The large red-eyed damselfly (Erythromma najas) is 30 to 36 mm long, with a wingspan of about 4.5 cm. It is thus somewhat larger and chunkier than the similarly colored blue damselfly (Ischnura elegans), which, however, has greenish-blue eyes.
For the male of the large red-eyed damselfly, blood-red eyes are very characteristic. Its legs are completely dark, likewise the dorsal part of the thorax is uniformly colored. The dorsal side of the abdomen is also almost entirely dark, not shiny black or grayish. The blue color in this species is limited to the thorax and the first and last two segments of the abdomen, where it is clearly separated from the black at a right angle. The ventral side of the remaining part of the abdomen is pale green.
The female appears darker than the females of other species of narrow-winged damselflies, and her brown eyes are often reddish. The dorsal side of her abdomen is entirely black, and the dorsal side of the thorax is adorned with only two, very short, green lines.
The large red-eyed damselfly is a species tied to standing and slow-flowing bodies of water, where floating aquatic plants thrive, as the females lay eggs on their leaves and stems, and the males likewise prefer to perch on them.
Adults can be observed from April to August. The largest numbers of large red-eyed damselflies occur on water bodies where yellow water-lily (Nuphar luteum), white water-lily (Nymphaea alba) or various kinds of pondweeds (Potamogeton sp.) with floating leaves occur; they appear in June.
