A dark, inconspicuous damselfly that lives concealed on marshy vegetation. At first glance, more reminiscent of an Ischnura than a Coenagrion. Both sexes can be identified by the black abdomen marked diagnostically with blue at the base and the tip, and are easily recognised by their different but equally distinctive markings. The male is armed (hence its name) with peculiar lower appendages, so large that they stand out even when seen with the naked eye.
Total: 31-34 mm.
Abdomen: 22-26 mm.
Hindwing: 16-21 mm.
Fields of sedges (Carex), Water Horsetail (Equisetum fluviatile) or similarly reedy plants, usually standing in shallow, mesothrophic water. Here, the vegetation is quite uniform, with a critical plant density (not too open, but also not too dense), offering adults both shelter and room to move.
From May to August.