The Western Tailed-blue (Cupido amyntula) is a member of the Lycaenidae

Lycaenidae

The Lycaenidae are the second-largest family of butterflies, with about 6000 species worldwide, whose members are also called gossamer-winged butterflies...

 family and is seen across western North America

North America

North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

 as far norther as Alaska

Alaska

Alaska is the largest state in the United States by area. It is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait...

. The upperside of the male butterfly is blue while the female has a darker brown band on the outer side of the wing. The underside is riddle with black spots, with a wingspan of 2.2 to 2.9 cm. The larvae feed on Thermopsis

Thermopsis

Thermopsis is a genus of 8 to 23 species of legumes, native to temperate North America and east Asia. They are herbaceous perennials and are known as goldenbanners.-Species:* Thermopsis alpina...

, Astragalus

Astragalus

Astragalus is a large genus of about 3,000 species of herbs and small shrubs, belonging to the legume family Fabaceae, subfamily Faboideae. The genus is native to temperate regions of the Northern Hemisphere...

, Oxytropis

Oxytropis

Oxytropis is a genus of plants in the legume family. It is one of two genera of plants known as locoweeds, and are notorious for being toxic to grazing animals. The other locoweed genus is the closely related Astragalus. There are about 300 species native to Eurasia and North America. Several...

, Vicia

Vicia

Vicia is a genus of about 140 species of flowering plants commonly known as vetches. It is in the legume family . Member species are native to Europe, North America, South America, Asia and Africa. Some other genera of their subfamily Faboideae also have names containing "vetch", for example the...

, and Lathyrus

Lathyrus

Lathyrus is a genus of flowering plant species known as sweet peas and vetchlings. Lathyrus is in the legume family Fabaceae and contains approximately 160 species. They are native to temperate areas, with a breakdown of 52 species in Europe, 30 species in North America, 78 in Asia, 24 in...

.