Caenis arunachalami sp. n.

(Figs. 100–142)

Materials examined. Holotype. L-S-IJ, South India, Tamil Nadu, Madurai District, Nagamalai, Pulluthu stream, 9°95′30′′N, 78°03′92′′E, 212 m, 13.X.2020, colls. P. Srinivasan & R. Isack (Reg. No. ZSI –SRC/I/E/740) . Paratypes. 2 L-SJ, 2 L-S-I ♀ (AMC /ZN/255), 7 larvae (AMC /ZN/256), 1 larva (Reg. No. ZSI –SRC/I/E/741) with same label data as holotype .

Diagnosis. C.arunachalami sp. n. can be distinguished from all other Caenis species by the following combination of characters: Imago: 1) dark brownish body coloration and terga III–VII and X with a darker longitudinal line medially (Fig. 100); 2) base of antennal flagellum well dilated (Fig. 101); 3) prosternal sclerites forming an open triangle with concave sides (Fig. 103); 4) foretarsus segments 2–4 each with an apico-median projection (Fig. 102); 5) contrasting coloration of forceps and penis (Fig. 104); 6) penis narrow without any bulging lobes; posterior margin nearly straight without any medial emargination (Fig. 105); 7) forceps short, subcylindrical, broad, apically with a long hook-like spine equipped with 2–3 small spines near the apex (Fig. 106). Larva: 1) genae distinctly bulged (Fig. 109); 2) femur of each leg ochre with contrasting dark brownish patch in the 3/4 th distal apex; tibia and tarsus of each leg ochre with dark brownish band at middle (Fig. 129); 3) mesonotum with margins denticulate; 4) forefemur with a transverse row of 6–7 strongly developed spatulate setae on ¾ of distal region (Fig. 122); 5) hindclaw long and slightly curved, with a row of very fine microdenticles, medial denticles larger compared to basal denticles (Fig. 131); 6) tergalius II with left margin of Y-shaped ridge bearing row of long simple setae in the proximal half and row of spatulate setae in the distal half (Fig. 132); 7) hind margin of sternum IX posteriorly protruding and blunt triangular with a rounded tip on the apex (Fig. 137). Egg: 1) chorion smooth with a flat epithema of coiled-rope type located at the pole (Figs. 140, 141); 2) micropyle with broadened mouth (Fig. 142).

Description.

Male imago (Fig. 100). Measurements. Body length: 2.5 mm; forewing length: 2.2 mm; foreleg length: 2.1 mm; midleg length: 1 mm; hindleg length: 1.2 mm; cercus length: 7.7 mm. Ratios. Head (see Malzacher 2015; Fig. 4k): c: a = 2.2, a: b = 1.3; Leg: fore femur: fore tibia = 0.58; fore tibia: fore tarsus = 1.27; fore leg: hind leg = 1.75; segments of fore tarsus 1st: 2nd: 3rd: 4th: 5th = 1: 3.1: 2.8: 2.3: 1.9

Coloration. Head: Vertex with intense blackish transverse band. Antenna: scape and pedicel light brownish, flagellum whitish shaded with gray at base. Thorax: Prothorax light brownish medially and dark brownish laterally; prosternum medially dark brownish and laterally yellowish brown. Mesonotum and metanotum dark brown; mesosternum and metasternum dark brownish. Wings hyaline, costa, and subcostal vein reddish-brown. Legs whitish, except apical part of femora and tibiae with a dark brownish patch. Abdomen: Terga I–II dark brownish, terga III–VIII with transverse light brownish tint medially and paler laterally and tergum IX & X darker; terga III– VII & X with a darker longitudinal line medially, in tergum VIII & IX, medial longitudinal line poorly expressed. Cerci translucent.

Head (Fig. 101). Foremargin between lateral and frontal ocelli convex. Pedicel twice the length of scape; pedicel well dilated laterally. Base of the antennal flagellum well dilated.

Thorax. Prosternal sclerites narrow, forming an open triangle with concave sides; transverse strip long, present just above the opening,prosternal triangle entirely washed with dark brownish pigmentation(Fig.103).Foretarsus segments 2–4 apically with a small tongue-shaped apico-median projection, equipped with small strong spines (Fig. 102).

Abdomen. Tergum II without a finger-like process. Lateral filaments short. Genitalia and sternum IX as in Fig. 104. Penis short, broad, without any bulging lobes, nearly in the posterior margin of styliger sclerite (Fig. 105). Styliger sclerite with apophyses of moderate length (Fig. 106). Forceps short, subcylindrical, broad, apically with a long hook-like spine equipped with 2–3 small spines near the apex (Fig. 106). Genitalia entirely washed with brownish pigmentation and with very minute hairs; penis and forceps translucent; styliger sclerite dark brownish; central sclerite light brownish.

Female imago (Fig. 107). Measurements. Body length: 3.8 mm; forewing length: 2.6 mm.

Coloration is similar to that of males. Terga III–VIII brownish tint strongly pronounced compared to that of males.

Mature larva (Fig. 108). Measurements. Body length: 3.3–3.5 mm in female; 2.4–2.5 mm in male. Cerci length: 3 mm in female; 1.3 mm in male. Antennae length: 0.7–0.8 mm. General body coloration is usually dark brown dorsally and pale brown ventrally.

Head. Length 0.37 mm, width 0.59 mm. General coloration dark brown, with dark blackish transverse band between lateral ocelli; vertex dark brown without any markings (Fig. 109). Hind margin without setae, antenna with fine, thin setae on each articulation at every 3 rd segment. Pedicel twice the length of scape and with 1–2 simple setae on lateral margins, genae distinctly bulged (Fig. 109). Mouthparts: Labrum (Fig. 110) twice as broad as long, medial emargination with thick setae, lateral margin with long simple setae, dorsal and ventral surface with scattered long, fine simple setae (Fig. 111). Hypopharynx (Fig. 112) with minute hair-like setae on the apical margin and emarginate, superlingua with long simple setae on the lateral margins (Fig. 113). Right mandible (Fig. 114). Outer incisor with three denticles in which the outermost denticle possesses 4–5 long, fine setae; inner incisor with two denticles; medial margin with a small process between mola and incisors. Left mandible (Fig. 115). Outer incisor with four denticles; inner incisor with three denticles; medial margin without any process between mola and incisors. Dorsal surface of both mandibles scattered with scales and row of long setae laterally and a few long setae near the surface of the outer margin. Maxilla (Fig. 116). Three-segmented maxillary palp and with ratio of 1: 0.60: 0.94; segment I with row of 4–6 spines on outer margin; segment III with longitudinal row of long setae near inner marginal surface. Labium (Fig. 117). Glossa with five small spines laterally; paraglossa with scattered long simple setae (Fig. 118); three-segmented labial palp with length ratio of 1: 0.81: 0.50; segment I with row of bipinnate setae in the outer margin (Fig. 119), segment II with scattered long hair-like setae all over the surface and in the outer margin; outer marginal surface with a row of 9–10 bipinnate setae, segment III with seven spine-like setae in the inner margin and two transverse rows of four spine-like setae on the apex.

Thorax. Pronotum dark brown medially with traphezoidal marking medially; mesonotum dark brown. Margins of mesonotum denticulate with slight bulging. Legs: Cuticular coloration (Fig. 120, 126, 129): Femur of each leg ochre with contrasting dark brownish patch in the 3/4 the distal apex; tibia and tarsus of each leg ochre with dark brownish band at middle. Hypodermal coloration: Coloration of legs coinciding with cuticular coloration. Coxal processes semicircular with denticulate margins. Trochanter with four small spine-like setae on the inner margin and 4–5 spatulate setae near the surface of the outer margin. Foreleg (Fig. 120): lengths of femur: tibia: tarsus: claw 0.30: 0.22: 0.17: 0.08 mm; femur (Fig. 121) with a transverse row of 6–7 strongly developed spatulate setae on ¾ of distal region (Fig. 122); distal outer margin consists of 4–5 slightly developed spatulate setae; femora covered with numerous scale bases; tibia with longitudinal row of strongly developed spatulate setae on dorsal surface (Fig. 123), cluster of a large pectinate seta, three bipectinate setae and two long strongly developed spatulate setae present on its distal region; tarsi with longitudinal row of spine-like setae, increasing in size towards the distal half (Fig. 124); proximal inner marginal surface bears two bipectinate setae; claw long, curved and pointed with 4–6 minute denticles, which are of different in size (Fig. 125). Midleg (Fig. 126): lengths of femur: tibia: tarsus: claw 0.28: 0.22: 0.18: 0.08 mm. Femur with scattered strongly developed spatulate setae all over the surface; distal outer margin consists of 4–5 slightly developed spatulate setae; tibia bears a longitudinal row of seven strongly developed spatulate setae on its surface; distal region of tibia with a large apically blunt bipinnate seta; tarsi with longitudinal row of bipinnate setae, increasing in size distally (Fig. 127); midclaw (Fig. 128) bears seven denticles (four on dorsal side and three on ventral side). Hindleg (Fig. 129): lengths of femur: tibia: tarsus: claw 0.30: 0.25: 0.20: 0.09 mm. Femur with strongly developed spatulate setae all over the surface; outer margin (both proximal and distal margin) with slightly developed spatulate setae; tibia same as of midleg; tarsi with two longitudinal rows of pectinate and bipinnate setae (Fig. 130); claw long and slightly curved, 2 basal denticles, and with a row of very fine microdenticles, medial denticles larger compared to basal one (Fig. 131).

Abdomen. Terga I, VIII, and IX dark brown; tergum X light brown; others whitish and laterally translucent. Tergum I lack posteromedian process; tergum II with a broadly blunt triangular posteromedian process and denticulate hind margin; posterolateral processes on terga I–II absent; tergum III with slightly marked posterolateral processes; terga IV–VII with well-developed posterolateral spines processes (Fig. 136); terga VIII–IX posterolateral spines subequal in size. Hind margins of terga VII and VIII with bristles and terga IX and X with small denticles. Tergalius II about twice the length of tergalius I. Tergalius II with complete and well-developed Y-shaped ridge (Fig. 132); dorsal surface with numerous scales and 4 spatulate setae on the 2/3 rd half of Y-shaped ridge (Fig. 133). Left margin of Y-shaped ridge with row of long simple setae in the proximal half and row of spatulate setae in the distal half margin. Right margin of Y-shaped ridge with row of long simple setae. Ventral row of microtrichia originates at anterior 1/5 th length of the tergalius II; microtrichia more elongated and pinnate (Fig. 134). Tergalius III–VI with numerous filaments (Fig. 135). Hind margin of sternum IX posteriorly protruding and triangular rounded on the apex (Fig. 137); shagreen on dorsal side of sternum IX with 4–5 irregular rows of very small denticles parallel to the hind margin (Fig. 138). Cerci translucent.

Egg. Length: 105–115 μm; width: 70–80 μm. Oval (Fig. 139); chorion smooth with a flat epithema of coiledrope type located at the pole (Figs. 140, 141); micropyle situated at the subequatorial plane and slightly bent with broadened mouth (Fig. 142).

Etymology. This species is named in honour of Dr. M. Arunachalam (Central University of Kerala, School of Biological Sciences, Department of Animal Science/Zoology, Periye, India) who is the mentor and guide for the authors T. Sivaruban and S. Barathy, for his outstanding contribution to the benthic macroinvertebrates.

Distribution. Nagamalai hills, Madurai, Tamil Nadu, India (Southern Eastern Ghats).

Ecology. Larvae of Caenis arunachalami sp. n. were collected in a freshwater stream of 10 m wide with 80% canopy cover. The water temperature ranges between 22°C–24°C; pH 7.1–7.3. Substratum is mainly made of cobbles, pebbles, leaf litter with silt and organic matter. The larvae of the Caenis arunachalami sp. n. shared their habitat with the larvae of Labiobaetis sp. (Baetidae) .

Discussion. The male imago of Caenis arunachalami sp. n. is distinguished from other Oriental species of Caenis by the unique coloration of the body and genitalia. The coloration of the adult closely resembles that of the Ethiopian species C. gretathunbergae Malzacher 2021 (Malzacher 2021, Fig. 15), however, the new species can be distinguished from C. gretathunbergae, by the shape of forceps and penis in the male imago and by the shape of sternum IX in the larva (Malzacher 2021).