Hydrodroma trigonometrica
Creators: name: Gerecke, Reinhard
Publisher: name: Zenodo
Year: 2017
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Arachnida
Trombidiformes
Hydrodromidae
Hydrodroma
Hydrodroma trigonometrica

description: Hydrodroma trigonometrica (Walter, 1928)   
(Figs 5 F-G)  
 Type series: Holotype ♀ NHMB " Diplodontus trigonometricus Walter, Typ. 1361 TYPUS Th. Monod Cameroun. Lac Nfou XXVI/72". State: specimen before slide-mounting obviously treated with clearing agent, partly dried, appendages possibly squeezed.   
 Material examined: NHMB " Diplodontus trigonometricus Walter ♂ Algerien 5.3.1926 Coll. Gauthier "; "1360 TYPUS leg. Gauthier Algérie. Oued Medjerala km 6 south Afras-Sedrata 5.3.1926 XXVI/70" [err., not part of the type series].   
 Diagnosis: Small in dimensions (e.g., L/W Cx-I+II, 160/123, Cx-III+IV, 208/168, genital plate L 157). Integument with closely arranged, flat and round papillae, hardly visible (Fig. 5 F). Cx-I medial setae not associated with tubercles; Cx-III+IV posterior apodemes long, parallel to body axis. Genital field with ca. 25 pairs of Ac, maximum number per transversal transect 3, and about 35 pairs of medial setae. Legs with relatively long claws (L ratio claw/segment 5, 10-15 %); II-L-5 with one posterior swimming seta, numerous swimming setae on III-L-4/5 restricted to posterior surface, on IV-L-4/5 on both surfaces.   
 Description: Colour not reported ("probablement rouge"). Integument structure no more visible, following Walter with closely arranged, flat and round papillae, in deeper integument strata a triangular pattern as described in the family diagnosis (Fig. 1 D). Coxal setation: Cx-I medial margin smooth, setae not associated with projections; groups of 4-5 long distal tip setae at Cx-I and Cx-II, 8 posterolateral setae at Cx-II, 13 at Cx–III; total setae number of Cx-IV, 13. Coxal plate Cx-III+IV with long posterior apodemes directed parallel to body axis. Genital plates slender, with 22-28 pairs of Ac, maximum number per transversal transect 3 (Fig. 5 G). Legs with relatively short claws (L I-III-L 25, IV-L 22, L ratio claw /segment 5, 12-15, IV-L 10 %). Leg segments rather normal in proportions (e.g., L/H ratio III-L-4, 3.3; III-L-5, 5.7; IV-L-4, 5.7; IV-L-5, 6.2; swimming setation not very rich (anterior/posterior): II-L-5 (0/1); III-L-4 (0/8); III-L-5 (0/4); IV-L-4 (7/6); IV-L-5 (8/5).   
 Measurements: Female holotype [if deviations remarkable, data from original description in brackets]: Idiosoma L/W 680/450 [930/825, obviously shrunk during slide mounting]; L/W Cx-I+II, 260/123, CxIII+IV, 208/168; genital plate 157/63[80], Ac pairs n 25 [20-27], genital setae pairs n = 34; gnathosoma L/W [175/130]; chelicera L 214 [240], basal segment/claw ratio 3.1 [3.6], L/H ratio 4.8 [5.3] (differences difficult to explain by damage, figure of original description in agreement with Walter's data); palp total L 334, L/H ratio (relative L %) P-1, 0.95 (11); P-2, 1.26 (16); P-3, 0.9 (11); P-4, 4.48 (42); P-5, 4.50 (20). L/H (ratio in parentheses) I-L-4, 119/40 (3.0); I-L-5, 168/34 (4.9); II-L-4, 168/42 (4.0); II-L-5, 208/33 (6.3); III-L-4, 153/46 (3.3); III-L-5, 193/34 (5.7); IV-L-4, 193/34 (5.7); IV-L-5, 224/36 (6.2).   
Male from Algeria (see below for species identity): Idiosoma L/W 760/670; L/W Cx-I+II, 233/157, CxIII+IV, 256/238; genital plate 184/94, Ac pairs n 27, genital setae pairs n 33; gnathosoma L 180; chelicera L 229, basal segment/claw ratio 4.1, L/H ratio 4.2; palp total L 336, L/H ratio (relative L %) P-1, 0.74 (13); P-2, 1.1 (18); P-3, 0.6 (10); P-4, 3.0 (41); P-5, 3.4 (18). L/H (ratio in parentheses) I-L-4, 139/63 (2.2); I-L-5, 198/54 (3.7); II-L-4, 198/67 (3.0); II-L-5, 247/50 (4.9); III-L-4, 190/61 (3.1); III-L-5, 240/51 (4.7); IV-L-4, 240/51 (4.7); IV-L-5, 280/52 (5.4).  
 Discussion: The original description of this species was based on a single female collected by T. Monod in a crater lake in Cameroon on 22.08.1926. For the moment it remains unclear if the author was correct when he later attributed a male from Algeria to this species. It shares with the holotype several important character states (rather small in dimensions, low number of acetabula and genital setae, well developed swimming setation comprising a group of anterior setae on IV-L-5), but differs in by far stouter leg segments. As these proportions could result from squeezing of the only available male specimen, the taxonomic weight of this difference remains unclear. I decide to accept the locality record with a question mark, but not to consider the specimen for the species diagnosis.   
 In the original description, Walter compared H. trigonometrica with the two other Hydrodroma species recorded at that time from Cameroon, H. despiciens and H. perreptans. Hydrodroma despiciens (my revision did not confirm any African record of this species) differs, among others, in distinctly higher number of acetabula (> 50 pairs), the absence of anterior swimming setae from IV-L-5 and relatively smaller leg claws (L ratio claw/segment 5 <10 %). As already observed by Walter, H. perreptans (and also H. ocellata) differs from H. trigonometrica among others in the morphology of legs with rather large claws (L ratio claw/segment 5> 14, in I-L> 20 %) and an extremely reduced swimming setation (not more than one seta per segment). In general, Walter stated correctly that H. trigonometrica is a rather small species. From both compared species it differs in distinctly minor dimensions, e.g. of the coxal and genital plates, but also of palp and leg segments.   
 Hydrodroma trigonometrica differs also from H. capensis (and from the similar H. zhokovi) in minor dimensions and a distinctly lower acetabula number, in addition in the presence of only one posterior swimming seta on II-L-5 and the presence of a group of anterior swimming setae on IV-L-5. Hydrodroma liberiensis agrees widely with H. trigonometrica in rather reduced dimensions and a low number of acetabula, but is well distinguished by its reduced swimming setation (in total, only 10 setae, IV-L-4/5 anteriorly without swimming setae).   
 All things considered, H trigonometrica is obviously a valid species, but additional investigation is required for understanding its diagnostic features and their variability. The name-giving character state, the inferior integument made up of a net with triangular meshes, is obviously typically found in many, if not all, Hydrodroma species (see introduction and Fig. 1 D).   
 Distribution: Cameroon, Algeria (?).
description: Published as part of Gerecke, Reinhard, 2017, The water mites of the genus Hydrodroma (Acari, Hydrachnidia, Hydrodromidae) in Europe and Africa, pp. 1-24 in Ecologica Montenegrina 13 on pages 14-15, DOI: 10.37828/em.2017.13.1, http://zenodo.org/record/13261268