Diagnosis: Tachidiidae. Rostrum elongate, delimited at base. Paired lateral integumental windows on P5- bearing somite present. Genital and first abdominal somites fused in ♀ original segmentation marked by transverse surface ridge dorsally and laterally; midventral copulatory pore not isolated from gonopores. Anal operculum spinulose. Caudal ramus setae IV – V well developed and pinnate. Sexual dimorphism in antennule, P2 endopod, P3 exopod, P5, P6, urosomal ornamentation and segmentation.

Antennule with numerous pinnate setae/spines; 7- segmented in both sexes; chirocer in ♂, with aesthetascs on segments 4, 6 and 7; segment 5 in ♂ with spinous outgrowth. Antenna with (indistinctly) 2-segmented exopod; exp-1 with one seta, exp-2 with one lateral and two apical setae; lateral endopodal spine proximally displaced. Mandible with two setae on basis, five setae on exopod and three lateral and nine apical setae on endopod. Maxillule with four setae and one geniculate spine on coxa, and six elements on basis; exopod represented by one seta, endopod 1-segmented with three setae. Maxillary syncoxa with enditic formula [4,3,3]; allobasis with three accessory setae; endopod indistinctly 2-segmented with one geniculate spine and five setae. Maxilliped with two accessory setae on endopod; claw minutely pinnate.

P1–P4 with 3-segmented rami; enp-1 of normal size and with inner seta. P1 exp-3 with two outer spines, enp-3 with one inner seta. P3 enp-3 with five setae/ spines. P4 enp-2 with one inner seta. Armature formula as follows:

Exopod Endopod P1 0.1.122 1.1.121 P2 1.1.222 1.2.221 P3 1.1.222 1.2.221 P4 1.1.122 1.1.221

P 2 ♂ enp-2 with outwardly directed spinous apophysis; enp-3 small, anterior surface with transverse spinular comb concealing tip of enp-2 apophysis, outer spine reduced in size, outer distal seta rudimentary and fused to segment, inner setae well developed. P3 exopod ♂ longer than in ♀ with exp-3 often bent inwards; inner setae of exp-1 and -2 smaller than in ♀ inner distal seta of exp-3 vestigial.

P 5 ♀ with outer concavity, separating outer lobe from distal portion; outer lobe with basal plumose seta and pinnate spine; distal portion with plumose seta flanked by strong pinnate spines around apex, and two (endopodal) pinnate spines along inner margin. P 5 ♂ medially fused; each with three pinnate spines and two setae.

P 6 ♀ represented by opercula closing off common genital slit, each with one seta. P 6 ♂ symmetrical; each member with two pinnate spines and naked outer basal seta.

Free-living. Freshwater or brackish habitats.

Type species: Tachidius (Neotachidius) triangularis Shen & Tai, 1963 = Neotachidius triangularis (Shen & Tai, 1963) comb. nov. [by monotypy]

Other species: N. coreanus sp. nov.; N. parvus sp. nov.