The male common blue butterfly is readily identified as it is the only one in the Highlands with iridescent blue upper wings. The blue colour contrasts with the white edging to all four wings. The under wings are grey with many tiny black and orange markings each with a white circle around them with some forming a line on the outer parts. The upper wings of the females are brown with tiny orange markings around the edge of the wings with a white outer edge. The markings on the under wings are similar to the male but the background is normally darker. The bright blue colouring of the males wings that make them glitter in sunshine is not caused by pigment. The thousands of corrugated scales on the wings absorb all colours of the spectrum except blue. In the Highlands and Islands there is also a form of the female that is as blue as the males. Not only are the upper wings clear deep blue, but the orange spots are also enlarged almost into a band and it is distinctly larger than southern specimens. Some butterfly enthusiast claim this is a distinct subspecies which is open to doubt but it has meant instances of mistaken identification. Such was the famous case on the Isle of Rum. In the 1940s a professor of botany J.W. Heslop Harrison was suspected of planting wild flowers on the island and claiming they were native. He also claimed to have taken three large blue butterflies from Glen Harris on the West side of the island. Large blues were only found in southern England and later became extinct. The ones on Rum would have been the large female common blue butterflies but it caused quite a stir at the time and many years later people were still looking for the famous “large blue” of Rum.
The common blue is one of the most widespread butterflies in the Highlands despite the fact that it suffered a decline in many places in the mid 1990s. It can be found in most parts of the mainland, the Western Isles and Orkney but apparently not on Shetland at present. One of the reasons for its success is the fact that although the main food plant of the caterpillars is birds foot trefoil there are several others including white clover and restharrow. There is growing evidence that there may be a link between what the caterpillars feed on and ants. When quite small the caterpillars drop to the ground and give off a sweet secretion that attracts ants that pick them up and carry them below ground. The caterpillars then feed on ant grubs and may hibernate underground. Above ground the ants have been known to accompany the caterpillars occasionally taking some of the secretion and actually defending the caterpillars. When a wood ant seemed to be taking on a caterpillar red ants tackled it and off it went. Although primarily a butterfly of open grassland it is frequently encountered in the glades and sunny rides of woodland. Around Inverness it can be found on any lightly grazed grasslands, open woodland and dunes such as those at Nairn. Unlike further south there is often only one generation each year with the first butterflies on the wing in early June and flying into September. The right level of grazing is an important factor as this gives a wide variety of flowers from which the adults take nectar. When myxymatosis decimated the rabbit population the common blues suffered because the vegetation grew tall and shaded out the wild flowers. On a minor scale this still happens.