Morphological adaptations of the semiaquatic millipede Aporodesminus wallacei Silvestri 1904 with notes on the taxonomy, distribution, habitats and ecology of this and a related species (Pyrgodesmidae Polydesmida Diplopoda)

The poorly known millipede Aporodesminus wallacei Silvestri 1904 appears to be a widespread pantropical species currently reported from St. Helena Island (southern Atlantic Ocean), the Hawaiian Islands, Tahiti (central Polynesia), and the vicinity of Sydney, Australia (Pacific Ocean). Of these records (the latter two are new), the Sydney adults and subadults have been taken underwater in a few creeks of a single small catchment area. This is only the third polydesmoid, and second Pyrgodesmidae, definitely attributable to semiaquatic millipedes. This habit is further proved by indirect evidence coming from the structure of the mouthparts and a cerotegument enabling plastron respiration. As virtually no record can readily be associated with man/human settlements, the hypothesis is put forth that, as is the case for numerous (sub)cosmopolitan water-dwellers, dispersal of this minute species (4.5-6.6 mm) could have been due to zoochory at least not less likely than to hydrochory or anthropochorism, whereas the remote and scattered islands can hardly be taken as the original source area for A. wallacei. Based solely on similar cerotegment structures as well as on a strikingly similar distribution pattern, but without direct evidence of hydrophily, the same habits if not dispersal mechanism can be suggested for one more pantropical pyrgodesmid, Cryptocorypha ornata (Attems 1938), which has been reported from St. Helena Island, the Hawaiis, the Cook Islands, the Marquesa Islands, Tahiti and Hong Kong. The four latter localities are likewise new.