Length 5-6 mm (Synopia ultramarina 1). Body strongly arched. Head triangular. Eyes more or less round. ANT I with accessory flagellum as long as or longer than ART 1 of main flagellum (the latter composed of several fused ART). Segment I of pereon is longer than segment II, VI is longer than VII, which is longest of all. ART 2 of P V and VI rounded, nearly circular (Synopia ultramarina 4). P VI longer than P VII. Telson cleft (Synopia ultramarina 3) (see key). Ordinary live animals and eggs intense blue, but may be hyaline (see Bovallius, 1886).