(Figs. 2, 6, 12–14, 17, 21, 25, 26)
Type material. Holotype: ɗ (PAH09_COE 6), Malaysia, Pahang, low pH swamp upstream on Sg Bebar, 22 ix 2009, leg. R.A. Dow, RMNH. Paratypes: 2 ɗɗ (PAH09_COE 7–8), Ψ (PAH09_COE 9), data as holotype; 3 ɗɗ (PAH09_COE 30–32), location as holotype, 23 ix 2009, leg. R.A. Dow; 3 ɗɗ, Ψ, location as holotype, 23 ix 2009, leg. C.Y. Choong. Paratypes in RMNH, coll. Choong and coll. Dow.
Etymology: Hoisen, a noun in apposition. Named for Professor Emeritus Dr. Yong Hoi Sen in recognition of his contributions to zoology in Malaysia.
Description of holotype male. Head: Labium pale. Mandible bases and genae yellow, labrum yellow except basally where dark brown. Anteclypeus pale along lower margin and more extensively centrally, brown above. Postclypeus very dark brown. Vertical face of frons shining black, with a pair of yellow transverse stripes, continuous with yellow on genae and nearly meeting centrally, yellow on genae extending narrowly beside eye margin to level of top of antennae bases, which are largely yellow. Horizontal surface of frons and vertex dark metallic green, ocelli whitish. Antennae dark brown except for yellow stripes along the anterior surface of the scape and pedicel and a yellow ring at top of scape. Occiput bronzy black except below genae where obscurely pale.
Thorax: Prothorax with pronotum almost entirely dark metallic green, becoming bronzy black to sides, with small obscurely pale areas laterally on anterior lobe. Propleuron bronzy black above, cream below. Synthorax with mesepisternum and mesepimeron dark metallic green, as is the upper part of the Mesinfraepisternum, the lower part is cream. Metallic green colouration extending onto upper part of metepisternum near mesinfraepisternum and more widely to rear, reaching to metepleural suture near wing bases, cream below as is entirety of metepimeron, metinfraepisternum and venter of synthorax. Legs with coxae cream coloured, trochanters cream above becoming yellow distally. Femur orange-yellow on all legs, except for small dark brown area immediately above tibia. Tibia pale yellow with a long dark streak on extensor surface of anterior tibia originating immediately below femur, smaller dark streaks below femur on flexor surface and laterally on extensor surface of other legs. Tarsi cream with brown spines and articulations, tarsal claws pale brown with darker tips and a tiny rudimentary tooth. Wings with 14 Px in Fw and 13 Px in Hw. Arculus at Ax 2 (left side) or slightly distal to it (right side). 1 A arising distal of Ac, IR 3 and R 4 arising distal of subnodus. Fw Pt (Fig. 12) covering slightly more than one underlying cell, almost square, dark brown, with a yellow outline, this best developed on distal and posterior margins. Hw Pt (Fig. 13) deeper than long, bright orange-yellow, covering slightly more than one underlying cell.
Abdomen: S 1 very dark shining brown above, pale cream to sides. S 2 bronzy brown dorsally, darker to rear, with a narrow pale basal annulus. S 3–7 brown dorsally, becoming darker on S 7, paler laterally (again S 7 darker), this extended dorsally as a narrow basal annulus, becoming less clear on successive segments. S 8–9 mostly bronzy brown to black, S 8 only narrowly pale laterally and S 9 mostly black laterally with an obscure paler area. S 10 small, entirely pale cream coloured. Genital valves (Fig. 17) very large with long posterior spines laying flat against sternum of S 10. Cerci (Figs. 21, 25) with upper branch with prominent inward and ventrally directed projection at ca mid length, after which the long tip turns abruptly downwards. Lower branch short, just reaching the level of the projection on the upper branch, of distinctive shape in ventral view (Fig. 26). Epiproct prominent, paraprocts barely visible.
Measurements (mm): Hw 19.5, abdomen without appendages 31.5, upper branch cercus ca 0.8.
Description of female paratype (PAH09_COE 9). As male except as noted. Head: Labrum only pale in anterior 1 / 3. Transverse stripes on vertical face of frons joined centrally. A narrow pale stripe along transverse occipital carina behind ocelli.
Thorax: Prothorax largely pale greenish blue, except along the lower margin of the propleuron, where cream, and along the anterior margin of the anterior pronotal lobe and centrally on the posterior pronotal lobe, where black (Fig. 2). Posterior pronotal lobe simple, shaped as in Figs. 2, 6. Mesepisternum pale greenish blue with a strong yellow tint, antealar triangles black. Mesepimeron and mesinfraepisternum pale greenish blue, this extending onto the upper half of the metepisternum, rest of synthorax cream. Legs cream except a black stripe along almost entire length of flexor surface of femurs, a small black lateral mark on femur just above joint, and obscure black marks below joint on tibia. Wings with 13 Px in Fw, 12 Px in Hw, R 4 arising at subnodus in all wings. Pt (Fig. 14) nearly square but deeper than long on costal side, covering one underlying cell, grey brown centrally with a wide cream border, especially pronounced on costal side.
Abdomen: Very similar to male, except more brown above, with pale basal annuli better developed on S 2– 7. S 8 dark brown above, obscurely pale laterally. S 9 dark brown above, with obscure pale patches laterally, especially to rear. S 10 entirely pale cream except ventrally. Cerci and paraprocts pale cream, as is the ovipositor, except the gonapophyses, which are brown.
Measurements (mm): Hw 20.5, abdomen without appendages or ovipositor 31.5.
Variation in paratypes. Teneral males have the Pt pale yellow in Fw and Hw, and the margins of the dark areas of the prothorax and adjacent parts of the synthorax, as well as the pale area laterally on the anterior lobe of the pronotum, are strongly red tinted.
Measurements (mm): Hw 19.5–20.5, abdomen without cerci 30.5–32.5.
Diagnosis. Both sexes of A. hoisen are clearly separated from all species known from Peninsular Malaysia and Singapore by the characters given in the keys. The closest allies of A. hoisen are the Bornean species A. martini Ris, 1911 and A. dactylostyla Lieftinck, 1953, from which the male differs in the details of the cerci, especially the extremely long down turned terminal part of the upper branch, and the shape of the lower branch seen in ventral view. The male differs from all other species in the remarkable long posterior spine on the genital valves (Fig. 17); this character is so distinctive that it would allow identification of a specimen so badly damaged that only S 9–10 remained intact.