Brachygonia oculata (Brauer, 1878)

Ngiam, Robin W. J., Chan, Juliana P. S., Khoo, Max D. Y., Kong, Eunice Y. L. & Low, Bing Wen, 2021, Description of the larva of Brachygonia oculata (Brauer, 1878) (Odonata Libellulidae) with notes on rearing technique, Zootaxa 4966 (5), pp. 563-570 : 564-569

Brachygonia oculata (Brauer, 1878) . Description of last instar larva. Figs. (1 to 4)

General:

Very small, overall dark brown with long dark banded legs. Head with prominent bulging eyes and dark banded antennae. Abdomen oviform ( Fig. 1 ).

Head:

Wider than long, somewhat pentangular. Eyes sloping rearwards. Long setae present on the posterior portion of occiput especially along occipital corner. A tuft of fine black setae on the postfrons centrally. Antennae sevensegmented with dark bands. Labium scoop-shaped ( Fig. 2A ). Prementum triangular ( Fig. 2B and 4A). A row of six long primary premental setae and four short secondary premental setae on either side of prementum. 12 to 14 short bristle-like setae present along the distal margin of prementum. Ligula pointed. Labial palps sub-triangular ( Fig. 2B ). Eight long palpal setae along dorsal rim. Moveable hook slender and as long as palpal setae. Six palpal crenations, each with one long palpal seta and one or two shorter setae. 10 to 12 setae along proximal margin of labial palp.

Thorax:

Prothorax narrow with short bristle-like setae along posterior margin. Sparse short bristle spiny setae on pterothorax. Wings sheaths parallel, dark, extending onto S6. Legs with distinctive dark bands on femur and tibia. Short spines throughout entire legs. Tarsus three segmented with sharp pretarsal claws.

Abdomen:

Abdomen oviform with dark mottled markings. Dorsal spines recurved, small from S2 to S5, becoming strongly prominent from S6 to S8 ( Figs. 3A and 4B ). Very small lateral spines from S3 to S7. Sharp protuberant lateral spines on S8 and S9. Lateral spine on S9 reach end of epiproct. Average length (measured dorsally with inner margin of lateral spine) ratio of S9 lateral spine to S9 is 1:0.84. Fine lateral setae present along abdomen. Measured dorsally, average length ratio of S9+S10 to anal appendages (measured to distal end of paraprocts) 1:1.67.

Small spiny setae on anal appendages. Epiproct triangular ( Figs. 3B and 4C ), shorter than paraprocts, average length ratio 1:1.4. Cerci slightly shorter than epiproct. Average length ratio of cerci to epiproct 1:1.3, ceri to paraprocts 1:1.9.

Measurement ranges in examined specimens (n=6) (mm):

TL 9 – 10.8; head width measured across widest section of occiput below compound eyes 2.39 – 3.43; prementum length 1.54 – 1.7; prementum width 1.94 – 2.71; epiproct length 0.59 – 0.64; paraproct length 0.81 – 0.88.

Habitat

In Singapore B. oculata is found exclusively in the western part of the island within a restricted area. One population is found at a shady young secondary forest swampy pool while the other is in a more open pond at a mosaic of forest and scrubland habitats ( Fig. 5 ). The tiny larva inhabits a substrate of submerged tangled leaves, twigs and branches. Throughout Sundaland, the species inhabits a wide range of alluvial forest, peat swamp forest and other forested marshy areas ( Orr 2003), and can thrive in secondary and disturbed forests ( Dow 2009).

Discussion

Brachygonia oculata belongs to the subfamily Brachydiplacinae which consists of 25 genera distributed in Africa, East Asia, Australia and South and Central America ( Silsby 2001). The species are generally small to medium-sized and mostly have primitive wing venation ( Silsby 2001). To the best of our knowledge from published material, larvae from 14 genera, including B. oculata , are now described or illustrated ( Table 1 ).

Silsby (2001) highlighted the genera in Brachydiplacinae is a heterogeneous collection of several unrelated groups. This is supported by the body of work on larval morphology ( Table 1 ). African genera Chalcostephia Kirby, 1889 and Hemistigma Kirby, 1889 differ in several morphological features. These two genera in turn differ from Neotropical Elga Ris, 1911 and Nephepeltia Kirby, 1889 ( Di Domenico et al. 2001). Furthermore, Argyrothemis Ris, 1911 and Oligoclada Karsch, 1890 have long spidery legs resembling corduliids ( Neiss et al. 2018). Additionally Fleck & Orr (2005) reported Nannophyopsis Lieftinck, 1935 sharing several morphological traits with Diastatops Rambur, 1842 from Palpopleurinae , possibly a result of evolutionary convergence.

By comparison Brachygonia is most similar to Brachydiplax Brauer, 1868 for which their distribution overlaps and can occupy the same habitat. They are alike in general habitus appearance. Taxonomically important labium features are also comparable in terms of shape, labial setae and palpal crenation. Morphological characters of four Brachydiplax species from Butler (2012) are combined and compared with B. oculata in Table 2 . Three main differences between B. oculata and Brachydiplax spp. are the smaller size of B. oculata , one less palpal seta and its hind legs reaching beyond the end of paraprocts when fully extended.

The phylogenetic relationship within Brachydiplacinae can only be resolved satisfactorily with a proper DNA analysis which is beyond the scope of this paper. To advance this work, Odonata researchers should search for larva of genera occurring in the same Sundaic region, namely Chalybeothemis Lieftinck, 1933 , Raphismia Kirby, 1889 and Tyriobapta Kirby, 1889 .

Figures

FIGURE 1. Brachygonia oculata F-0 larva, habitus.

FIGURE 2A. Labium dorsal view FIGURE 2B. Prementum dorsal view

FIGURE 3A. Abdominal spines and anal appendages, dorsal lateral view FIGURE 3B. Anal appendages dorsal view

FIGURE 4A. Illustration of prementum. Scale bar = 1mm

FIGURE 5. A shady forest pool and a more open habitat both inhabited by Brachygonia oculata.