Key to both sexes of the species of Longiflagrum

1 Antennule outer flagellum with eight articles .............. L. amphibium sp. n.

– Antennule outer flagellum with more than 10 articles................................ 2

2 Flagellum of the antenna with more than 10 articles .................................. 3

– Flagellum of the antenna with less than 10 articles..................................... 4

3 Antenna article 5 short (as long as wide); spiniform setae of the propodus of pereopod 1 large (more than five times as long as wide) ............ L. caeruleus

– Antenna article 5 long (twice as long as wide); spiniform setae of the propodus of pereopod 1 short (about three times as long as wide)................. ................................................................................................. L. estuarius

4 Pereopods 3–4 carpus short (about 1.5 times as long as wide).. L. koyonense

– Pereopods 3–4 carpus long (about three times as long as wide) .... L. nasutus