Syntypes of Orthomorpha hydrobiologica spinala: 4 ♂, 1 ♀ (NHMW–3510), Indonesia, “Karimon Djawa Inseln” (= Pulau Karimunjawa Island, north of Java), V.1926, leg. Dammerman; 1 ♀ of Orthomorpha hydrobiologica spinala (NHMW–8001), Indonesia, Java, Tjibodas, no date, leg. W. S. S. van Benthem-Jutting, det. C. Attems.

Length 33–38 mm (♂), 36–38 mm (♀), width of midbody pro- and metazona 2.7–2.8 and 4.0–4.2 mm (♂), 3.1–3.3 and 4.3–4.4 mm (♀), respectively. Coloration of alcohol material upon long-term preservation rather uniformly brown with contrasting pale yellowish paraterga, venter and legs light yellow-brown (Fig. 2).

Head usual, clypeolabral region sparsely setose, surface of vertex smooth; epicranial suture distinct. Antennae moderately long (Fig. 2A & B), reaching behind midway of body segment 3 (♂) or beyond segment 2 (♀). Head in width < collum < segments 3 and 4 < segment 2 < segments 5–16(17), gently and gradually tapering thereafter. Collum with three transverse rows of setae, 4+4 anterior, 2+2 intermediate, and 3+3 posterior setae; caudal corner of paraterga dentiform, pointed, directed caually (Fig. 2A, B & J). Tegument smooth and shining, prozona very finely shagreened, metaterga slightly rugulose; surface below paraterga smooth. Postcollum metaterga with two transverse rows of setae, these being always abraded and traceable as insertion points: 2+2 in anterior (pre-sulcus) row, 3+3 in posterior (postsulcus) one. Axial line barely visible both on pro- and metazona. Paraterga very strongly developed (Fig. 2A-G, J-L), especially so in ♂, subhorizontal, always lying below dorsum, thin in lateral view, like blunt blades, a little thicker only on pore-bearing segments, always clearly projecting well behind tergal margin. Calluses delimited both dorsally and ventrally, only on segment 2 without ventral sulcus, thin, especially so on poreless segments. Paraterga 2 broad, anterior edge rounded, lateral edge with two small, but evident incisions in anterior 1/3; posterior edge evidently concave (Fig. 2A & J). Paraterga 3 and 4 subequal, like subsequent paraterga, anterior edge slightly rounded, bordered and fused to callus, lateral edge with a small incision in anterior third. Paraterga 15–19 with tip of caudal corner evidently curved mesad. Ozopores evident, lateral, lying in an ovoid groove at about 1/3 of metazonital length. Transverse sulcus present on metaterga 5–18, shallow, not reaching bases of paraterga, finely beaded at bottom (Fig. 2A, C, F, J-L). Stricture between pro- and metazona narrow, shallow, beaded at bottom down to base of paraterga (Fig. 2D & E). Pleurosternal carinae complete crests only on segment 2 or segments 2 and 3, with a small, sharp, caudal tooth on segments 3–7(8) (♂) or 4–6 (♀), thereafter with a very small caudal denticle until segment 15 (♂, ♀). Epiproct (Fig. 2E-G & L) conical, flattened dorsoventrally, apical papillae well-developed, acute and directed ventrad; tip subtruncate; pre-apical papillae small, but visible. Hypoproct (Fig. 2G) subtriangular, setiferous knobs at caudal edge well-separated.

Sterna sparsely setose, without modifications, but with a pair of small, rounded, completely separated, setose cones between ♂ coxae 4 (Fig. 2H & I). No conspicuous ridge in front of gonopod aperture. Legs long and slender, slightly incrassate in ♂, midbody ones ca 1.2–1.3 (♂) or 0.8–0.9 times (♀) as long as body height, prefemora without modifications, tarsal brushes present until legs of segment 9.

Gonopods (Fig. 3) simple. Coxa long and slender, with several setae distodorsally. Prefemoral (= densely setose) portion more than 3 times shorter than femorite (measured until beginning of solenomere, including “postfemoral” part lying beyond lateral sulcus). Femorite slender, slightly curved and not enlarged distad, “postfemoral” part demarcated by an oblique lateral sulcus; tip of solenophore evidently trifid, middle denticle much smaller than both a terminal tooth and a subterminal lobule; solenomere about as long as solenophore, flagelliform.

A complete historical review of the typification of Orthomorpha has long been provided by Jeekel (1963). Despite some confusion, Orthomorpha was properly typified by Pocock (1909), with Orthomorpha beaumontii (Le Guillou, 1841) serving as the type-species.

Originally described as a subspecies of Orthomorpha hydrobiologica (see Attems 1932), not as a variety as mistakenly quoted by Jeekel (1968), spinala has since been treated as a full species (Jeekel 1968). The above samples, especially the only available ♀ syntype, agree in almost every detail with the very accurate redescription of the Orthomorpha beaumontii holotype provided by Jeekel (1963), making a restudy of the holotype superfluous. The few differences, such as size (4.4 vs 5.0 mm), coloration (brown vs blackish), the shape of the caudal tooth on pleurosternal carinae (sharp teeth vs triangular lappets), the presence of an anterolateral denticle on paraterga (very small vs virtually missing) etc., are deemed too minor, rather reflecting individual or population-level variation, to consider Orthomorpha spinala as being distinct from Orthomorpha beaumontii at the species level. Hence the new synonymy advanced. In addition, the type series of Orthomorpha spinala derives from an islet lying nearly halfway between Java and the beaumontii type locality, Borneo.

Jeekel (1963), when trying to find the closest match among the known Orthomorpha species to the holotype of Orthomorpha beaumontii, emphasized its especially strong similarities to Orthomorpha weberi. Slight differences were only noted in the shape of the paraterga. It was this that allowed Jeekel to unequivocally conserve the concept of Orthomorpha. Zoogeographically, the strong morphological similarities between Orthomorpha beaumontii and Orthomorpha weberi make sense, because the latter species is endemic to Java, Indonesia.

With the above synonymization, the nomenclature of Orthomorpha becomes stabilized, confirming this genus’ present scope. The identity of its type-species, Orthomorpha beaumontii, has been refined, based on male characters as well.

Bollman (1893), when proposing Orthomorpha as a replacement name for the preoccupied Paradesmus De Saussure, 1859, synonymized Orthomorpha beaumontii with Orthomorpha spectabilis (Karsch, 1881), the latter species from Java. Apparently because he provided no evidence whatsoever to substantiate his synonymization, it has since been neglected, spectabilis still remaining a dubious name (e.g. Attems 1937; Jeekel 1968).

(Attems (1898, 1914, 1937) referred to beaumontii a sample from Java, Indonesia which he had received from the Berlin Museum, thus providing a second record of this species. However, because the gonopod tip of that sample shows a remarkably small subterminal lappet, while the paraterga and pleurosternal carinae slightly differ in shape from those of beaumontii, there can be no doubt of Attems’ misidentification. Hence our references to it as such in the catalogue section above.