(Figs. 2, 5, 6, 7, 8, 9, 10, 13, 14)
Type material. Holotype: ♂ (BRU 13 _COE 1, RMNH.INS. 509684), Brunei, Belait district, Sungai Belait, Sungai Malilit, freshwater mangrove close to tidally influenced creek, 4.552 N 114.209 E, 26 vi 2013, leg. RAD, in RMNH. Paratypes. 2 ♂ (BRU 13 _COE2, 3), ♀ (BRU 13 _COE 4), location as holotype, 23 vi 2013, leg. RAD, in coll. RAD.
Etymology. Megabinluyog, a noun in apposition. Named for Haji Mohammed Hassanal Mega bin Abdullah Luyog, the extremely able boatman and field assistant who brought the first author to the site where the species was discovered.
Description of holotype male. Head: Labium entirely pale. Labrum and mandible bases almost entirely cream. Anteclypeus yellowish, postclypeus very dark brown, becoming paler laterally. Genae pale, this extending to eye margin, along eye margin narrowly to level of top of clypeus, and below well onto underside of head. Ocelli whitish, antennae with pedicel greyish brown, rest of dorsum of head bronzy black except for pair of large bilobed pale blue postocular markings (Fig. 2), with outer lobe broad, inner lobe narrow and faded.
Thorax: Prothorax bronzy black except centrally on pronotum, including entire posterior lobe, where rich dark brown. Posterior pronotal lobe with lateral parts narrow, middle part deeper, raised (Fig. 5). Synthorax: Mesepisternum bronzy black except for pair of narrow blue antehumeral stripes (Fig. 7), widely separated into two parts, anterior part widest near prothorax, narrowly separated from mesostigmal plates, tapering gently toward wing bases, and occupying ca four-tenths of length; posterior part much shorter, occupying a little more than onetenth of length, terminating shortly before antealar carina. Mesepimeron and metepisternum (Fig. 9) bronzy black and dark brown, well-defined blue stripe originating at middle coxa runs above spiracle toward, but not reaching wing bases on lower part of mesepimeon; ca trapezoidal blue mark narrowly separated from antealar carina on metepisternum (Fig. 9). Metepimeron and venter of synthorax almost entirely dirty pale. Mesinfraepisternum bronzy black except narrow stripe along lower margin; metinfraepisternum entirely dirty pale. Legs with coxae and trochanters pale, femora same except above joints. Tibiae pale but darker than femora, with darker area below joint, tarsi same colour, dark at joints. Wings as for genus, with 9 Px in left Fw, 10 in right, 8 Px in Hw. Fw quadrilateral with costal side slightly less than half anal side, slightly longer than proximal side and longer than distal side; Hw quadrilateral with costal side just over two-thirds anal side, ca twice times proximal side, which is slightly shorter than distal side. Three post-quadrilateral cells. Pt black, with narrow pale margin, covering slightly less than one underlying cell.
Abdomen: Largely brown laterally, black above except S 8–10 where ground colour of entire segment is black, with pale markings as follows: S 2 with pair of very short dorsal basal streaks; S 8 (Fig. 10) with subrectangular blue mark occupying basal ca two-thirds of dorsum. S 10 entirely very dark brown, apical margin produced into pair of rearward and slightly upward directed spikes dorsally, clearly visible in lateral view. Anal appendages black, cerci much shorter than paraprocts, and just shorter than S 10, of complicated shape (Figs. 13–14), with short interior lobe and long down-turned outer lobe almost resembling an elephant’s trunk in lateral view (Fig. 14), reaching the paraprocts at ca one-third of their length. Paraprocts tapering from base to tip in ventral view; in lateral view, after narrowing from base to ca one-third length, of approximately constant width until near tip, tip turned slightly inward. Small interior tooth at a little less than one third length of each paraproct from base (Fig. 13). Genital ligula typical for Mortonagrion s.s.
Measurements (mm): Abdomen without anal appendages 22, superior anal appendage ca 0.3, Hw 12.
Description of female paratype (BRU 13 _COE 4). As male except as noted. Head: Pale markings on front of head dirty blue. Postocular spots greenish, well-defined.
Thorax: Posterior pronotal lobe short, lateral parts not extending below level of mesostigmal plates, central part short and broad (Fig. 6). Synthorax (Fig. 8) with mesostigmal plates raised up, longest laterally, where folded forwards. Rest of mesepisternum dark brown, pale blue antehumeral markings not interrupted. Mesepimeron and metepisternum with stripe joined to marking beside antealar carina on metepisternum. Metepimeron, venter of synthorax, mesinfraepisternum and metinfraepisternum mostly brown. Legs mostly brown with paler areas on trochanters and distally on tibiae. Wings with 10 Px in Fw, 8 Px in Hw. Fw quadrilateral with costal side ca twothirds length of anal side. Hw quadrilateral with costal side ca three-quarters anal side.
Abdomen: Similar to male, very dark brown to black above, brown laterally, blue basal marks on S 2 larger, almost joined centrally, also present on S 3–5, where joined in centre but smaller on successive segments. S 8 without pale dorsal marking. Apical margin of S 10 simple. Cerci short, very dark brown, shorter than S 10, paraprocts very dark brown, slightly shorter than cerci, ca triangular. Ovipositor brown with obscure paler areas, valves terminating before level of tips of paraprocts.
Measurements (mm). Abdomen without anal appendages or ovipositor ca 23.5, Hw 14.5.
Variation in paratype males. One paratype is teneral and has a blue postclypeus. Both male paratypes have postocular markings with the inner lobe not faded and blue lateral mark on the apical half of S 1, smaller in the nonteneral individual. The dorsal markings on S 2 and S 3 are more conspicuous in both male paratypes, and the more mature individual also has these markings on S 4; in this specimen the dorsal mark on S 8 is longer and has its apical margin reaching almost to S 9 laterally, but excised centrally. In the same specimen the cerci are rotated slightly relative to their position in the holotype, so that the outer lobe appears shorter and more pointed apically.
Measurements (mm): Males: Abdomen without anal appendages 21–21.5, Hw 12, 9– 10 Px in Fw, 7–9 Px in Hw.
Diagnosis. The male is easily separated from all other species of Mortonagrion s.s. except M. astamii, M. appendiculatum and M. forficulatum by having paraprocts clearly longer than two times the length of S 10. It is separated from M. appendiculatum and M. forficulatum by the down-turned outer lobe of the cercus. It differs from M. astamii in details of the anal appendages, for instance the shape and length of the outer lobe of the cerci (compare with Fig. 12) and the smaller interior teeth on the paraprocts (compare with Fig. 11), but is most easily separated from it by the differently orientated postocular markings (compare with Fig. 1) and interrupted antehumeral stripes. Amongst regional species of Mortonagrion s.s., the female is readily separated from those of M. appendiculatum, M. arthuri, M. astamii and M. forficulatum by its shorter and broader central part of the posterior pronotal lobe, from M. indraneil by its complete antehumeral stripes, M. amoenum differs in having an even shorter posterior pronotal lobe with a central peak, and M. falcatum has narrower but more strongly raised mesostigmal plates.
Remarks. Mortonagrion megabinluyog was found perched low amongst the roots of Rhizophora mangrove trees in fresh, but tidally influenced, water close to a creek running into the Sungai Belait. The site was accessible because bird trappers (acting illegally) had cleared a channel into the mangrove, cutting the roots that normally make movement in this habitat extremely difficult. During rain M. megabinluyog was observed to retreat to perches underneath the arching roots for shelter.
At present M. megabinluyog is only known from one site in Brunei, however it is likely to be much more widespread around the coast of Borneo. It has probably remained undetected until now because its habitat is usually difficult to enter and work in and is not usually considered as likely habitat for Odonata. It is entirely possible that a number of other presently unknown odonate species could be found in habitats of this type.