The Long-tailed Blue butterfly can be distinguished from Lang's Short-tailed Blue by the broad white streak near the border of the underside wings.
The Long-tailed Blue above left still has its long tails and "eye-spots" but the faded one above right has lost its long tail (to a bird or to wear and tear).
Lang's Short-tailed Blue butterfly doesn't have the broad white streak near the border of the underside wings visible in the Long-tailed Blue
and, as its name suggests, has a somewhat shorter "tail".
Pair of Lang's Short-tailed Blue butterflies
Lang's male (photos above), a little similar to a dull Common Blue with eyespots and tail from the topside.
Lang's female (the one in the foreground) with an iridescent blue patch on topside of wings.