Orthetrum borneense, Kimmins, 1936

Steinhoff, Philip O. M., Butler, Stephen G. & Dow, Rory A., 2016, Description of the final instar larva of Orthetrum borneense Kimmins, 1936 (Odonata, Libellulidae), using rearing and molecular methods, Zootaxa 4083 (1), pp. 99-108 : 100-105

Description of the larva of Orthetrum borneense

Table 3. List of larval material examined (all from Gunung Mulu National Park, Sarawak)

Description of a male larva reared in the lab (POMS485-2014)

Diagnosis (For habitus see Figs. 2 , 3 ). A rather hairy, long legged larva; head with prominent eyes, abdomen rather elongate.

Head ( Figs. 2 , 3 ). Wider than long, widens from base to eyes. Eyes large, about the same length as lateral margins. Postocular lobes rounded, suboval glabrous areas longer than broad, touching hind margin. Antennae seven-segmented. Clypeus and frons well developed, labrum not visible in dorsal view but covered by palps of the spoon-shaped labium. Labium with prementum short and broad, reaching well behind the posterior margin of the front coxae ( Fig. 3 ). Prementum with an oval-shaped depression mediodorsally, a wave-shaped row of 15–20 setae on each side of the depression ( Fig. 4a ). Setae gradually shorter and more densely packed towards the depression ( Fig. 4a ). Anterior margin of prementum rounded, but pointed medially with 13 very weak crenations and several small, spine-like setae ( Figs. 2 , 3 ). Labial palps subtriangular, with seven long setae on dorsal margin and strong single end-spines ( Fig. 4a,b ). Medial margins of labial palps with ten fine rounded crenations ( Fig. 4b ).

Thorax ( Fig. 2 , 3 ). Prothorax narrow, in dorsal view partially obscured by long setae originating on the hind margin of the head. Synthorax covered by setae except for bare area in dorsal anterior part, and bare areas laterally between coxae. Wing cases dark with long, thin and erect setae on margins and one row of setae medially on surfaces ( Fig. 4f ). Wing cases reaching well onto the 5th segment ( Fig. 2 ). Legs with long setae on flexor and extensor margins of femora and tibiae, hind legs additionally with strong spines on flexor margins of tibiae; all tarsi with short stout spines on flexor surfaces.

Abdomen ( Figs. 2 , 3 ). Moderately high in profile, rather elongate, broadest part somewhat caudally from middle. S8–9 with short lateral spines, S4–7 with very strong dorsal spines ( Fig. 4e ). Spines most strongly developed and nearly erect on S4 and S5, gradually decreasing in size rearwards ( Fig. 4e ). Spine on S4 broken off in specimen described here, but present in all other examined material ( Figs. 2 , 4e ). Dorsal spines on S5 and S6 overlap intersegmental skins ( Figs. 1 , 3e ). Dorsal spines accompanied by tufts of long setae ( Fig. 2 ). Caudal margins of S5–9 with long setae that cover intersegmental skins; lateral margins of S5–9 with short setae ( Fig. 2 ). Anal pyramid large and elongate, paraprocts slightly inward curved ( Figs. 2 , 3 , 4c,d ). Paraprocts slightly shorter than epiproct (1:1.1), cerci shorter than paraprocts (1:1.6). Margins of paraprocts and epiproct with long setae ( Figs. 2 , 3 ).

Measurements (mm): Total length 32.4; paraproct length (ventral) 1.8; head width 8.2; prementum width 6.6; prementum length (ventral) 5.8.

Variation among reared specimens. One female with two additional small setae on dorsal margin of right labial palp, positioned anteriorly of the seven other setae and oriented somewhat more dorsally. One female with nine instead of ten crenations on medial margin of palps. Number of weak crenations on anterior margin of prementum varies between 12 and 15 among specimens. Paraprocts as long as epiproct in females (1:1) and cerci with half the length of paraprocts in females (1:2). Dorsal spine on S 7 in some females very small and procumbent.

Measurement ranges in examined specimen (n=8) (mm): Total length 29.7–33.3; paraproct length (ventral) 1.6–2.3; head width 7.7–8.5; prementum width 6.4–7.3; prementum length (ventral) 5.8–6.8.

Comparison between reared specimen and the early instar matched to adults using DNA barcoding. The early instar has a less elongate shape, with the abdomen more compact than in the final instar. Setae are about the same length in the early instar as in the final instar, which gives the much smaller larva a more setose overall appearance. However, the arrangement of setae and the bare spots on the thorax are the same as in the final instar. The eyes seem somewhat more protruding in the early instar than in the final instar, but the general head form and proportions agree well. Prementum and labial palps as in final instar. Wing cases tiny with few long setae. Dorsal and lateral spines on abdomen not yet developed. Paraprocts slightly longer than epiproct, setae on base of terminal appendages as in the final instar.

Measurements (mm): Total length 7.8; paraproct length 0.7; head width 2.4; prementum width 1.9; prementum length (ventral) 2.3.

Habitat. Orthetrum borneense is known exclusively from the Bornean highlands above 1000 m a.s.l. Most records are from primary (but in some cases disturbed by logging) forest, but at 1600 m a.s.l. at Paya Maga in Sarawak’s Limbang Division adults were found around small pools of water in open areas (Chee Yen Choong, personal communication). At Gunung Mulu the larva inhabits shallow, shady forest pools whose substrates usually consist of many layers of leaf litter. It can often be found in large numbers at such sites. In forest pools at high altitude on Gunung Mulu, O. borneense larvae were one of only a few species found along with larvae of Indaeshna grubaueri and Coeliccia cf. nemoricola ( Steinhoff 2015).

Figures

FIGURE 1. Neighbour joining COI gene tree using uncorrected p – distance for adults of nine species and subspecies of Orthetrum plus two outgroup taXa, and a larva of O. borneense.

FIGURE 2. Orthetrum borneense ♂ eXuvia dorsal, Visionary Digital stacking photograph

FIGURE 3. Orthetrum borneense ♂ eXuvia ventral, Visionary Digital stacking photograph.

FIGURE 4. Orthetrum borneense ♂ larva detail drawings: (a) prementum dorsal (b) labial palp frontal (c) anal pyramid dorsal (d) anal pyramid lateral (e) abdomen (S 4 – 10) lateral (f) wing cases lateral; scale bars 1 mm.