Figures: 16-17, 32, 43, 102, 110, 130, 163-164, 209-210, 234
In addition to the Ecdyonurinae characteristics, the following combination of characteristics will identify the genus:
Larvae of Nixe are differentiated from those of other Ecdyonurinae genera, except the simplicioides group of Ecdyonurus, by the presence of interfacing setae on the caudal filaments (Fig. 102), fimbriate setae on the ventral surface of the maxillae (as in Fig. 51), and the absence of posterolateral projections on the pronotum (Fig. 16). In North America, Nixe can be differentiated from larvae of Ecdyonurus simplicioides , which is sympatric with Nixe in the central part of the continent, by having the median pair of pale spots on the anterior margin of the head capsule separated by a distance less than the distance between the bases of the antennae (Fig. 35).
Males of Nixe are characterized by having ventral spines on the penes (Fig. 163), titallators that are robust, spindle shaped and usually apically denticulate (Fig. 164), and eyes that are contiguous dorsally or separated by distance less than diameter of median ocellus. The simplicioides group of Ecdyonurus are similar but have slender titillators. Females of North American Nixe differ from other Ecdyonurinae by having eggs with a distinct mesh-like pattern of ridges surrounding the knob-terminated coiled threads (Fig. 110). The subgenital plate of females has the posterior margin thickened, forming a ridge (Figs. 209-210), which is absent in Ecdyonurus simplicioides, but present in E. criddlei.
Holarctic; Oriental (Fig. 234).