid
The caterpillar of the Common Blue butterfly hires an army of ants as personal bodyguards, paying them with sweet, sugary liquid oozing from special glands on its back, prompting the ants to fiercely defend the caterpillar from predatory wasps and even tuck it into their underground ant nest for bed.