Acanthagrion trilobatum Leonard, 1977

Román-Heracleo, Jareth, Springer, Monika & Novelo-Gutiérrez, Rodolfo, 2019, Descriptions of the larvae of Acanthagrion speculum and A. trilobatum from Costa Rica (Odonata: Coenagrionidae), Zootaxa 4624 (2), pp. 219-229 : 220-223

Acanthagrion trilobatum Leonard, 1977

Figs. 1a , 2b, d , 3c , e–f, 4a, c, 5a–b, 6a–b, 7

Specimens examined: Two exuviae (1♂, 1♀ both emerged), seven F-0 larvae (3♂♂, 4♀♀). COSTA RICA: San José, Ciudad Colón, El Rodeo (9.9219°, -84.2552°), elevation 746 m, 07 September 2018, J. Román-Heracleo leg., 2 F-0 larvae (emerged on 10-Sep-2018, 1♀, 12-Sep-2018, 1♂); same data but 5 F-0 larvae (3♂♂, 2♀♀), 24 October 2018, and 2 F-0 larvae (♀♀), 16 January 2019. Three larvae deposited at IEXA; all other specimens in MZUCR.

Description. Exuviae light brown to brown, mature larvae mostly yellowish-brown; slender; femora banded; caudal lamellae very long, 65% of total body length ( Fig. 1a ).

Head. Subpentagonal, almost twice as wide as long, wider than thorax and abdomen ( Fig. 1a ). Labrum with ventral margin concave at middle and scattered setae on surface; clypeus mostly glabrous with some long and short delicate setae; frons and vertex flat, mostly glabrous with some scattered long setae. Occipital margin widely concave, straight at middle; cephalic lobes bulging and rounded, with abundant, short, stout spines (as Fig. 2a ). Antenna ( Fig. 2b ) long and filiform, 7-segmented, scape and pedicel brown, cylindrical, flagellomeres elongated, 3rd antennomere longest and uniformly yellowish-brown, antennomeres 4–6 with the basal half light brown, the distal half yellow pale, 7th antennomere pale yellow, proportional lengths of antennomeres: 0.50, 0.70, 1.0, 0.85, 0.50, 0.30, 0.20. Compound eyes moderately developed, wider than long, laterally prominent. Mandibles (as Fig. 3a ) with formula R 1+2345 y a / L 1+2345 0 a b, b>a. Maxilla: Galeolacinia with six teeth, three dorsal teeth slightly incurved, similar length and robustness, with a basal row of long thin setae, three ventral teeth of different size and robustness (as in Fig. 3b ), size proportions: apical tooth 1.0, median one 0.50, basal tooth 0.15, with a row of setae on ventral side and a basal row of strong, stiff setae which increase in size and robustness apically; maxillary palp gently incurved, ending in a stout, round-tipped spine, with long setae on the external surface. Ventral pad of hypopharynx subpentagonal, anterior margin convex, anterior half with large setae, distal half glabrous. Prementum-postmentum articulation reaching posterior margin of prothoracic coxae; prementum longer than wide ( Fig. 3c ), sub-rhomboidal, lateral margins slightly concave, widely divergent apically, laterodorsally with a row of eight spiniform setae and a group of three very tiny basidorsal spiniform setae; ligula with tip prominent, widely rounded, finely serrulate, with a minute sub-marginal spine on each side of midline; 2 long premental setae to each side of midline, rarely 2+1, on one side only. Labial palp ( Figs. 3c, e ) with 4 long setae, apical lobe ( Fig. 3e ) divided into a squarely truncate dorsal branch composed of four small teeth, the ventral three more or less of same size and robustness, dorsal tooth widest, obtuse and with minute denticles on distal margin, and a ventral branch with a well-developed end hook; ventral margin of palp finely serrate; movable hook almost half the length of labial palp, sharply pointed; two spines at base of palpal articulation. One specimen with a remarkable teratology, in which the prementum has the ligula divided into two lobes and with only 3 palpal setae ( Fig. 3f ).

Thorax. Pronotum light yellowish-brown, subtrapezoidal, posterolateral margin slightly angulated, serrate with the most lateral spiniform setae the longest; posterior margin straight at median. Synthorax yellow dorsally, yellowish-brown laterally; anterior and posterior wing sheaths reaching anterior margin and basal third of S4, respectively. Legs long (when fully extended, tip of metathoracic tarsi reaching S10), slender, pale with subapical bands on femora; dorsal and ventral borders of femora with a row of spiniform setae, an external row of spiniform seta only on prothoracic femora ( Fig. 1a ). Tibiae spiny on internal surface, external surface with long delicate setae, apical internal third with abundant tridentate setae ( Fig. 4 ). Tarsi with two ventral rows of spiniform setae, claws simple, with pulvilliform empodium.

Abdomen. Light brown ( Fig. 1a ); S2–9 with a thin pale middorsal line gradually widening on S8–9, a narrow, reddish brown stripe on each side of pale middorsal line. S1–10 gradually narrowing posteriorly, S2–8 with a lateral carina which bears a row of spiniform setae, increasing in number and robustness posteriorly, S9–10 without such carina but with a row of spiniform setae. Dorsal surface of S5–10 with spiniform setae which increase in abundance and robustness caudally, and a transversal, subapical row of spiniform setae which increase in robustness on posteri- or segments ( Fig. 1a ). Sterna 1–6 lacking spiniform setae, sterna 7–10 with spiniform setae, increasing in abundance and robustness caudally. Posterior margin of sterna 1–9 smooth, that on 10 with a row of stout spiniform setae. Male gonapophyses ( Fig. 4a ) long, surpassing well beyond the posterior margin of sternite 9, slightly incurved in ventral view, sharply pointed, ventral border with a row of five spines and a long seta at the base of each spine. Female gonapophyses ( Fig. 4c ) surpassing well beyond the posterior margin of sternite 10, external valve longer than internal ones, ventral border with a row of five spines and a long seta at the base of each spine; external valve sharply pointed, internal valve roundly tipped. Caudal lamellae (Figs. 5a–b) membranous with visible tracheae, lanceolate, seven times longer than their widest part, mostly translucid, creamy pale basally, ending in a short, sharply pointed tip, without filament, measuring 41.1–44.7% of total body length, with a row of spines on lateral carina; nodus approximately at midlength in median and lateral lamellae; basal half of dorsal margin of median lamella with 26 small spines, basal 0.25 of ventral margin with 10 minute spines (Fig. 5a); basal 0.25 of dorsal margin of lateral lamellae with nine minute spines, basal 0.54 of ventral margin with 41 small spines (Fig. 5b), remainder of margins in both median and lateral lamellae with a row of minute and very fine setae. Cerci slightly longer than wide, widely round-tipped in male (Fig. 6a), conical in female (Fig. 6b).

Measurements (mm): Larvae only: TL 17–19; AL 7–8.5; PfL 1.8─2; MsfL 2.4─2.5; MtfL 3; MWh 2.8; CeL 0.2; MgL 0.5; FgL 0.9. Lateral lamella length 7–7.5, maximum width 1.0. Largest measurements correspond to males.

Remarks: The recreational park El Rodeo is located to the west of the Central Valley of Costa Rica. It stands out for being the greatest fragment of forest in this area of the country. It is located within the forest reserve of the University for Peace. A description of the physical characteristics is provided in Cascante-Marín (2012). The samples were collected in an artificial lake lacking aquatic vegetation that is filled with rainwater ( Fig. 7 ); for this reason, there is variation in depth throughout the year. The lake has a heterogeneous margin consisting of an abrupt edge on one side, while the rest is beach; the border is surrounded mostly by grasses ( Poaceae ). The larvae were found into the sediments. Larvae were collected during the months of August to January, with the highest emergence activity in September. The highest incidence of adult activity was observed during September to December.

Figures

FIGURE 1. Dorsal habitus of the larvae of: a) A. trilobatum; b) A. speculum.

FIGURE 2. Morphological details of the larvae. Figs. a and c, A. speculum; b and d, A. trilobatum. a) Head, dorsal view; b) Right antenna, dorsal view; c) Left antenna, dorsal view; d) Distal apex of metatibiae showing tridentate setae.

FIGURE 3. Morphological details of the larvae. Figs. a, b and d, A. speculum; c, e and f, A. trilobatum. a) Mandibles, ventrointernal view; b) Right galeolacinia, ventral view; c) and d) Prementa, dorsal view; e) Right labial palp, anterior view; f) Prementum teratology, ventral view, showing bilobate ligula.

FIGURE 4. Morphological details of the abdominal segments 9–10 of the larvae. Figs. a and c, A. trilobatum; b and d, A. speculum. a–b, Male gonapophyses, ventral view, c–d, Female gonapophyses, ventral view.

FIGURES 7–8. FIGURE 7. Lake El Rodeo, San José, Costa Rica. Habitat of Acanthagrion trilobatum. FIGURE 8. Habitat of Acanthagrion speculum: a) Lake CATIE, Turrialba, Costa Rica; b) La Tirimbina, Sarapiquí, Costa Rica.