A new cavernicolous, semiaquatic species of Serradium (Diplopoda, Polydesmidae) and its terrestrial, sympatric congener. With notes on the genus Serradium

Serradium semiaquaticum sp. n. is described from several North Italian caves. The new species is very similar to, and probably the closest relative of S. hirsutipes Verhoeff, 1941, which lives in the same caves as S. semiaquaticum but has a wider distribution. Serradium hirsutipes is redescribed and newly recorded from several caves. Serradium semiaquaticum is remarkable by its semiaquatic habits and correspondingly modified spiracles and mouthparts, whereas S. hirsutipes is normal in these respects. The genus Serradium is reviewed, and a key to the known species is given. Serradium as currently defined may not be a monophyletic group. Serradium semiaquaticum and S. hirsutipes show strong gonopodal similarity (synapomorphy?) with species of Polydesmus, subgenus Acanthotarsius.