Vestalis gracilis (Rambur, 1842)

Rattanachan, Kaewpawika, Sangpradub, Narumon & Keetapithchayakul, Tosaphol Saetung, 2022, Description of the larva of Vestalis gracilis (Rambur, 1842) (Zygoptera: Calopterygidae) from Thailand, International Journal of Odonatology 25, pp. 1-6 : 2-5

publication ID	https://doi.org/10.48156/1388.2022.1917151
DOI	https://doi.org/10.5281/zenodo.15564738
persistent identifier	https://treatment.plazi.org/id/03DC879F-FF94-DA13-FF33-6FCC7801FA62
treatment provided by	Felipe
scientific name	Vestalis gracilis
status

Treatment

Description of the larva of Vestalis gracilis View in CoL

( Figures 1 View Figure 1 ─6, 8C, 9B, 10─11)

Larva yellowish brown to dark brown, long, slender, length of body 7.43× as long as maximum body width ( Figure 1 View Figure 1 ).

Head: subpentagonal, with marked pattern, 1.18× wider than head length, head wider than thorax and abdomen. Distal half of labrum covered with dense setae, anterior margin flattened ventrally with sparse setae. Clypeus with small, sparse setae. Frons and vertex glabrous. Occiput concave, mostly glabrous. Postocular lobes curvilinear in outline with several scattered spiniform setae, with posterolaterally directed protuberance at the middle of each side. Compound eyes narrow and rounded, protruding posterolaterally. Antennae ( Figure 2A View Figure 2 ) filiform, 1.87× longer than head length, 8-segmented with A1 (scape) the longest; relative lengths of antennomeres 1.00 (0.5 mm): 0.17: 0.60: 0.48: 0.24: 0.20: 0.11: 0.13; A1 concave laterally, triangular, its maximum width 2.20 and 2.75 × A2 and A3 width, respectively, with small setae on lateral side and distally on mesal side, and long setae on the proximal to middle inner side; A3 with small scattered setae; A3–A8 with small setae. Genae ( Figure 2B View Figure 2 ) with scattered setae. Labium: prementum-postmentum articulation extended at level of anterior margin of mesocoxae; prementum ( Figure 2C View Figure 2 ) with scattered setae, lobes of ligula separated by a deep, ovate, median clef, maximum width between lobes 0.53× the clef length, tips of lobes truncated and touching each other mesally ( Figure 2D View Figure 2 ); two setae on mesal side of each lobe of ligula, and 4–5 minute scattered setae in basal area; lateral margin of ligula lobe finely serrate; sub-quadrangular postmentum with scattered setae on ventral side. Labial palp ( Figure 3A View Figure 3 ) as long as 0.38× prementum length, with two short palpal setae, one seta near articulation of movable hook, the other one close to the palp’s articulation, mesal margin serrulate, a row of delicate setae along lateral margin, distal end with three hooks ( Figure 3C View Figure 3 ), outer and middle hook approximately of the same size and inner hook shortest, outer hook with a row of 5–6 small protuberances ( Figure 3B View Figure 3 ); movable hook slender and sharply pointed, 0.71× as long as labial palp length. Maxilla ( Figures 3D, E View Figure 3 ): galeolacinia covered with dense long setae, with seven teeth, three small ventral teeth of different size and robustness, apical tooth largest, three incurved dorsal teeth of similar size. Mandibles ( Figures 4A–D View Figure 4 ) asymmetrical, brawny, with well-developed large teeth, with molar crest; mandibular formula:

L 1+1’234 0 a(m 1-6)b / R 1+1’234 y a(m 0)b,

a> b in lef mandible, a = b in right mandible.

Thorax: Pronotum subtrapezoidal with scattered setae, lateral margins almost concave, pronotal disc smooth with a small shallow groove. Pterothorax with scattered setae. Wing sheaths parallel; anterior and posterior wing sheaths reaching posterior margin of abdominal S4. Legs almost flat and very long: femora thin with a dark band on the posterior side and small setae, the hind femora 1.62× and 1.24 × longer than the fore and mid femora; tibial comb ( Figure 4E View Figure 4 ) with dense spiniform setae on its distal ends; tarsi with two rows of scattered setae, tarsal formula 3–3–3, with 2 simple claws and pulvilliform empodium.

Abdomen: slender and cylindrical, narrowed caudally, abdominal terga S1–3 smooth, abdominal terga S4–10 scattered setae; abdominal sterna S1–10 with a pale network of tracheoles, with scattered setae; posterolateral ends of S9–10 with blunted spines. Male gonapophyses ( Figure 5A, C View Figure 5 ) triangular, roundly pointed, widely divergent in ventral view, extending to anterior margin of S10; gonopore O-shaped, embossed, and with a fissure from its middle to its posterior end. Male cerci digitiform, concave on its inner surface and roundly tipped. Female gonapophyses ( Figure 5B, D View Figure 5 ) almost reaching posterior margin of S10; lateral valves covered with small setae, parallel in lateral view ( Figure 5D View Figure 5 ), divided into two lobes, the dorsal lobe digitiform, bluntly tipped, largest, the ventral lobe short, spine-like, sharply pointed, central valves slender, apically rounded, and slightly shorter than lateral valves; female cerci small, cone-shaped and bluntly tipped. Caudal lamellae ( Figure 6 View Figure 6 ) long triquetral, covered with small setae along their margins, lateral lamellae longer than median lamellae, median tracheae largest, distinct, reaching 80% of caudal lamellae, indistinct secondary branches, irregularly branched, a little undulate and extending to the distal margin, tertiary branches arising from margin; median lamellae obliquely truncated, subtrapezoid, with a row of 17–21 spiniform setae in the center on both sides; lateral lamellae oblong, apex with a small pointed tip, with a row of 14–16 spiniform setae on the lateral side of the median trachea.

Measurements. [in mm; n = 8; 2 exuviae and 6 larval specimens in alcohol] total length of body without caudal lamellae = 22.99–23.88; length of lateral lamellae = 8.26–8.50; length of median lamellae = 7.4–7.78; width of head = 2.85–3.00; length of antenna = 4.76–5.10; width and length of prementum = 2.90–3.00 and 3.90– 4.15; length of labial palp = 1.50–1.55; length of movable hook = 1.60–1.70; length of inner and outer wing sheaths = 5.82–7.31 and 5.97–7.01; length of femora (fore: mid: hind) = 3.73–3.75: 4.63–5.08: 5.82–6.27; length of tibiae (fore: mid: hind) = 4.78–4.93: 5.81–5.84: 6.27–6.72; length of tarsi (fore: mid: hind) 1.05–1.19: 1.34–1.49: 1.34–1.64.

Diagnosis. The larva of V. gracilis is similar to that of V. amoena ( Table 1 View Table 1 ). They share posterlaterally directed protuberance on their postocular lobes, obliquely truncate on the posterior margin of median lamella, and two setae on the labial palp, but V. gracilis bears eight antennal segments whereas seven antennal segments are found in V. amoena . Vestalis luctuosa shares eight antennal segments with V. gracilis . Moreover, V. luctuosa differs from the other two Vestalis spp. by having an upward-directed protuberance on its postocular lobe and a pointed posterior margin of its median lamella. Liefinck (1965) mentioned a possibly diagnostic character of V. amoena on A2, which has an extra joint between the pedicel and the second segment of the antenna ( Figure 10 View Figure 10 ), but did not provide sufficient characteristics for comparisons with other species. This character on A2 might have been overlooked in other calopterygid species. The larva of V. amoena should be reanalyzed from the original specimens to confirm the character described above and add more diagnostic characters.

Biological notes. The larvae of V. gracilis inhabit forest streams. They are usually concealed among vegetation in riffle zones ( Figure 10 View Figure 10 ) and are generally found together with other calopterygid larvae such as those of Neurobasis chinensis (Linnaeus, 1758) . When the larvae felt threatened, they displayed agonistic behavior in that they lifed the abdomen and pointed the caudal lamellae forward in a scorpion-like posture ( Figure 11 View Figure 11 ). These agonistic displays are similar to those observed in other calopterygid species ( Ryazanova & Mazokhin-Poshnyakov, 1992) and seem to be a characteristic behavior of damselfly larvae ( Rowe, 1992, 2002, 2004; Saetung et al., 2020).