Onychargia atrocyana Selys

Orr, Albert G. & Dow, Rory A., 2015, Description of the final stadium larvae of Onychargia atrocyana Selys, 1865 from Sarawak, identified using DNA barcoding (Odonata: Zygoptera: Platycnemididae), with an overview of larval characters in the Platycnemididae, Zootaxa 4040 (3), pp. 384-392 : 387-391

Onychargia atrocyana Selys Figs 2; 3a, b, c, d, e, f, g, h; 4a, b, c

Material. Two F larvae, ♀, both Gunung Mulu National Park, 4.04318° N 114.8104° E 14-15.i.2008, 45– 55 m a.s.l., leg S.G. Butler. Both deposited in RHNH, Leiden, The Netherlands.

Description. In general appearance a small, moderately long-legged zygopteran of medium build and typical coenagrionoid appearance. Postocular lobes of head angulated and bearing a cluster of small, robust processes posterolaterally. Caudal lamellae foliate, with distinct petiole, the central lamella considerably longer and broader than the laterals ( Fig. 2). Coloration overall neutral brown, with darker brown mottling at the extremities, especially the antennae, legs and caudal lamellae. Body surface relatively smooth, bearing only sparse, very fine setae.

Head in dorsal view ( Fig. 3 a) rather squat and angular in outline, especially at posterolateral margins, which bear patches of short robust processes, best seen in ventral view ( Fig. 3 b, c). Labrum produced and slightly angulated medially. Labial palps clearly visible projecting beyond labrum in dorsal view. Upper surface of head with low profile and no distinctive features. Occipital triangle (which bears the ocelli) only a slight bump. Antennae long, seven segmented, final segment very short, segments 4 and 6 dark brown ( Fig 3 d). Compound eyes moderately large. Mandibles not evident in dorsal view. In ventral view ( Fig. 3 e) mandibles rather squat, with four strong well-developed short teeth (central pair partly fused) on each incisor lobe and a vestigial lower tooth; molar lobe unarmed except for small thorn-like processes near inner margin; outer angle of each mandible rounded, bearing sparse tuft of long setae. Galeolacinea of maxilla seen in ventral view with five thin outer (ventral) spines and three strong inner spines; maxillary palp narrow, curved and bearing long, dense, fine setae ( Fig. 3 f). Articulation of labial mask reaching midpoint of mesocoxae when retracted ( Fig. 3 b). Prementum ( Fig. 3 g) flat ventrally, only slightly concave dorsally, with a single pair of long, strong setae seated post medially. In outline broad distally and tapered to a narrow base a little less than one third its maximum distal breadth; median lobe (ligula) produced to form an obtuse triangle, the two slightly convex sides forming an angle of about 100°; median cleft absent; lateral margins of prementum bearing short spines in distal half, smooth in basal half. Labial palp long, terminating in a strong ventral hook and a dorsal straight, slightly serrate edge ( Figure 3 h), the two separated by a short notch. Palp bearing five long, strong, inwardly directed setae and a rather short movable hook, seated well back from its apex. Prothorax with two small supracoxal spines best seen in ventral view ( Fig 3 c). Legs moderately long; femora stout and somewhat angular, with distinct dark markings and short spines; tibiae thin with only very fine sparse setae. Wing pads just reaching hind margin of S4, with intricate pattern of dark markings. Abdomen unmarked, smooth, gradually tapered posteriorly. Caudal lamellae clearly dorsoventrally flattened, foliate with elongate petiole; median lamella ( Fig 4 a) about two-thirds length of abdomen, broadly foliate and acuminate, well tracheated and with mottled dark markings; petiole with distinct dorsal and ventral spines. Lateral lamellae ( Fig 4 b) narrower, about five-sevenths length of median lamella; petiole with outer lateral, as well as dorsal and ventral spines. Cerci small and budlike. Female gonapophyses extending well beyond posterior margin of S10 ( Fig 4 c). Inner and outer gonapophyses subequal in length; outer gonapophyses with well-developed subbasal shoulder.

Measurements (mm): Total length (without caudal lamellae) 10.2–11.3; maximum width of head 3.14–3.25; hind femur 2.5–2.6; cercus 0.1; right lamella 3.45; median lamella 4.8.

Remarks. Male larva unknown. Apart from a slight difference in size and the maturity of the wing pads, the two specimens show no significant variation.

The larva of O. atrocyana presents a combination of characters typically found in both Platycnemididae and Coenagrionidae . Notably the anterior border to the prementum is entire, and long papal setae are present ( Dijkstra et al. 2014). It may best be distinguished by having just a single pair of setae on the prementum together with the angulate form of the postocular lobes and the distinctive group of blunt processes on the posterolateral margin of the head. The two small supracoxal spurs may also be unique but are not an obvious diagnostic feature. All these characters were also present on a small F-2 or F-3 larva collected in a stagnant drain bordering secondary forest on the campus of Universiti Kebangsaan Malaysia by AGO in 2007, hence it is now confidently assigned to O. atrocyana . In particular, the posterolateral processes on the head were very well developed and, moreover, the form of the caudal lamellae was identical to those of the mature larvae. Similar arrangements of setae on the prementum and labial palps do occur in the larvae of some coenagrionids occurring within the range of Onychargia , but in these the legs are distinctly shorter (see e.g. Ishida 1996). The other platycnemidid (sensu lato) group in which the prementum bears just one pair of setae and that occurs within the range of Onychargia is the Disparoneurinae, including Prodasineura Cowley, 1934 (figured by Matsuki 1991). However, as far as we know larvae of this genus have short legs, short rounded caudal lamellae, a moderately broad base to the prementum and a smooth posterolateral margin to the head. It is unlikely within its range the larva could be mistaken for any other species.

Within the Platycnemididae as defined by Dijkstra et al. (2014) there is considerable variation in larval characters, as summarized in Table 3 . Many of these characters also occur in the Coenagrionidae , and their significance in higher classification is debatable, but others are not found elsewhere within the Coenagrionoidea. For example, all known larvae of Idiocnemidinae possess unique caudal lamellae, in which the vertical membrane is strongly pleated to form a frilly ruff around its margins ( Lieftinck 1957; Corbet 1999, p 83). There are no obvious setae either on the prementum or labial palps, as probably occur, sometimes much reduced, in all other Coenagrionoidea ( Dijkstra et al. 2014). In the African subfamily Allocnemidinae considerable variation occurs in the form of the caudal lamellae. Those of Allocnemis leucosticta Sélys, 1863 , are triquetral and inflated ( Samways and Wilmot 2003, Samways 2008) as are those of Stenocnemis pachystigma (Sélys, 1863) , (figured by Vick 1998), whereas those of Mesocnemis singularis Karsch, 1891 , are lamellate. All allocnemidine larvae known have a robust build with head both long and broad, resulting in a much less tapered prementum. Premental and palpal setae are also reduced in this subfamily, as also occurs to an even greater extent in the Neotropical coenagrionid genus Argia ( Novelo-Gutiérrez 1992) . Disparoneurinae and Calicnemiinae, while clearly recognizable, do not exhibit a definitive set of characters. Platycnemidinae seem universally to have a terminal filament at the end of long lamellate caudal lamellae (elaborated to many marginal filaments in Copera Kirby, 1890 ) and angular postocular lobes on the head, together with fairly long legs and well-developed setae on the prementum and palps. We are not aware of this combination of characters occurring elsewhere, but individually all may be found in the Coenagrionidae .

In summary, character combinations within subfamilies of the Platycnemididae appear mostly to be highly consistent ( Table 3 ), but in some cases they do not necessarily support the arrangement of these subfamilies relative to one another presented by Dijkstra et al. (2014). As already implied, based on larval morphology Onychargia might equally be placed in Platycnemididae or Coenagrionidae , but as there is no simple combination of characters which can separate the larvae of the two families, this may simply reflect the limitations of using potentially plastic and widespread characters to elucidate higher-order relationships. We would not on this basis advocate any changes in present higher classification but do suggest that the Platycnemididae as presently constituted perhaps should be regarded as a work in progress.

Figures

FIGURE 3. Anterior details of Onychargia atrocyana: (a) dorsal detail head; (b) ventral detail head and anterior thoracic segments; (c) ventral detail posterior margin of head and supracoxal spurs; (d) detail of antenna; (e) mandibles ventral view; (f) right maxilla ventral view; (g) labial mask dorsal view; (h) detail labial palp in dorsolateral view.

FIGURE 4. Posterior details of Onychargia atrocyana: (a) median caudal lamella; (b) right caudal lamella; (c) ventral view of terminal abdominal segments showing female gonapophyses.

Tables

TABLE 3. A summary of main diagnostic characters occurring in the various subfamilies of the Platycnemiidae sensu lato, as defined by Dijkstra et al. (2014).

	Caudal lamellae	Premental setae	Palpal setae	Median lobe prementum	Postocular lobes of head	Relative leg length	Authority
Onchargiinae Onychargia atrocyana	lamellate, foliate with clear petiole	1+1	6	strongly produced, angular	well developed, angular, bearing strong spiny processes	moderate- ly long	This paper
Allocnemidinae Stenocnemis pachystigma	triquetral, saccoid, very short, small terminal filament	0	2 (+ 6 very short setae)	weakly produced, angular	well developed, rounded	short	Vick 1998
Allocnemidinae Allocnemis leucosticta	triquetral, saccoid	1+1	2	weakly produced, angular	well developed, rounded	moderate- ly long	Samways & Wilmot 2003 Samways 2008
Allocnemidinae Mesocnemis singularis	lamellate	0	2	weakly produced, angular	well developed, rounded	moderate- ly long	Samways & Wilmot 2003 Suhling & Martens 2007
Idiocnemidinae Hylaeargia simulatrix , Palaiargia ceyx ,	short bulbous petiole with membranous frills	0	0	strongly produced, rounded	rounded	very long	Lieftinck 1957
Idiocnemidinae Lieftinckia kimminsi	with membranous frills	-	-	-	-	-	Corbet 1999
Calicnemiinae Coeliccia sp.	lamellate, foliate to ovate, with or without petiole	3+3 to 5+5	4–6	strongly produced, angular	weakly developed, rounded	moderate- ly long to long	Matsuki & Lien 1984 Ishida 1996 Xu 2013 Orr & Dow unpublished
Disparoneurinae Prodasineura coconotata Nososticta sp.	lamellate, short, ovate	1+1	2–4	very weakly produced, angular	well developed, rounded	medium length	Matsuki 1991 Theischinger & Hawking 2006
Platycnemidinae Platycnemis sp. Pseudocopera sp. Copera sp.	elongate, lamellate, with terminal filament or fringe of filaments ( Copera )	2+2	3–4	moderately produced, angular	fairly weakly developed, rounded or with distinct angle	medium - long	Needham 1930 Matsuki 1993 Ishida 1996 Brochard & van der Ploeg 2014
Discussion