The Brown Argus has a warm, chocolate brown, ground colour to the upper wings, and striking white wing margins when fresh. The butterflies average wingspan is 28mm. There may well be a visible discal spot on each forewing, sometimes just black and sometimes with a white circle around it. The orange crescents (“lunules”) on all four wings vary in number an intensity of colour. The females of the species typically have more pronounced lunules and more of them particularly on the forewings. It is lively in flight showing unexpected shimmer of silver as it flies.
The males are often seen patrolling with a very darting movement over a small area of land flying low to the ground and landing often on prominent twig or piece of grass to survey his territory for incoming females. They perch usually with their wings wide open basking in the sun. If disturbed they take flight but often return to the same location. If another male or a female or another similar sized butterfly, Common Blue or Dingy Skipper comes into view, the male launches into aerial combat and the two head spiraling upwards before the intruder is chased off. In the early mornings they are often found perched on a prominent blade of grass wings closed. Where the populations are of a reasonable number they can be found roosting together. The adults feed on a variety of nectar sources including Common Bird’s-foot trefoil, dandelions, buttercups, Common rock-rose and ragworts.
The two species often inhabit similar places so caution is required with regards to identification, photographs are always very helpful, particularly underwing shots.
The best way to tell the Brown Argus and female Common Blue apart is to look at the underwing. The Brown Argus has two of its rear underwing dots in a figure of 8 or colon (:) arrangement. The Common blue has them in a continuous arc the two dots being in a line (. .) There is also a dot missing on the forewing of the Brown Argus when compared to the Common Blue. (see photos)
Separating the Brown Argus (A.agestis) from the Northern Brown Argus (A.artaxerxes) is now almost impossible in our region unless you are in a geographic area which is known only to have Brown Argus or Northern Brown Argus populations. In areas where they now may overlap in VC65 and VC66 only genetic analysis is likely to be able to tell them apart.
In the past there were a number of accepted methods to distinguish the two, but these are all now considered unreliable. These include using differences in the upper wing discal spots. The Brown Argus A. agestis (with black spots), Northern Brown Argus A. artaxerxes (with white spots) and Northern Brown Argus subspecies A. salmacis (with black and white spots) which was in the past recognized as coming from the Castle Eden Dene area in VC66 and stretching over to southern Cumbria.
White scales, and even the all-white discal spot, can occasionally appear throughout the Brown Argus range becoming more frequent as one heads north. Likewise, within the former A. salmacis range, a proportion of butterflies still show black discal spots. The number of white scales appearing in either group varies from a single scale to a white patch or even a complete ring of scales around the black discal spot (ab. snelleni Ter Haar). The Brown Argus tends to be more strongly marked than its northern counterpart, with more prominent orange lunules, and bigger black spots in the dot pattern of the underwings. The upper wing ground colour also tends to be a more reddish brown in the Brown Argus compared to a much darker brown in Northern Brown Argus. However, the range of variation means that individuals cannot be cited as having diagnostic features either in Yorkshire or elsewhere.
It is often noted that the Brown Argus is double brooded (Bivoltine) and the Northern Brown Argus is single brooded (Univoltine) but this is not a reliable indicator.