The Brown Argus butterfly is much less common than the Common Blue, so most of the time you see a brown butterfly like this it will be a female Common Blue. This one was found in the meadows near the lake. The Brown Argus, like the Common Blue is quite a tiny butterfly, and you will best spot them in flight as a shimmering silver.

One of the most reliable ways to tell the difference from the Common Blue is in the patterns on the under wing, which are unique to each species. The Common Blue under wing has spots on the forewing close to the body (see inset image below). The Brown Argus has no spots on the forewing nearer than halfway to the body.