A genus of Orthomorphini with 20 segments. Body medium- to large-sized, adults ca 15–50 mm long, ca 1.1–3.1 and 1.5–6.7 mm wide on midbody pro- and metazona, respectively. Paraterga invariably well-developed, metazonite to prozonite width ratio being ca 1.6–1.7. Adenostyles on ♂ legs 1 missing. Sternal lobe or cone(s) between ♂ coxae 4 present or absent.

Gonopod with a long, subcylindrical, distodorsally usually setose coxite and a normal, cylindrical cannula. Telopodite mostly very slender and long, modestly curved. Prefemoral portion densely setose, about as long as (rarely) to ca 2–3 times (usually) shorter than femorite (measured together with “postfemoral” part lying distal to lateral sulcus). Femorite without evidence of torsion (= seminal groove running only mesally), often slightly enlarged distally, mostly with a clear-cut, oblique, distolateral sulcus demarcating a “postfemoral” part. Solenophore only moderately strongly curved mesad or caudomesad, consisting of modestly developed laminae lateralis and medialis, yet with lamina lateralis somewhat larger than lamina medialis, both sheathing a similarly long, simple, flagelliform solenomere with a barely exposed tip; tip of solenophore never deeply split, normally poorly bi- or trifid, some of apical prongs being either minute denticles or lappets, or small teeth, or completely reduced.

Type-species: Polydesmus beaumontii Le Guillou, 1841, by subsequent designation by Pocock (1909).

The Orthomorphini is certainly the most diverse tribe of Oriental Paradoxosomatinae both at the generic and species levels. It is generally characterized by the gonopod showing a simple, usually subcylindrical (= normally not excavate mesally) and elongate femorite devoid both of torsion (= the seminal groove running entirely or nearly entirely on the mesal side) and processes/outgrowths. A distolateral sulcus demarcating a “postfemoral” part is usually, but not always, present. A solenomere is always long and flagelliform, starting on top of the femorite (+ “postfemoral” part, if any) at the base of a more or less elaborate solenophore, the latter being demarcated by an evident cingulum or mesal sulcus. The solenophore always consists of well- to moderately well developed, often subequal, lamellar lamina lateralis and lamina medialis, both only modestly curved mesad or caudomesad and both supporting and sheathing at least most of the solenomere. It is the solenophore that provides most of the generic characters in Orthomorphini, such as the presence or absence of additional structures (processes or lobes) at its base, near midway and/or at its tip (Jeekel 1968; Golovatch 1997a, 2000, 2009).

Orthomorpha is basically characterized by very broad paraterga, coupled with the gonopod showing mostly an elongate, slender femorite (+ “postfemoral” part, if any) and a long, modestly curved solenophore bearing additional structures neither at its midway nor near its base; the tip of the solenophore is never deeply split, normally poorly bi- or trifid, some of the apical prongs being either minute denticles or lappets, or small teeth, or completely reduced.

Based on the broad paraterga and the conformation of the solenophore, Orthomorpha comes closest to the continental Southeast Asian Antheromorpha Jeekel, 1968 (see above), the Philippine Luzonomorpha Hoffman, 1973 and the basically Bornean Gigantomorpha Jeekel, 1963 (Jeekel 1963, 1980a; Hoffman 1973; Golovatch 1996, 1997a). However, Antheromorpha species show a very deeply split tip of the solenophore, Gigantomorpha species demonstrate a somewhat flattened and bisinuate gonofemorite, usually also a more elaborate, often deeply split solenophore tip, whereas in Luzonomorpha the tip is deeply bispinose. A similarly poorly bi- or trifid gonopod is only observed in Orthomorphoides gen. n., but its species differ from Orthomorpha in much smaller bodies, in the rather poorly developed paraterga and at least in some of the apical prongs of the solenophore being short spines (see below).