Sunday, January 17, 2010

Tetracanthagyna larva

An impressive larva I’m currently rearing has provided very fascinating insights into terrestrialism in dragonflies larvae.
This aspect of larvae ecology is rather well known. There are several studies showing larvae inhabiting small streams or seepages in forest can have a diminished dependence on water. Some species in Japan live among wet rock and mosses beside streams, a species in Himalaya hang around wet ferns near waterfalls and one species from New Caledonia clings onto the underside of moist leaves away from any water body.

Closer to home, larvae of the genus Tetracanthagyna exhibits semi-terrestrial behaviour. Tetracanthagyna degorsi from Borneo apparently leaves the water at night to perch head down near water surface, seemingly to catch surface swimming fishes. T. waterhousei is another species with similar habits. There’s a picture of its larva catching a fish while completely out of water in the Field Guide to Dragonflies of Hong Kong 2nd edition (page 211).

The larva I have now also shows identical behaviour. Its hunting strategy is ambush. Most of the time it clings out of water with only the head partially submerged. From this position it waits, and when a shrimp moves near, wham! The forceful impact of its labium capturing the hapless shrimp fatally.
After capturing, the larva will occasionally move completely out of water to consume the prey. I notice the bigger the shrimp, the more likely it’ll move above water for consumption. And this becomes more frequent as the larva grows.
Those with a phobia for things creepy and crawly will find this larva utterly hideous. It is now almost entirely black, having adapted its colour to the surroundings; it currently measures about 6cm in length; and has a dragon-like frontal appearance. It might look monstrous but once emerged into adult, it will become the biggest dragonfly in the world in terms of wingspan and weight.

Tetracanthagyna plagiata is the only species from the genus recorded in Singapore so this larva should be of the species. I didn’t have the chance to establish its gender accurately so hopefully it’ll emerge into a male as very few male specimens of T. plagiata exists in museums worldwide. It should emerge very soon as it has stopped feeding for the past few days.




Pictures taken at: ex-situ, November, December 2009 & January 2010.


References:

Corbet, P. S., 1999. Dragonflies: Behaviour and Ecology of Odonata. Cornell University Press, New York.

Norma-Rashid, Y., L. F. Cheong, H. K. Lua & D. H. Murphy, 2008. The dragonflies (Odonata) of Singapore: Current status records and collections of the Raffles Museum of Biodiversity Research. Raffles Museum of Biodiversity Research, Singapore. 24 pp. Uploaded 07 Nov 2008 [http://rmbr.nus.edu.sg/raffles_museum_pub/Dragonfly_of_Singapore.pdf]

Orr, A. G., 2003. A Guide to the Dragonflies of Borneo. Their Identification and Biology. Natural History Publications (Borneo) Sdn. Bhd, Malaysia.

Orr, A. G., 2005. Dragonflies of Peninsular Malaysia and Singapore. Natural History Publications (Borneo) Sdn. Bhd, Malaysia

Wilson, K. D. P., 2004. Field Guide to the Dragonflies of Hong Kong 2nd edn. Agriculture, Fisheries and Conservation Department, Hong Kong SAR.

10 comments:

Noppadon Makbun said...

fascinating! thanks for sharing.

Yen Kheng said...

Oops, so this nymph will grow up and if it's a male, it will become a museum specimen? Sigh... fate....

C.Y. Choong said...

Amazing indeed.

The gender of aeshnid larvae is easily recognized by looking at the ventral part of the abdominal segments 7-9. Female has protuding structure that is lacking in male.

Ian

WChinner said...
This comment has been removed by the author.
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