Holotype : 1 male larva Thailand, Chiang Rai Province, Mueang Chiang Rai, Pong Phra Bat waterfall, 20°00'41.0"N, 99°48'15.0"E, 470 m, 6.V.2019, D. Chainthong leg. [ZMKU]. Paratypes: 2 larvae same data as holotype; 1 larva on slide [ZMKU] and another in ethanol [MZL GBIFCH00673059]. Both paratypes were used for DNA extraction.

Mature larva (in alcohol, Fig. 1A). Body length 11.25 mm without cerci; cerci 4.8 mm; body brown with dark brown markings on thorax and femora.

Figure 1. 

Vietnamella spp. A Vietnamella maculosa sp. nov., habitus in dorsal view B Vietnamella sp. B, habitus in dorsal view. Scale bars: 1 mm.

Head. Brown with a pair of occipital tubercles, a single sub-occipital tubercle medially; two projections below eyes; inner pairs of projections small, spine-like and sharp (Fig. 2A); outer pair large, triangular, cone shaped without any serrated spines (Fig. 4A, D). Labrum, similar to other vietnamellid mayflies, anterior half of dorsal surface and margins with relatively long setae, ventral surface with short setae (Fig. 2B). Labium, glossae width greater than length, glossae and paraglossae with dense setae on surface, setae on dorsal surface and margins longer; labial palpi three segmented, basal segment broader and longer than the second, apical segment very small; palpi with tiny setae (Fig. 2C). Left mandible, slender, a little concave at sub-median area; molar block-like shape with a tuff of short setae below inner molar margin (Fig. 2D). Right mandible, slender, slightly concave at sub-median area; molar block-like shape with a row of setae below inner molar margin (Fig. 2E). Maxillae slender; maxillary palpi three segmented, with tiny setae; length ratio from basal to apical = 1.3:1.2:1 (Fig. 2F). Hypopharynx, lingua square and superlinguae nearly round, with setae on surface (Fig. 2G)

Figure 2. 

Vietnamella maculosa sp. nov. A ventral view of head with outer projection and small inner projection (arrow) B labrum C labium D left mandible E right mandible F maxilla G hypopharynx. Scale bars: 1 mm (A); 0.1 mm (B–G).

Thorax. Pronotum with small moderately sharp anterolateral projections, and slightly pointed protuberances below the anterolateral projection (Fig. 3A). Forefemur, strongly expanded with serrations or teeth projections on ventral margin (Fig. 3B); transverse ridge serrated with small rounded setae (Fig. 5A, B) and long thin setae near inner dorsal margin. Midfemur without any projection, expanded, dorsal margin convex apically and with a row of hair-like setae (Fig. 3C). Hindfemur without any projection, expanded, longer than midfemur, dorsal margin with a row of hair-like setae (Fig. 3D). All claws similar, strongly hooked with a single small denticle basally (Fig. 3E).

Abdomen. Tergites I–X with a pair of median ridges or tubercles; posterolateral angles of tergites II–X extended into sharp projection; tergite VII with a pair of tubercles (Fig. 4G) and tergite X with well-developed a pair of tubercles (Fig. 4J); lateral margins of tergite with dense setae. Gill on segments I-VII: gill I finger-like with setae (Fig. 3F); gills on segments II–VI similar in structure, with dorsal and ventral lamellae, the latter further divided into two clusters, each with several smaller lobes (Fig. 3G–K); gill on segment VII small, with two lamellae but ventral lamella divided into three lobes (Fig. 3L). Caudal filaments with dense lateral setae on inner and outer margins of middle part.

Figure 3. 

Vietnamella maculosa sp. nov. A thorax B forefemur C middle femur D hind femur E foreleg claw F first gills on segment I (arrow) G gill II H gill III I gill IV J gill V K gill V L gill VII. Scale bars: 1 mm (A–D, F); 0.2 mm (G–L); 0.1 mm (E).

Figure 4. 

Vietnamella spp. Vietnamella maculosa sp. nov. (A, D, G, J): A outer projection on head (arrow) D inner and outer projections on head G, J tubercle on tergite VII and X; Vietnamella sp. B (B, E, H, K): B outer projection on head (arrow) E inner and outer projections with serration on head H, K tubercles on tergite VII and X; Vietnamella thani (C, F, I, L): C outer projection on head (arrow) F inner and outer projections on head I, L tubercles on tergite VII and X. Scale bars: 1 mm (A–C); 0.5 mm (G); 0.1 mm (D–F, H, I); 0.05 mm (J–L).

Figure 5. 

Forefemur and setae A forefemur of Vietnamella maculosa sp. nov. with serration of transverse ridge (arrow) B setae on transverse ridge of Vietnamella maculosa sp. nov. C setae on transverse ridge of V. thani. Scale bars: 1 mm (A); 0.01 mm (B, C).

Eggs. (dissected from mature larva). Length 200 µm, width 144 µm; oval shape, chorionic surface with small protuberances, half of egg covered with helmet-shaped polar cap (Fig. 6A); rod shaped KCT (Knob Terminated Coiled Thread) around egg body (Fig. 6B); 2 or 3 tagenoform-type micropyles at centre (Fig. 6C).

Figure 6. 

SEM of egg structure of Vietnamella maculosa sp. nov. A overview B detail of Knob Terminated Coiled Thread (KCT) C micropyle and chorionic surface. Scale bars: 50 µm (A); 20 µm (B); 10 µm (C).

The larva of Vietnamella maculosa sp. nov. is most similar to that of V. thani and V. sinensis in the lack of serrations on the outer projection on the head, but it can be separated from V. thani based on the following characteristics: i) the pattern of serration on the ventral margin of the forefemur, ii) the setae on the transverse ridge of the forefemur, iii) a well-developed pair of median ridge projections of tergite X. It can be separated from V. sinensis by the proportion of the maxillary palp on the second segment, which is slightly longer than on the third segment (1.2:1), whereas in V. sinensis it is clearly longer (1.6:1).

The larvae of Vietnamella maculosa sp. nov. have a unique colour pattern, with brown and dark brown banding spread over the body and femurs. However, this colour can change in intensity depending on the life stage and environment. Other species, like V. thani, have variation in body colour but do not have this pattern.

The name maculosa (Latin for having spot or macula), refers to the brown and dark brown banding of the species.

Chiang Rai Province

Unknown

Thailand; 1 larva (immature) on slide, Tak Province, Mae Ra-Merng, 17°31'18.7248"N, 98°3'36.8064"E, 26.X.2015, A. Watcharangkool leg. (ZMKU).

Larva (in alcohol, Fig. 1B). Body length 6 mm without cerci; body reddish with dense setae lateral and on margin of head.

Head. Reddish with a sharp pair of occipital tubercles and a single sub-occipital tubercle medially; two projections below eyes: inner pairs of projections small, spine-like and sharp; outer pair large, triangular, cone shaped with five unequal serrated spines (Fig. 4B, E), basal spine the largest. Labrum, similar to V. thani and V. maculosa sp. nov. Labium, glossae width greater than length, glossae and paraglossae with dense setae on surface, labial palpi three segmented, basal segment broader and longer than the second, apical segment small; palpi with tiny setae. Left mandible and right mandible slender, mostly similar to the other Vietnamella species. Maxillae slender, maxillary palpi three segmented with tiny setae, length ratio from basal to apical = 1.3:1:1. Hypopharynx, lingua and superlinguae nearly round with setae on surface.

Thorax. Pronotum with small sharp anterolateral projections, slight protuberances below the anterolateral projections. Forefemur strongly expanded with serrations or teeth projections on ventral margin; transverse ridge serrated with small setae. Midfemur without any projection. Hindfemur without any projection, expanded, longer than midfemur; dorsal margin with a row of hair-like setae. All claws similar, with one small denticle basally.

Abdomen. Tergite I–VI and VIII–X with pair of median ridges or tubercles progressively; tergite VII with a single tubercle (Fig. 4H); posterolateral angles of terga II–IX extended into sharp projection; tergite X with a pair of moderately-developed tubercles (Fig. 4K); lateral margins of tergite with dense setae. Gills on segments III–VI similar in structure with dorsal and ventral lamella, the latter further divided into two clusters, each with several smaller lobes; gill VII small, with two lamellae but ventral lamella divided into three lobes.

The larva of Vietnamella sp. B can be separated from those of other species based on the following characteristics: i) outer pairs of projections on the head are large and stout, triangular, and cone shaped, with five unequally serrated spines (one large spine + four small spines), and ii) the abdominal tergites II–IX have a pair of projections or tubercles at the posterolateral margin except for tergite VII which has only a single projection (Fig. 4H).

The larval description given herein agrees with larvae of other species belonging to the genus Vietnamella, including the presence of a pair of projections on the head, the expanded femur and the forefemur with serrations on the outer dorsal margin. The larva described here has serrated spines on the outer projection of the head that differ from the other valid species of Vietnamella.The outer serrated projection is similar to that of Vietnamella sp. A from India (Selvakuma et al. 2018), but it differs in the number and character of the outer projection spines (four equal serrated spines in Vietnamella sp. A, five unequal serrated spines in Vietnamella sp. B). Although Vietnamella sp. B was not a mature larva, the phylogenetic analysis showed it belonged to a clearly different clade and had a high genetic distance compared to the other species (Fig. 12; Table 2). Formal description of this species is pending more material.

Tak Province.

Unknown.

Thailand; Kanchanaburi Province, Thong Pha Phum, Huai Pak Kok, 14°35'01.4"N, 98°34'54.0"E, 161 m, 15.X.2015, 1 larva; 20.II.2016, 1 larva; 21.II.2016, 3 larvae, 1 female imago; 31.I.2019, 9 larvae, 1 male subimago (reared), 1 male imago (reared). Huai Khayeng, Ban Prachum Mai, 14°39'34.0"N, 98°32'02.0"E, 233 m, 20.II.2016, 4 larvae, [ZMKU]; 13.XII.2014, 1 larva, all B. Boonsoong leg; 15.X.2015, 3 larvae, B. Boonsoong & M. Sartori leg. [MZL]. Prachuap Khiri Khan Province, Kui Buri, Huai Samrong, 12°03'55.0"N, 99°37'38.0"E, 103 m, 11 larvae, D. Chainthong leg. [ZMKU].

The larva of Vietnamella thani can be distinguished from those of other Vietnamella based on the following characteristics: i) outer pairs of projections on the head are long, triangular, and cone shaped without serrated spines; ii) the first and second segments of the maxillary palpi have an equal length ratio; iii) the forefemur has a serrated transverse ridge with spatulate setae (Fig. 5C); and iv) the abdominal tergite X either lacks or has a poorly-developed pair of tubercles (Fig. 4L).

Male imago (in alcohol, Figs 7, 10C). Head. Eyes rounded with ventral part brown-yellowish and dorsal part yellowish (Fig. 7A, B). Thorax. Forelegs (8.07 mm), length ratio of femur and tibia = 1:1.45; length ratio of four tarsal segments is 3:2:1.5:1 (Fig. 7C). Midlegs (4.76 mm), length ratio of femur and tibia = 1.2:1; length ratio of four tarsal segments is 1:1:1:3 (Fig. 7D). Hindlegs (4.97 mm), length ratio of femur and tibia = 1.4:1; length ratio of four tarsal segments is 1:1:1:4 (Fig. 7E). Mesonotum with a notable median longitudinal suture, two medioparapsidal sutures (Fig. 7F). Mesosternum with a square basisternum, broad furcasternum (Fig. 7G). Forewings, numerous crossviens. MA forked middle of wing, MP forked basally, three intercalaries between MP1 and MP2; CuA and CuP adjacent at base; cubital field with three bifurcate veins arising from CuA (Fig. 7H). Hind wings rounded, leading margin slightly concave, with clear crossveins; seven crossveins with one bifurcate between Sc and RA; three crossveins between MA and MP (Fig. 7I). Abdomen. Genitalia with three-segmented forceps (1.2 mm), first segment = 0.6 mm, second segment = 0.45 mm, apical segment = 0.15, small and nearly rounded; penes (0.76 mm) totally fused with a shallow median cleft; subgenital plate slightly convex (Fig. 7J–L). Abdominal segment IX with lateral projection, white stripe on sternites VII–IX (Fig. 7M–O).

Figure 7. 

Male imago of Vietnamella thani A dorsal view of head B lateral view of head C foreleg D middle leg E hindleg F, G dorsal and ventral view of thorax H forewing I hindwing J ventral view of genitalia K dorsal view of genitalia L lateral view of genitalia M dorsal view of abdomen N lateral view of abdomen O ventral view of abdomen. Scale bars: 1 mm.

Male subimago. (in alcohol, Fig. 8). Head. Eyes rounded; dorsal part dark grey; ventral part brown (Fig. 8A, B). Thorax. Forelegs (6.38 mm), length ratio of femur and tibia = 1:1.1; length ratio of four tarsal segments is 1.6:1:1:1.3 (Fig. 8C). Midlegs (5.41 mm), length ratio of femur and tibia = 1.4:1; length ratio of four tarsal segments is 1.3:1:1.1:3.4 (Fig. 8D). Hindlegs (5.58 mm), length ratio of femur and tibia = 1.6:1; length ratio of four tarsal segments is 1.4:1:1.2:3.4 (Fig. 8E). Mesonotum, brown with a notable median longitudinal suture (Fig. 8F). Mesosternum, pale red with a square basisternum, broad furcasternum (Fig. 8G). Forewing and hindwing are similar to imago but more opaque and having more visible crossveins (Fig. 8H, I). Abdomen. Genitalia similar to those of imago but forceps and penes shorter and broader; forceps with total length = 1.0 mm, first segment = 0.53 mm, second segment= 0.40 mm and third segment = 0.07 mm; penes length = 0.7 mm (Fig. 8J–L). Abdomen brown and pale red dorsally, segment IX with notable lateral projections (Fig. 8M–O).

Figure 8. 

Male subimago of Vietnamella thani A dorsal view of head B lateral view of head C foreleg D middle leg E hindleg F, G dorsal and ventral view of thorax H forewing I hindwing J ventral view of genitalia K dorsal view of genitalia L lateral view of genitalia M dorsal view of abdomen N lateral view of abdomen O ventral view of abdomen. Scale bars: 1 mm.

Female imago. (in alcohol, Fig. 9). Head. Eyes rounded, dark brown without dorsal eyes (Fig. 9A, B). Thorax. Forelegs (7.2 mm), length ratio of femur and tibia = 1.4:1; length ratio of four tarsal segments is 1:1:1:3.7 (Fig. 9C). Midlegs (6.4 mm), length ratio of femur and tibia = 1.2:1; length ratio of four tarsal segments is 1:1:1.2:2.8 (Fig. 9D). Hindlegs (7.2 mm), length ratio of femur and tibia = 1:1, length ratio of four tarsal segments is 1:1.2:1.1:2.6 (Fig. 9E). Mesonotum brown with a notable median longitudinal suture (Fig. 9F). Mesoternum pale red with rectangle basisternum, broad furcasternum (Fig. 9G). Forewing, 14 crossveins in stigmatic area; MA forked middle of wing; MP forked basally, 3 intercalaries between MP1 and MP2; CuA and CuP adjacent at base (Fig. 9H). Hindwing rounded, leading margin slightly concave, with clear crossveins, 7 crossveins between MA and MP (Fig. 9I). Abdomen. Tergites brown, sternites pale red, sternites VIII-IX brown (Fig. 9K–N). Subanal plate brown with shallow median cleft (Fig. 9J). Subgenital plate weakly developed, pale, with shallow median emargination (Fig. 9N)

Figure 9. 

Female imago of Vietnamella thani A dorsal view of head B lateral view of head C foreleg D middle leg E hindleg F, G dorsal and ventral view of thorax H forewing I hindwing J, K ventral and dorsal view of genitalia L dorsal view of abdomen M lateral view of abdomen N ventral view of abdomen. Scale bars: 1 mm.

Figure 10. 

Vietnamella thani A habitat B habitus of larva C habitus of male imago (incomplete molting).

Eggs. (dissected from female imago). Length 175 µm, width 125 µm; oval shape, chorionic surface with small protuberances, half of egg covered with helmet-shaped polar cap (Fig. 11A); many KCT around egg body (Fig. 11B); 1 or 2 tagenoform-type of micropyles at centre (Fig. 11C).

Figure 11. 

SEM of egg structure of Vietnamella thani A overview B detail of Knob Terminated Coiled Thread (KCT) and chorionic surface C micropyle. Scale bars: 50 µm.

Kanchanaburi and Prachuap Khiri Khan Provinces, Thailand.

The larvae of Vietnamella thani are widely distributed in Thailand. They inhabit fast-flowing streams (Fig. 10A) but have never been reported in southern Thailand. Vietnamella thani have notable outer pairs of projections on head without serrated spines that differ from Vietnamella sp. B. The larvae show colour variations and can be greenish, yellow or brown. The imaginal stages of V. thani have similar characters to those of V. sinensis but lack the longitudinal vein on the stigmatic area of the forewing and they have less crossveins between Sc and RA on the hindwings (Table 4). The egg structure is covered by a membrane, which leads to unclear sculpturing of the surface, especially in the posterior part of the egg. Thus, the egg from this study showed little difference from the egg structure of V. sinensis.