Caudal gills are diverse and species specific, and their shape and marking can be used for identification. However, caudal gills may be easily shed. The regenerated gills may be smaller and different from their original form.

For families or genera identification, the examination of larval features should be conducted as follow:
1. Shape of head & angle of protruding eyes e.g. pentagonal with postocular lobes rounded & large eyes in Coenagrionidae
2. Any distinct markings or setae on head e.g. white curved marking at the centre of frons in Crocothemis servilia servillia
3. Shape of antennae e.g. elongated 1st antennal segment in Calopterygidae & stout club-shaped 3rd antennal segement in Gomphidae
4. Shape of labium, arrangement of labial hooks, setae & spines e.g. prementum long, flat in Aeshnidae & mask-shaped in Libellulidae
5. Any distinct markings or setae on legs
6. Number of tarsal segments & shape of claws
7. Wing buds developing in parallel or divergent
8. Shape of abdomen, color pattern & presence of setae, spines or projections on lateral margin
9. Shape of gills, pattern of trachea, presence of markings, setae & spines e.g. long, notate lamellae gills in Pseudagrion
or shape of anal pyramid, length of epiproct, paraproct and cerci e.g. to distinguish species in Aeshnidae

Apart from the examination of larvae, we could collect the exuviae from vegetation near their habitats in some occasions. It is another way to find out some lurking species. Since the characteristics would be fully developed in the final instar larvae, it would be reliable for species identification by examination of exuviae. However, it would be still difficult for doing this particularly for damselflies because their fragile caudal gills might be lost easily.

By comparison of pictures in each families, it is not difficult to match the corresponding genus or species. However, Libellulidae is the largest odonate family in Hong Kong. Here, the characteristics of local species of libellulids are compared to indicate their diversity.

Comparison of some heads in Libellulidae

The examination of head including inspecting eye size and angle of eye projection should be needed for species identification. Besides, the other character traits like any distinct marking or setae should be considered as well. Coloration in this case might not be a reliable clue because some specimens have been preservated, their colorations are quite different from the originals.

The following tentative key for Libellulidae has been prepared using available descriptions from other keys and in-house species emergence for confirmation.

****** Presence of dorsal spine on abdomen ******

Features for ID	Scientific name
* Dorsal spines on abdomen segments IV - VII (S4 - S7)
--> 8 palpal setae; 4 or more outer premental setae on each side; large dark marking between eyes	Orthetrum glaucum
--> 5 palpal setae; 3 outer premental setae	Orthetrum luzonicum
--> 8 palpal setae; 3 outer premental setae; abdominal segments 6-10 reddish brown	Orthetrum sabina
--> 7 palpal setae; 3-4 outer premental setae	Orthetrum triangulare
* Dorsal spines on S3 - S8
--> head triangular shaped; long lateral spines, nearly double length of each segment on S8 and S9	Hydrobasileus croceus
--> head triangular shaped; short lateral spines on S8 and S9	Urothemis signata
--> head pentagonal shaped; lateral spines nearly reaching middle way of anal pyramid	Macrodiplax cora
* Dorsal spines on S4 - S8
--> head elongated laterally, almost in abdominal width	Brachythemis chalybea
--> head quadragular shaped; abdomen oval shaped, the latter abdominal segments seem squeezed forward & upwardly; 6 palpal setae, 3 long outer premental setae on each side	Orthetrum chrysis
--> head quadragular shaped; abdomen comparatively elongated; 5 or 5+1 palpal setae, 2-3 long outer premental setae on each side	Orthetrum pruinosum
Larval species not yet described/ unavailable, but some suspicious individuals having 7 palpal setae & 3 long premental setae are found	Orthetrum melania (S? - S8)
* Dorsal spines on S2 - S9
--> head quadragular shaped; 3 palpal setae; dorsal spines large and hooked	Orthetrum poecilops
* Dorsal spines on S3 - S9
--> body color reddish brown; head with large area of reddish brown markings behind eyes; less markings on wing sheaths	Trithemis aurora
--> body color greenish brown; small or scattered reddish brown markings on head, but getting larger at late stage; conspicuous dark spots pairs on abdomen; more darken markings on wing sheaths	Trithemis festiva
Larval species not yet described/ unavailable	Trithemis pallidinervis
* Dorsal spines on S4 - S9
--> anal pyramid large & protruding	Brachythemis contaminata
--> anal pyramid short & somewhat embraced by abdominal segment IX --> 1 premental seta (in addition 1 to 2 short setae on inner side); 5 palpal setae	Rhyothemis variegata
--> anal pyramid short & somewhat embraced by abdominal segment IX --> 8 premental setae (in addition 3 short setae on inner side); 6 - 7 palpal setae	Rhyothemis fuligonosa
Larval species not yet described/ unavailable	Rhyothemis triangularis
* Dorsal spines on S5 - S9
--> lateral spines long; 9 palpal setae; 10 - 11 prementum setae	Lyriothemis elegantisima
--> head with posterior occipital lobe highly concave; legs with dark markings	Onychothemis tonkinensis (S?-S9)
* Dorsal spines on S2 - S10
--> abdomen wide & large (cf. to its head), ovate shaped with some pale spots on each sides	Pseudothemis zonata
--> head with 4 conspicuous proturberances on occipital margin --> lateral spines long on S9 nearly to the length of its abdominal segment	Zygonyx iris
--> length of lateral spines much shorter than Z. iris	Zygonyx asahinai
* Dorsal spines on S3 - S10
body relatively large sized, up to about 22mm long; head with small eyes and conspicuous large area of dark marking; legs short, slender with conspicuous dark markings on femur; abdomen comparatively large, ovate shaped with dark median markings, especially noticeable black stripes on abdominal segment IX; dorsal spines on abdominal segments III - X and lateral spines on abdominal segements VIII - IX	Tholymis tillarga
* Dorsal spines on S4 - S10
body medium sized, up to 17mm long; abdomen elongated with pair of white markings on abdominal segment IX; dorsal spines on abdominal segments IV - X (tiny on S10) and lateral spines comparatively long on abdominal segments VIII - IX with pale tip	Zyxomma petiolatum

****** Absence of dorsal spine on abdomen ******

* Absence of lateral spine on abdomen body small sized, about 10mm long; head trapezoid-shaped with cluster of setae at posterior margin; legs short, last tarsal segement darken; abdomen oval shaped having transverse row of long setae situated at middle of dorsum and near tip of abdomen; no dorsal spine or lateral spine Acisoma panorpoides   * Presence of LARGE/LONG lateral spines on abdomen --> epiprot as long as, or longer than paraprots Pantala flavescens --> epiprot short, about 2/3 length of paraprots; 10-11 palpal setae, 12-13 premental setae on each side Tramea virginia --> epiprot short, about 2/3 length of paraprots; 10-11 palpal setae, 13-15 premental setae on each side Tramea transmarina euryale   * Presence of SMALL lateral spines on abdomen --> body relatively small sized, <10mm long; head wide with bulbous eyes protruding anterolaterally; abdomen with longitudinal dark band across S6 - S9 in which a pale spot on each side; conspicuous lateral spines on S7 - S9 and no dorsal spine Nannophyopsis clara --> body relatively small sized, <10mm long; head wide with bulbous eyes protruding anterolaterally; abdomen with whitish midline and some pale markings on each side; conspicuous lateral spines on S8 - S9 and no dorsal spine Nannophya pygmea --> body relatively small sized, about 10mm long; eyes bulging anterolaterally; spidery legs with dark tarsus near claw on hind legs; abdomen short and round shaped; inconspicuous lateral spine on S9 only, but without dorsal spine Rhodothemis rufa --> body relatively small sized, about 10mm long; head very wide with eyes protruding laterally; abdomen with purplish midline and some markings on each side, tip of abdomen curled upward, lateral spines inconspicuous on S8 - S9 and no dorsal spine Diplacodes trivialis larval species not yet described/ unavailable Diplacodes nebulosa --> body relatively medium sized, about 15mm long; head pentagonal with two clusters of setae behing eyes; two markings on femur; wing sheath straight up to abdominal segment VII; abdomen ovate shaped with prominent longitudinal pale strip enclosed by dark bands; lateral spines on S8 - S9, but no dorsal spine; long setae present near anal pyramid Neurothemis fulvia larval species not yet described/ unavailable Neurothemis tullia --> body relatively medium sized, up to 20mm long; head wide with bulbous eyes; legs with faint dark markings; abdomen oval shaped, conspicuous lateral spines on S8 - S9 and no dorsal spine Crocothemis servilia --> body up to 21.5mm long, hairy, head rectangular, eyes projected anterolaterally Potamarcha congener larval species not yet described/ unavailable Aethriamanta brevipennis --> body body relatively small sized, about 10mm long; Palopopleura sexmaculata

Acknowledgements

I would like to thank Graham Reels generously shared a lot of references and carried out strenuous field work for species collection. Ken So and Alphonse Tsang kindly gave me invaluable individuals in alive, that was the best way to record larval development and to confirm species ID by rearing larvae to adult emergence. Nicole Kit and Kestrel Lam provided a high quality of species imaging, underwent tedious image cleanup work and well-managed our inventory of odonate species. Philip Yip who is working in Allied Environmental Consultants Limited for sharing their specimens for imaging. e.g. Epophthalmia elegans and Rhyothemis variegata. Last but the least, Professor David Dudgeon is gratefully acknowledged for his full support and patience. Without him, this species guide never happened.

Lily Ng Chiu Yee
Webmaster

Some references

Shida, S., Ishida, K., Kojima, K. Sugimura, M. (1988) Illustrated guide for identification of the Japanese Odonata. Tokyo University Press, 328pp.

Soltesz, K. (1996) Identification Keys to Northeastern Anisoptera Larvae. Workshops on Invertebrate Biodiversity: Dragonflies and Damselflies
Theischinger, G. and Hawking, J. (2009) Identification guide to the Australian Odonata. Department of Environment, Climate Change and Water NSW, 283pp.

Theischinger, G. and Endersby, I. (2014) Australian Dragonfly (Odonata) Larvae: Descriptive history and identification. Memoirs of Museum Victoria 72: 73–120.

Kawai, T. (1992) An illustrated Book of Aquatic Insects of Japan. Tokyo University Press, 409pp.

Wilson, K.D.P. (1995) Hong Kong Dragonflies. Urban Council, Hong Kong, 211pp.

Wilson, K.D.P. (1995) The Gomphid dragonflies of Hong Kong, with descriptions of two new species (Anisoptera: Gomphidae). Odonatologica 24(3): 319-340.

Yeh, W-C. & Lien, J-C. (1995) Proven distribution of Nannophyopsis clara in Taiwan and morphological description of the ultimate instar larva. Tombo XXXVIII 1995 Tokyo: 24-26.