Common argus / Brown blue !!!
When out and about I often hear comment from inexperienced observers on the difficulty in separating the Brown Argus Aricia agestis (Lepidoptera: Lycaenidae) from the ‘brown’ form of the female Common Blue Polyommatus icarus (Lepidoptera: Lycaenidae) in both field and photographic situations.
The blue present in female P. icarus is highly variable, with individuals ranging from almost completely blue through to almost, though extremely rarely, completely brown. In icarus some degree of blue scaling is typically present on the body, around the bases of the wings, and along the hindwing margins. It is this ‘brown’ form that causes the most confusion. A. agestis has no blue scales but may, in certain light, give off a bluish-green sheen from the upperside of its wings and the hairs found on the thorax and abdomen. Upperside diagnostic features include (i) the prominent dark discoidal spot normally found in the centre of the forewings of agestis, which, if visible, is usually much reduced in icarus, (ii) typically strongly marked orange lunules along the wing edges in agestis, and (iii), the outer margin fringes of the wings which generally appear as a light two-tone fringe in icarus typically appearing more whitish in agestis. A small dark extension of the venation into the border fringe is also characteristically present in agestis and, where present in icarus, typically much reduced.
Differentiating icarus and agestis from their undersides can be more problematic and here we need to resort to the pattern of spots. We have two main distinguishing features and a third less obvious. These are (i) icarus has a spot on the underside of the forewing that is absent in agestis, though this is not always visible if the wings are not fully open, and (ii), two of the spots on the leading edge of the hindwing are relatively-close in agestis, almost forming a figure of eight, but are more spaced apart in icarus. This second feature is particularly useful if the full underside of the forewing isn't visible. A third guide to determination, as per the upperside, is that the outer margin fringes of the wings which generally appear as a light two-tone fringe in icarus typically appear whitish in agestis; the small dark extension of the venation in agestis is also typically present.
Finally, and one mistake that many people make, is jumping to conclusions and thinking they have a brown ‘female’ icarus without first checking the abdominal length and form. If the specimen clearly shows an abdominal structure that is male, long and slender and extending just beyond the wing margin, as opposed to being more rounded and pointed in form and typically not extending beyond the hindwing margin, then it can’t possibly be a ‘brown’ female icarus.
The above images (top to bottom) show (i) a female agestis upperside, (ii) a female ‘brown’ icarus upperside, (iii) a strongly blue form female icarus, (iv) female agestis underside, and (v), a female icarus underside.