Plebejus argyrognomon

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Reverdin's blue (Plebejus argyrognomon) is a small butterfly in the Lycaenidae family. It is named after Jacques-Louis Reverdin. The species lives across Central and Southern Europe, the Caucasus, the mountains of Southern Siberia, Mongolia, and along the Amur River; in Japan it can be found in agricultural areas. It prefers grassy, flowery places at 200–1,500 meters above sea level.

The wingspan is about 28–34 mm. Males have a violet-blue upperside with a thin black border and a white fringe; females are dark brown with black-bordered orange lunules along the outer edge, especially on the hindwings. The underside of both wings is light beige with several black spots surrounded by white, an orange stripe, and a row of black spots near the edge that look silvery blue-green.

Caterpillars are pale green with orange-brown stripes and grow up to about 20 mm long. This species looks similar to Plebejus argus, Plebejus idas, Plebejus pylaon and Polyommatus escheri; on the underside, argyrognomon is greyer and has smaller black spots than argus and idas.

Plebejus argyrognomon flies in two generations per year, from May to August depending on the location. The larvae feed on several plants, including Coronilla varia, Securigera varia, Trifolium repens (white clover), Melilotus officinalis, Medicago sativa (alfalfa), Lotus corniculatus, Onobrychis viciifolia, Vicia sativa and Astragalus glycyphyllos. The caterpillars are tended by a variety of ants, such as Lasius niger, Lasius alienus, Myrmica sabuleti, Myrmica scabrinodis and Camponotus vagus. The name argyrognomon comes from Latin, meaning a silver pointer on a sundial.