Spesbona angusta Selys, 1863

Charl Deacon & Michael J. Samways, 2016, Larva of one of the world’s rarest and most threatened damselflies: Spesbona angusta (Odonata: Platycnemididae), Odonatologica 43 (3), pp. 225-234 : 226-233

Spesbona angusta Selys, 1863

( Figs 1–3 )

Material studied. Four F larvae (2 ♂, 2 ♀), Theewaterskloof Conservancy , Western Cape, South Africa (33°58’37’’S, 19°08’31’’E), viii-2015, leg. Ch. Deacon.

Description. Overall shape distinctly slender and elongate, total length 14.5mm ( Fig. 1 ; Table 1 ). Body dorso-ventrally flattened with distinctive colour patterns; overall light brown with head slightly darker brown. Dorsal, ventral, and lateral surfaces of body covered with almost bilaterally symmetrical pale markings, becoming denser posteriorly, which do not coincide with setae distributed across the surface of the body. Entire body with dark banded pattern, most evident on head, legs and lamellae ( Fig. 1 ).

Head in dorsal view roughly pentagonal, with well-developed post-ocular lobes, and with the postero-lateral corners sharply rounded ( Fig. 2 ). In addition, a post-ocular horn-like structure is present, protruding upwards and unique in shape to each individual. There are two unique darker bands on the head – a broader band ventrally and a narrower band dorsally. Head with pale white dorsal stripe, continuous with pale white stripe on thorax. Horn-like structures and post-ocular lobes sparsely covered by short tuπs of setae. Eyes wide but not very prominent in dorsal view and well-rounded on sides. Colour of eyes distinct, with clear black bands on golden yellowish ground colour in live specimens.

Antennae 6-segmented, with segment 5 being the shortest, about four times shorter than segments 2, 3, and 4, and twice as short as segments 1 and 6. Antennae mounted on protruding base, appearing to be the first segment fused to head. General structure long and slender, densely covered by setae on segments 1 and 2, and sparsely covered by setae on segments 3–6. Broad black bands present in the middle of segment 4 and at the origin of segment 6.

Prementum elongate pentagonal. Premental setae 2 +2, arranged in straight line, not reaching from one side to the other ( Fig. 2 ). Distal margin of prementum conspicuously crenulated and bulging at distal extremity. Sides with two rows of spiniform setae along distal quarter. Proximal to rows of setae, is a distinct notch and blunt dentate process facing outwards at point ⅔rd distally from base. Undersurface of prementum lacking processes. When at rest articulation of prementum and postmentum reaches the boundary of meso- and metasternum.

Palpal setae usually 4+ 4, but may be 5+ 4 in some cases ( Fig. 2 ). Spacing between each palpal seta is irregular. Short setae present on distal margin. Palpal setae are as long as movable hook.

Thorax almost glabrous, far less hirsute than most other parts of the body. Wing sheaths long and slender, extending to the intersection of abdominal segments 5 and 6.

Legs very long and slender, densely covered by setae varying in length with no clear pattern. Legs lineate (angular in section), due to strong keels on tibiae and femora. Two brown rings present on each femur and tibia, with paired-spotted pattern between each pair of rings.

Abdomen dorso-ventrally flattened, with lateral carina on S1 to S10. Setae do not coincide with lateral carina, and are denser and longer posteriorly. White spotted pattern visible in live specimens, with two pairs of larger blackish spots at each abdominal intersection dorsally.

Lamellae exceptionally long and slender, close to the entire length of the abdomen ( Table 1 ). With highly frilled ventral margins ( Fig. 3 ), varying in shape between individuals. Medial lamella slightly shorter than lateral lamellae and frilled on both dorsal and ventral margin. Very clear sulcus on the lamellae insertion on S10. Principal tracheae are clearly distinguishable and following contour of dorsal margin. Secondary tracheae not clearly visible, but emerge from primary tracheae at an angle close to 45°. Secondary tracheae short and straight.

Biological notes. All larvae of Spesbona angusta were collected by netting through banks of fennel-leaved pondweed Stockenia sp. ( Aponogetonaceae ) in well vegetated, shallow pools rich in organic matter. Larvae were not found within sediment, or in clear, non-vegetated sections of pools. The pools were small (<3.5 m diameter), and shallow (<0.7 m deep) in the dry month of March. The lamellae of S. angusta do not seem to be important for swimming, although they may be more important for predator avoidance as they easily separate from the body. The frilled borders of the lamellae also suggest that at least some time during development the larvae are well adapted to survive oxygen-deficient conditions. Such conditions are expected as the temperature of the pools varies greatly, with a water temperature of 21°C recorded in high summer when adults are ovipositing and 8°C during winter months when larvae are approaching emergence ( Table 2 ). With the exception of Dytiscidae and Notonectidae, roughly with the same abundance as S.angusta and averaging around five individuals per sample, no other macroinvertebrates were found in the pools. The long and spiny leg structure of the larvae also suggests movement by crawling and attachment to aquatic vegetation, rather than actively swimming.

Discussion

The larva of Spesbona angusta is easy to distinguish from any other South African species of damselfly outside the family of Platycnemididae. Oth- er genera within the same family are also characterized by slender bodies, with a body length of 16 mm in Mesocnemis singularis Karsch, 1891 ( CORBET 1956), and 16–17 mm in Allocnemis leucosticta Selys, 1863 ( BARNARD 1937). The distinct banded pattern on the femora and tibiae is shared by M.singularis. The bands are not mentioned in the description of A.leucosticta by BARNARD (1937), but are mentioned by CORBET (1956). The head morphology is easily distinguished from other South African platycnemidids by not being bulky in relation to the rest of the body. The combination of the angular post-ocular lobes on the head and the long and spiny legs are characteristic of Platycnemidinae ( ORR & DOw 2015). Antennae can be distinguished from that of M.singularis by being 6-segmented instead of 7-segmented. However, the antennae of A.leucosticta are also 6-segmented. The antennae are also banded, although there is variation in the consistency of the banded pattern between individuals.

The lamellae of M. singularis are long, smooth, and parallel-sided (COR- BET 1956) and those of A. leucosticta are slender, thickened at the insertion, and triquetral in cross-section ( BARNARD 1937; SAMwAYs & WHITELEY 1997). S.angusta also posesses long slender lamellae (8 mm in length), but none of the other above-mentioned features are shared. The lamellae of S.angusta are frilled to varying degrees, a feature easily distinguishable from any other South African odonate. The lamellae are also not parallelsided. Finally, the palpal setae of S. angusta are 4 + 4, easily distinguishable from the palpal setae of M. singularis, being 2 + 2. The premental setae are 2 + 2, which corresponds to the premental setae of Platycnemis, Pseudocopera, and Copera sp. (DIjKsTRA et al. 2014; ORR & DOw 2015) and is contrasting to the premental setae of A. leucosticta (1 + 1) and M. singularis (0) (DIjKsTRA et al. 2014). The well-developed setae on the palps and prementum further add to the set of characteristics which distinguishes Platycnemidinae from the other subfamilies of Platycnemididae ( ORR & DOw 2015).

Several characteristics are also shared between Spesbona and its sister group, Copera, as defined by DIjKsTRA et al. (2014). One key feature is the elongate, frilled lamellae as seen in Copera vittata Selys, 1863, among others (NEEDHAM 1930). Further diagnostic features contributing to the placement of S. angusta within the phylogenetic reconstruction by DIjKsTRA et al. (2014) is the 6-segmented antennae as seen in Copera (contrasting to 7-segmented antennae seen in platycnemidids in general), premental setae formula (2 + 2), and the palpal setae formula (3–4), as seen in Copera marginipes Rambur, 1842 (NEEDHAM 1930), the bulging distal margin of the prementum ( Fig. 2 ), and the long and slender legs seen in the larval stage, all of which are coincidental with the diagnostic features of Copera.

The larvae are highly habitat specific yet this habitat is highly dynamic with challenges for survival, especially the greatly fluctuating water levels. Water temperature of pools fluctuated significantly from summer to winter ( Table 2 ), and along with great seasonal differences in rainfall (hot, dry summers and wet, cool winters), provided a specialized habitat, possibly to reduce interspecific competition and predation. Lower dissolved oxygen content during the summer is presumably countered by the frills on the lamellae, which are probably capable of extracting more oxygen from the surrounding water than would be the case without them.

Heterogeneity of abiotic water conditions, with specific reference to fluctuating dissolved oxygen and pH, is highly significant for the conservation of the species, as homogenizing these conditions would probably create the opportunity for competing species to establish in and around the remnant pools. As the larvae are sluggish, faster moving species could possibly exert competitive pressure on S. angusta.

Figures

Figure 1. SpesbonD DngustD, larva. a − habitus, dorsal view; b − lamellae insertion, dorsal view; c − lamellae insertion, lateral view.

Figure 2. SpesbonD DngustD, larva. a − head, dorsal view; b − head, lateral view; c − prementum, dorsal view; d – palpus.

Figure 3. SpesbonD DngustD, lamellae. a − lat- eral lamella with more complex frills; b − lat- eral lamella with less complex frills; c − me- dial lamella.

Tables

Table 1. Measurements [mm] from four specimens of SpesbonD DngustD, collect- ed from the field at Theewaterskloof Conservancy, Western Cape, South Africa in August 2015. S. D. − standard deviation.

Character	Mean	S.D.
Head width	3.2	0.02
Distance between antenna insertions	0.9	0.04
Antenna length	1.8	0.02
Wing-sheath length	3.4	0.1
Hind femur length	3.5	0.1
Hind tibia length	2.6	0.1
Hind tarsus length	1.3	0.1
Mask width	1.4	0.05
Body length	14.5	1.5
Medial lamella length	6.6	0.2
Lateral lamella length	7.6	0.2

Table 2. Average water conditions during summer (November) and winter (April) of shallow pools at Theewaterskloof Conservancy, Western Cape, South Africa, where larval development of SpesbonD DngustD was recorded.

	November	April
Temperature [°C]	19.95	8.7
Conductivity [mS mg-1]	0.03	0.3
Dissolved oxygen [%]	50.64	58.43
pH	6.68	4.46