Northern Brown Argus
- Latin synonym: Plebejus artaxerxes
- Family: Gossamer-winged Butterflies – Lycaenidae
- Subfamily: Blues – Polyommatinae
- Wing span: Small, 26–31 mm (1.01–1.21 in.). Females slightly larger than males.
- Wing upper side: Brown, edges with varying amount of red blotches that can form also continuous stripe. Females’ blotches are usually slightly larger and they have more of them. Middle of forewing with a small, black, blotch or stripe that is sometimes difficult to see.
- Wing underside: Brownish grey, with white-edged black blotches. Margin with red blotches that can also form a continuous stripe. White wedge-shaped pattern reaches from middle of hind wing to edge of wing.
- Habitat: Unfertilised, abundantly-flowered meadows, grazing land and power lines.
- Flying time: Mid-June–mid-July.
- Overwintering form: Caterpillar.
- Larval foodplant: Wood cranesbill (Geranium sylvaticum), bloody cranesbill (G. sanguineum) and common rockrose (Helianthemum nummularium).
The northern brown argus is quite common in southern and central Finland and rather rare in northern Finland. It can be confused with the female geranium argus, Amanda’s blue and the common blue. All of these species have a black blotch on the underside of their wings and a row of red blotches at the edge. The northern brown argus differs from these with the white wedge-shaped stripe that runs from the middle to the edge of the hind wing. This wedge-shaped pattern does not feature on the geranium argus or Amanda’s blue, and it is larger and more distinct than the one on the common blue. The black blotches on the northern brown argus’s forewing are larger than those on the hind wing, while on the common blue they are virtually the same size.
Males defend their territory, but also leave it to look for females, who lay their eggs individually on the leaves of the host plant. The larvae have a gland that secretes a sweet, nourishing liquid that ants love to lick. This means that there are often ants around the larvae, and their presence protects them.