Dasystigma, Mesibov, 2003

Mesibov, Robert, 2003, A new genus of Tasmanian millipedes (Diplopoda: Polydesmida: Dalodesmidae) with unusual spiracles and a mosaic distribution, Memoirs of Museum Victoria 60 (2), pp. 197-206 : 198-202

Dasystigma gen. nov.

Type species. Lissodesmus margaretae Jeekel, 1984 , by present designation.

Diagnosis. Differs from Lissodesmus and other known Australian dalodesmids in (a) the dense brush of hair-like structures emerging from each spiracle and (b) the unusually wide separation between the bases of the solenomerite and tibiotarsus on the gonopod telopodite.

Description of males. Adult length 18–22 mm when contracted in alcohol, diameter of midbody metazonite c. 1.8 mm. Overall colour pale yellow-brown to deep chestnut brown. Head with labrum weakly emarginate in center; clypeus very weakly convex in lateral view, moderately setose; vertex bare, strongly convex in lateral view, vertigial sulcus extending forward to a point about one antennal socket width from an imaginary line joining socket centres. Antennal sockets separated by about twice the diameter of a socket, weakly impressed; antennomeres ( Fig. 2) setose, more densely and finely so on 5–8, antennomere lengths decreasing in the order 2, (3, 6), (5, 4), antennomere 6 the widest. Collum slightly wider than head in front, widening posteriorly, anterior margin broadly convex, lateral margin with typically 3 small, seta-bearing teeth, posterior corner rounded and not projecting, posterior margin squarely transverse laterally but with the central third slightly emarginate; several transverse rows of sparse, long setae anteriorly on collum; a long seta extending posteriolaterally from point near posterior corner of collum (= posterior corner seta, PCS). Paranota inflated ( Fig. 2), maximum width at about one-third the ring diameter from the dorsum in midbody segments. Somites 2–4 from above about equal in width and slightly wider than collum; somites 5–17 about equal in width and slightly wider than 2–4; somite 18 narrower than 17. Tergites unsculptured, bare apart from PCS near posterior corners ( Fig. 3). Paranota on most somites with 4 or 5 (3–6) small marginal teeth, each bearing a seta ( Fig. 3); paranotal margin a straight line in lateral view, rising posteriorly; margin in dorsal view either nearly straight (parallel to long body axis) or slightly convex (see also “Derwent form” under D. margaretae ( Jeekel, 1984) comb. nov., below); posterior corner variably projected ( Fig. 3), with minute terminal seta. Ozopores on somites 5, 7, 9, 10, 12, 13, 15–19; pore opening dorsally on paranotum, just mesal to marginal thickening and typically about one-fourth of lateral margin length from tip of posterior corner. Spiracles ( Fig. 5) variably enlarged, all with hair-like structures variably emergent ( Fig. 1A) (at low magnification, the swollen, “hairy” spiracles in D. bonhami and D. margaretae resemble ectoparasitic mites). Legs ( Fig. 2) incrassate, much more so anteriorly beginning with leg-pair 3, prefemur and femur dorsally swollen, tibia on anterior legs in D. bonhami , D. margaretae and D. tyleri with slight ventrodistal swelling; tarsus about as long as or slightly longer than femur; dense pubescence ventrally on coxa, prefemur, femur and postfemur; numerous sphaerotrichomes ventrally on postfemur, tibia and tarsus; long, prominent seta at ventrodistal end of coxa and prefemur and at dorsodistal end of tibia. Genital opening inconspicuous on slight distal swelling of leg 2 coxa. Preanal ring with numerous long setae, densest dorsally; epiproct in dorsal view a truncated triangle with weakly concave sides; hypoproct broadly paraboloid in ventral view.

Gonopod aperture one-third to one-half ring 7 prozonite diameter in width, about 1.5 times as wide as long; in ventral view with straight anterior and lateral margins, posterior margin slightly curved, concave anteriorly; in lateral view anterior aperture margin not raised, lateral margin not raised or slightly convex upwards, and higher than slightly raised posterior margin.

Gonopods ( Fig. 4) retracted reaching as far as leg-pair 5 bases on ring 5, solenomerites and tibiotarsi of opposing gonopods interlaced. Telopodite in posterior view more or less straight, posterior and mesal faces sparsely setose from base to about level of solenomerite origin. Solenomerite arising at about half telopodite length on anteriomesal face, just proximal to origin of prefemoral process, directed first distad and mesad, then curving caudad and distad, tapering to a sharp point from about two-thirds its length and terminating at about three-quarters length of telopodite. A thin, curved ridge of cuticle on anterior surface of telopodite appearing to extend the line of the solenomerite proximad and strengthening it at its base; prostatic groove running along anterior surface of telopodite just lateral of this ridge. Tibiotarsus arising on posterior face of telopodite at about level where prefemoral process arises, smoothly curving mesad and distad, tapering near its apex to a blunt point on the telopodite just proximal to tip of solenomerite. Prefemoral process arising about midlength on telopodite, curved (concave posteriorly) and flattened anterioposteriorly, bearing a large uncus on posterior surface at about half its length, tip of uncus pointed caudad and mesad. Femoral process arising from lateral surface of prefemoral process proximal to uncus, variably shaped, not extending further distad than prefemoral process.

Females longer and heavier-bodied than males. Legs not swollen apart from slight dorsal swelling on prefemur and femur on anterior leg-pairs; no ventrodistal swelling of tibia; no sphaerotrichomes or ventral pubescence. Cyphopods not examined.

Juveniles considerably smaller than adults, midbody metazonite diameters c. 1 mm in stadium VII and c. 0.8 mm in stadium VI. Paranotal teeth much more prominent than in adults, spiracles generally placed as in adults (see species descriptions, below) in stadium VII, but typically wellseparated in stadium VI and younger.

Remarks. The four species of Dasystigma recognised here are very similar in overall appearance ( Fig. 1), distinguished partly on differences in the size, placement and “hairiness” of spiracles on diplosegments, but primarily on details of gonopod structure. The various processes on the gonopod are named here in accordance with the terminology used by Jeekel (1984) for Lissodesmus margaretae . Interspecific differences in gonopod structure are consistent in Dasystigma but subtle, and I have therefore provided for each species three different views of the gonopod, emphasising the prefemoral and femoral processes.

The dense spiracular “brush” of hairs is present in the type specimens of D. margaretae but appears to have been overlooked by Jeekel (1984). In 1972, P.M. Johns collected specimens of D. bonhami near Triabunna, in south-eastern Tasmania and later noted “spiracles greatly swollen, densely setose, the setae fine and short” (Johns, in litt., 15 Oct 1991). The function of the hair-like structures is unknown. Throughout their ranges, Dasystigma species co-occur in forest litter with dalodesmids of similar size and habits but with non-hairy spiracles.

Distribution and microhabitat. Tasmania south of 41ºS; in and under rotting wood, in leaf litter and in humus.

Etymology. Greek dasys (“hairy”) + stigma (in entomology, “opening to tracheal system”), neuter.