After discussing on twitter with another moth mad butterfly chaser the ident points of Brown Argus (Aricia agestis) and Common Blue (Polyommatus icarus) butterflies, I decided to do a series of posts about confusion species within Lepidoptera, starting with the aforementioned species.
The separation of the two is best done (if possible) using the underwing of each butterfly, and using the upperwing should be avoided when making a decision as variations in individual butterflies of any species is notorious.
As you can see from the comparison image below (Fig:1) there are a few diagnostic traits on each species that should be taken together to make a conclusive identification.
1, Size. Size is on its own can be a little misleading as several factors can come into play. It would be easy to say that Brown Argus is in general smaller than Common Blue, but both broods and affects of a poor year can both influence size. In a year with an early and constant summer, a particularly early emergence of Common Blue may result in smaller individuals, as can a very late brood.
In some years other blues like Holly Blue (Celastrina argiolus) have been observed to have a wingspan anywhere between 11 & 14mm, and if they had a partner in the confusion game, they too would create headaches for Lepidopterists. It is worth noting that when I had to identify some Brown Argus in early September, I originally thought I was chasing Little (Small) Blue (Cupido minimus), such was the small size of that brood’s butterflies.
2, Cell spot. On the underside of the forewing of the Common Blue there is a lower black mark below the ringed mark near the top of the wing, this does not usually appear on Brown Argus. It is worth noting that if the wings are closed with the hind-wing covering a lot of the forewing this spot may not be observed and may lead to an error in identification.
3, Two vertical spots. From the top edge of the underside of the hind-wing of the Brown Argus are two vertical black spots with white boarders close together. These spots are not present of Common Blue and their spots may appear horizontal.
4, Orange. The Brown Argus tends to show much more orange on the edges of the underwing than does the Common Blue. This tends to be with both the length of the orange segments and the richness towards the front of the wings.
Please feel free to comment below if you feel I am incorrect in any of the points I have made or wish to add to the points?
Here is British Butterflies own guide to seperation of Brown Argus and Common Blue.
an english birder in france