Underside strongly recalling icarus, but male as well as female with distinct white median streak on the hindwing.

(80 d) are females with bright blue upperside, which occur singly and rarely in Europe, but are said to be a constant form in West China; this, however, is very improbable, and Leech says nothing about it.

e. eroides (Frivaldszky, 1835)] is more sky-blue than greenish blue and the largest form of the species after the following one; in Eastern Europe, particularly in Russia, the Balkan Peninsula, westwards extending to Germany, where it occurs in East Prussia and Posen, though only rarely.

— sutleja Moore [now full species Polyommatus sutleja (Moore, 1882)], from Kashmir, is extremely close to boisduvalii, having the same tint of blue, but its black margin is somewhat narrower and the black veins in the apical area of the forewing are not quite so sharply defined.

[now full species Polyommatus amor (Lang, 1884)] (80 e), from Ferghana and the Tian-shan, the blue of the upperside of the male has a violet sheen, nearly as in icarus, and the black margin is narrower.

[P. e. napaea (Grum-Grshimailo, 1891)] has likewise the same blue tint above as icarus, but the black margin is much broader, as is also the costal area of the hindwing.

[now full species Polyommatus erigone (Grum-Grshimailo, 1890)] (80 e), from the Pamir, is considerably smaller; beneath the ocelli are less prominent and the white streak of the hindwing is very distinct.

Also the female bears a superficial resemblance to icarus, whose blue colour is almost exactly the same as that of the erigone males obtained at a considerable altitude (13,000 ft.).

[now P. e. amdoensis (Wnukowsky, 1929)]from the Sinin Mts., resembles East Russian eroides, but the blue has already a slight tinge of violet, and the black margin is narrower.

The butterflies occur from June until August; they are not rare and are in the Alps among the most frequent visitors of damp places on the roads.

Upon reaching a length of up to 16 mm, the caterpillars stop feeding, seek shelter in the litter, hide under fallen leaves, which are attached to the substrate with threads.