Common Blue Butterfly

Polyommatus icarus — the common blue butterfly — is the most familiar 'blue' butterfly in Europe and one of about 6,000 species in family Lycaenidae (the gossamer-winged butterflies). The species is widespread across Europe, North Africa, and parts of western and central Asia, and has been introduced to several other regions. Adults are small (28-35 mm wingspan); males have brilliant iridescent sky-blue wings with thin black borders, females have brown wings with orange marginal spots. Caterpillars feed on legumes (Fabaceae) — primarily bird's-foot trefoil (Lotus corniculatus), white clover, and rest-harrow. The species' most extraordinary biology — characteristic of much of family Lycaenidae — is myrmecophily (ant association). About 75% of Lycaenidae species worldwide have some level of mutualistic or parasitic relationship with ants. The basic Lycaenidae caterpillar architecture includes a 'dorsal nectary organ' on the abdomen that secretes a sugar-and-amino-acid honeydew, plus paired 'tentacle organs' that release attractant pheromones. Ants are attracted by the chemical signals, drink the honeydew, and in exchange protect the caterpillar from parasitoid wasps and predator insects. Common blue caterpillars are facultatively ant-tended (the relationship is helpful but not obligate). Other Lycaenidae have evolved much more extreme strategies. Maculinea ('large blue') species are SOCIAL PARASITES of ants — caterpillars are picked up by Myrmica ants and carried into the ant nest, where the caterpillar lives for months consuming ant brood while being treated as a colony member. The Maculinea-Myrmica-Thymus mutualistic-parasitic-host triangle is one of the most-studied relationships in evolutionary biology, and the British Maculinea arion was famously extirpated, then successfully reintroduced after the ecology was understood. The common blue is a flagship pollinator and meadow indicator species across Europe.