1	Gonopod tip like a single, very small, rounded lobule (Figs 5 & 6). Virtually no modifications between ♂ coxae 4 (Fig. 4H & I). Pantropical	Orthomorpha coarctata
–	Gonopod tip mainly bi- to trifid. Modifications between ♂ coxae 4 mostly present. Local in Southeast Asia	2
2	Gonopod femorite about as long as prefemoral portion (Fig. 113)	Orthomorpha butteli
–	Gonopod femorite ca 2–3 times longer than prefemoral portion (e.g. Figs 83, 94 & 99)	3
3	No distinct lateral sulcus on gonopod femorite demarcating a “postfemoral” region, at most a very faint one; solenophore tip neither acuminate nor branching. Thailand and northernmost Malaysia	4
–	A distinct lateral sulcus on gonopod femorite demarcating a “postfemoral” region; solenophore tip either acuminate or rather evidently branching (e.g. Figs 30, 33 & 46)	9
4	At least surface of metaterga leathery, dull	5
–	Dorsal surface shining	7
5	Only caudal parts of paraterga contrasting light. Paraterga less strongly developed, narrower, with only a small incision at best in front 1/3 extent. No tergal setae borne on evident knobs/tubercles	Orthomorpha sericata
–	Entire paraterga contrastingly light, much broader, largely with an evident, dentiform incision in front 1/3 extent. At least some tergal setae borne on evident knobs/tubercles	6
6	Surface of metaterga rough, microgranulate and rugulose. Metatergal setigerous tubercles higher, present also on collum. Paratergal incisions especially prominent	Orthomorpha enghoffi
–	Surface of metaterga nearly smooth, microgranulate. Metatergal setigerous tubercles evidently smaller, absent from collum. Paratergal incisions evident, but not so prominent	Orthomorpha alutaria
7	Adults > 30 mm long and > 4 mm wide. Collum broadly rounded laterally. Paraterga set high, at least some upturned above dorsum. Sternal cones between ♂coxae 4 isolated. Lateral sulcus on gonofemorite traceable, but very faint	Orthomorpha parasericata
–	Adults < 30 mm long and < 4 mm wide. Collum narrowly rounded to angulate laterally. Paraterga set high, but remaining below dorsum even in male. Sternal cones between ♂ coxae 4 fused basally into a single lamina. Lateral sulcus on gonofemorite absent	8
8	Bases of paraterga more broadly light. Legs shorter, midbody ones ca 1.2 (♂) or 1.0 times (♀) as long as body height. Tip of solenophore like an elongate and undulate lobe	Orthomorpha subsericata
–	Bases of paraterga more narrowly light. Legs longer, midbody ones ca 1.5 (♂) or 1.2–1.3 times (♀) as long as body height. Tip of solenophore tridentate	Orthomorpha asticta
9	Gonopod tip bifid (Figs 10, 12, 18 & 36)	10
–	Gonopod tip trifid, albeit middle prong can be very small (Figs 44, 46, 52 & 61)	18
10	Metaterga smooth and shining, at most faintly rugulose (Figs 11 & 25)	11
–	Metaterga at least with one rather evident, transverse row of tubercles near caudal margin (Figs 9, 13, 28 & 34)	13
11	Gonopod femorite slightly, but evidently twisted near base; tip rather deeply split, both terminal and subterminal prongs spiniform	Orthomorpha paviei
–	Gonopod femorite not twisted near base; tip shallowly bifid, both terminal and subterminal prongs dentiform (e.g. Figs 12, 26 & 27)	12
12	Length ca 19–23 mm, width of midbody pro- and metazona 1.8 and 2.7 mm (♂), 2.3–2.5 and 3.4–3.5 mm (♀), respectively. Coloration of paraterga and epiproct creamy orange (live material) to faded pinkish or pale yellow (alcohol-preserved material) (Fig. 11). Similan Islands, Thailand (Map 2)	Orthomorpha picturata sp. n.
–	Length 31–38 mm, width of midbody pro- and metazona 2.6–4.0 and 4.2–4.7 mm (♂), 3.2–3.8 and 4.8–5.3 mm (♀), respectively. Coloration of paraterga and epiproct creamy yellow (both live and alcohol-preserved material) (Fig. 25). Continental Thailand (Map 2)	Orthomorpha communis sp. n.
13	Metaterga with only a single row of very small tubercle near caudal margin (Figs 28, 31 & 34)	14
–	Metaterga with two rows of tubercles: small ones in front of, larger ones behind sulcus (near caudal margin) (Figs 9, 13 & 22)	16
14	Caudal corners of anterior and midbody paraterga not extended behind tergal margin (Fig. 34A-D). Metaterga mostly flavous (Fig. 34A-F). Sternal cones between ♂ coxae 4 widely separated (Fig. 34H & I)	Orthomorpha suberecta sp. n.
–	Caudal corners of all postcollum paraterga evidently produced behind tergal margin. Metaterga mostly infuscate. Sternal cones between ♂ coxae 4 subcontiguous	15
15	Pleurosternal carinae complete high crests with a sharp caudal tooth on segments 2–7 (♂). Transverse sulcus visible on metaterga 5–18, narrow, not reaching bases of paraterga, ribbed at bottom. Stricture between pro- and metazona narrow and rather shallow, faintly beaded at bottom down to base of paraterga (Fig. 34B-F & K)	Orthomorpha latiterga sp. n.
–	Pleurosternal carinae complete high crests with a sharp caudal tooth on segments 2–4(5) (♂, ♀). Transverse sulcus evident (Fig. 28B-F & H), thin, deep and only slightly incomplete on metaterga 2–4, complete, at most very faintly beaded at bottom, reaching bases of paraterga on metaterga 5–18 (Fig. 28B, D & F)	Orthomorpha atypica sp. n.
16	A single sternal cone between ♂ coxae 4 (Fig. 9H & I). Vietnam	Orthomorpha arboricola
–	Two sternal cones between ♂ coxae 4 (Figs 13H, I, 22H & I). Thailand	17
17	Postcollum paraterga with a very strong front indentation laterally (Fig. 22a-F & J-L)	Orthomorpha subtuberculifera sp. n.
–	Postcollum paraterga with only a very small front indentation laterally (Fig. 13B-F)	Orthomorpha tuberculifera sp. n.
18	Paraterga mostly level to or even above dorsum	19
–	All paraterga below dorsum	28
19	Colour pattern not strikingly contrasting, calluses only being inconspicuously paler than a dark remaining background (Figs 41 & 109)	20
–	Coloration of metaterga more or less strongly contrasting, with very pale calluses against a very dark remaining background (Figs 49, 51 & 106)	21
20	Metaterga with two transverse rows of evident tubercles (Fig. 41A-F). Vietnam	Orthomorpha scabra
–	Metaterga quite smooth, without transverse rows of evident tubercles (Fig. 109B-F). Northern Malaysia	Orthomorpha spiniformis sp. n.
21	Calluses on metaterga rather thin (Figs 51A-F & 100A-F)	22
–	Calluses on metaterga rather thick (Fig. 49A-F)	23
22	Sternal cone between ♂ coxae 4 single, large (Fig. 51H & I). Southern Vietnam	Orthomorpha glandulosa
–	Two small sternal cones between ♂ coxae 4 (Fig. 100I & J). Northern Malaysia	Orthomorpha elevata sp. n.
23	Metaterga virtually smooth, at most faintly rugulose, only seldom (Orthomorpha weberi) with minute knobs caudolaterally (Fig. 49A-F & J-L)	24
–	Metaterga with a caudal row of evident tubercles	26
24	♂ tarsal brushes entirely wanting. Epiproct with very distinct apical papillae. Lombok Island, Indonesia	Orthomorpha francisca
–	♂ tarsal brushes present. Epiproct with small apical papillae. Java Island, Indonesia	25
25	Tarsal brushes present until ♂ midbody legs	Orthomorpha weberi
–	Tarsal brushes present until ♂ legs 7	Orthomorpha tenuipes
26	Adult body particularly small, only ca 2.6 mm in width (♂). Bali Island, Indonesia	Orthomorpha baliorum
–	Adult body larger, at least 3.5 mm in width. Java Island, Indonesia	27
27	Paraterga relatively strongly produced caudally, paratergal calluses in dorsal view rather strongly sinuate near ozopores	Orthomorpha conspicua
–	Paraterga relatively slightly produced caudally, paratergal calluses in dorsal view rather poorly sinuate near ozopores	Orthomorpha beroni
28	A row of very small tubercles near caudal margin of some metaterga (Figs 39A, C, F, 43A, C, F, J-L)	29
–	Metaterga virtually smooth, at best only slightly rugulose near caudal margin (Figs 53A-F, 58B-F)	31
29	Small tubercles present only on metaterga 10–18(19)	Orthomorpha flaviventer
–	Small tubercles present on all postcollum metaterga (Figs 39A-F, 43A-F & J-L)	30
30	Paraterga more strongly produced caudally (Fig. 39A-G). Java Island, Indonesia	Orthomorpha unicolor
–	Paraterga less strongly produced caudally (Fig. 43A-G & J-L). Vietnam	Orthomorpha rotundicollis
31	A single sternal lobe between ♂ coxae 41 (Fig. 53H & I)	32
–	Two sternal cones, sometimes very low ones, between ♂ coxae 4 (Figs 65I, J, 81H, I, 106I & J)	33
32	Sternal lobe between ♂ coxae 4 subquadrate. Perak State, southern Malaysia	Orthomorpha bipunctata
–	Sternal lobe between ♂ coxae 4 divided into two lobules only apically (Fig. 53H & I). Java Island, Indonesia	Orthomorpha zehntneri
33	Metaterga and collum usually more or less flavous, with lighter bands, spots or markings, at least in caudal parts (Figs 64A-F, 70A-F, 75A-G & K-M)	34
–	Metaterga and collum between paraterga usually completely dark (Figs 37A-F, J-L, 106A-F)	41
34	Pore-bearing midbody paraterga particularly thick in lateral view (Figs 75E & 91E)	35
–	Pore-bearing midbody paraterga considerably thinner in lateral view (Figs 65E, 69D & 86D)	36
35	Paraterga set mostly slightly higher (Figs 91E, H, 92E & H). Transverse sulcus present on metaterga 4–19. Tarsal brushes present until ♂ legs of segment 16	Orthomorpha insularis
–	Paraterga set mostly slightly lower (Figs 74B, D, E, 75C, E & H). Transverse sulcus present on metaterga 5–18(19). Tarsal brushes present until ♂ legs of segment 13	Orthomorpha subkarschi
36	Entire collum yellow, colour pattern like in Figs 70, 87 or 97, with at least a small patch remaining light near caudal margin	Orthomorpha karschi
–	Collum and subsequent tergites between paraterga at least partly infuscate, only postsulcus halves of metaterga often pallid to pale brown	37
37	No dark axial stripe on metaterga. Paraterga less prominent, their anterolateral margin relatively narrowly rounded (Figs 87 & 97)	38
–	A dark axial stripe or line on metaterga evident. Paraterga much more prominent, their anterolateral margin very broadly rounded (Figs 65, 70 & 81)	39
38	Colour pattern as in Fig. 97. Calluses on paraterga narrow. Sternal cones between ♂ coxae 4 very small and broadly separated (Fig. 97H & I). Central Thailand	Orthomorpha isarankurai sp. n.
–	Colour pattern as in Figs 86 & 87. Calluses on paraterga considerably broader. Sternal cones between ♂ coxae 4 much higher and poorly separated (Fig. 87G & H). Southern Thailand	Orthomorpha lauta
39	Sternal cones between ♂ coxae 4 smaller (Figs 81H, I, 82I & J). Caudal denticle of pleurosternal carinae traceable until ♂ segment 16 or 17	Orthomorpha thalebanica
–	Sternal cones between ♂ coxae 4 larger (Figs 64I & 70I). Caudal denticle of pleurosternal carinae traceable until ♂ segment 13 or 14	40
40	Colour pattern of live animals as in Fig. 70A. Bases of paraterga in alcohol material mostly infuscate, brown to dark brown (Figs 69 & 70)	Orthomorpha horologiformis
–	Colour pattern of live animals as in Fig. 65A. Bases of paraterga in alcohol material flavous, brown to dark brown (Figs 64 & 65)	Orthomorpha pterygota
41	Postcollum metaterga flavous behind transverse sulcus (Figs 79 & 103)	42
–	Postcollum metaterga dark on both pre-sulcus and postsulcus halves (Figs 37, 58 & 106)	45
42	Paraterga with anterolateral corner considerably more angulate, more narrowly rounded	Orthomorpha fuscocollaris
–	Paraterga with anterolateral corner considerably more broadly rounded (Figs 79 & 103)	43
43	Metaterga nearly completely flavous, only a thin axial line and transverse sulcus forming an infuscate cross. King Island, Mergui Archipelago, Myanmar	Orthomorpha crucifer
–	Metaterga mostly dark, including bases of paraterga (Figs 79 & 103). Southern mainland Thailand	44
44	Pleurosternal carinae present at least as a caudal denticle until segment 12 (♂). Tarsal brushes present until about ♂ midbody legs	Orthomorpha banglangensis
–	Pleurosternal carinae present at least as a caudal denticle until segment 16 (♂) or 13 (♀). Tarsal brushes present until about ♂ legs 8	Orthomorpha subelevata sp. n.
45	Caudal corner of midbody calluses only modestly produced behind tergal margin (Figs 37C, D, K, 106D & E). Sternal cones between ♂ coxae 4 very small, nearly indistinguishable, widely separated (Figs 37H, I, 106I & J)	46
–	Caudal corner of midbody calluses very strongly produced behind tergal margin (Figs 2A-F, 47C, D, 55C, D, 57C & D). Sternal cones between ♂ coxae 4 mostly evident, higher and less strongly separated (Figs 2H, I, 57H, I, 62H & I)	47
46	Pleurosternal carinae complete crests on segments 2–4 (♂, ♀) (Fig. 37B, D & E), each with an evident sharp denticle caudally, thereafter increasingly strongly reduced until segment 17 (♂) or 16 (♀). Tarsal brushes present on ♂ legs 1–7	Orthomorpha hydrobiologica
–	Pleurosternal carinae complete crests bulged anteriorly and with a sharp caudal tooth on segments 2–7, thereafter only a sharp caudal tooth on segments 8–16 (♂), or crests bulged anteriorly and with a sharp caudal tooth on segments 2–4, thereafter only a small sharp caudal tooth on segments 5–14 (♀). Tarsal brushes present until legs of ♂ segment 13	Orthomorpha similanensis sp. n.
47	Caudal corner of paraterga evidently rounded (Fig. 55A-F). Sternal cones between ♂ coxae 4 high and strongly separated (Fig. 55H & I). Environs of Kuala Lumpur, Malaysia	Orthomorpha fluminoris
–	Caudal corner of paraterga evidently pointed (Figs 2A, C, F, 47A-G, J-L & 57A-G). Sternal cones between ♂ coxae 4 usually much lower, if not, then both fused at base (Figs 47H, I, 57H, I, 62H & I)	48
48	Paraterga especially strongly produced behind tergal margin (Figs 57 & 58). Sternal cones fused at base (Figs 57H, I, 58I & J). Singapore	Orthomorpha murphyi
–	Paraterga much less strongly produced behind tergal margin (Figs 2A-G, 47A-G & 62A-G). Sternal cones not fused at base (Figs 2H, I, 47H, I, 62H & I)	49
49	Transverse metatergal sulcus present only on segments 5–18 (Fig. 2A, C & F). Pulau Karimunjawa and ?Java islands, Indonesia	Orthomorpha beaumontii
–	An incomplete transverse metatergal sulcus traceable also either on segments 3 and 4 or only on segment 4	50
50	Transverse metatergal sulcus traceable on segments 3 and 4, fully developed on segments 5–18 (Fig. 62A, C & F). Sternal cones between ♂ coxae 4 rather poorly separated (Fig. 62H & I). Sumatra Island, Indonesia	Orthomorpha melischi
–	Transverse metatergal sulcus incomplete only on segment 4, complete on segments 5–18 (Fig. 47A, C, F & J-L). Sternal cones between ♂ coxae 4 strongly separated (Fig. 47H & I). Cambodia	Orthomorpha cambodjana

1 Unknown in Orthomorpha fuscocollaris, a species based only on ♀♀.