Some Animals from Hill Streams (Sri Lanka) (long!)


The south-western Wet Zone of Sri Lanka, as has often been pointed out, is in many respects equivalent to India's Malabar Tract. On gaining some familiarity with the biota of one area an obvious question that occurs is: Are there corresponding species or communities in the other, equivalent, area? For a systematist this 'correspondence' has the implication of phylogenetic relationship, while from an ecological perspective it might (perhaps) imply similarities in community structure /membership. Some examples from birds may illustrate this 'equivalence': _Columba torringtonii_ (Ceylon Wood Pigeon)/_Columba elphinstonii_ (Nilgiri Wood Pigeon); _Psittacula calthropae_ (Layard's Parakeet)/ _Psittacula columboides_ (Blue-winged Parakeet); _Megalaima rubricapilla_ (Ceylon Small Barbet)/_Megalaima malabarica_ (Malabar Crimson-throated Barbet); _Lonchura kelaarti_ (Ceylon Hill Munia)/_Lonchura jerdoni_ (Jerdon's Rufous-bellied Munia). At an ecological level one may, for example, compare mixed hunting parties in the forests of the Sri Lanka's Wet Zone with those observed in India's Malabar Tract.

A characteristic feature of the forest-clad hills of Sri Lanka's Wet Zone is the presence of many rocky streams. They are the habitat of various fishes, amphibians, aquatic insects and other kinds of invertebrate animals, including freshwater prawns and crabs, snails, flatworms, etc. These streams run over a rocky substrate and are strewn with rocks and boulders. Water flows from one rock pool to another via a series of cascades, swirling round emergent rocks and over submerged algae-covered ones. The velocity of the water depends on the gradient (cascades) or position (middle or sides of pools), and a diversity of microhabitats is seen based on water flow, substrate (rock, pebbles, sand, mud), light, etc.

While the animals that are found in or on (e.g., whirligig beetles and pond skaters) the water are fascinating enough, I would like to draw attention to a small but distinctive assemblage of species that occur on the dry or water-sprayed emergent rocks in or bordering these streams.

The most conspicuous member of this community of rock loving creatures is the damselfly _Euphaea splendens_ (Odonata: Euphaeidae), a beautiful dark purplish-blue insect which is very characteristic of these rocky hill streams. The nymphs are, of course, aquatic and not noticed by the casual observer but the adults are impossible to miss as they flutter and dart in the sunlight and settle on the rocks. Next is the grouse locust _Scelimena gavialis_ (Orthoptera: Tetrigidae), an amphibious insect which is equally at home on dry rocks and water. Much less conspicuous is the cricket _Paranemobius pictus_ (Orthoptera: Gryllidae). Slinging its horizontal orb web over pools, a few inches above the water and anchored to emergent rocks, is a spider of the genus _Orsinome_ (Araneae: Tetragnathidae), apparently an undescribed species related to the Indian species _Orsinome marmorea_. Some extracts from my notebooks concerning these animals may not be out of place here:

Morawaka Hill, Morawaka, Matara District. 11 March 1984: "_Orsinome_ sp. 6 females and 1 male from fairly strong horizontal orb webs spun between boulders in stream, with spider about 3-4 inches above water at hub. If disturbed spider either runs to different part of web or lets itself down into water from hub on dragline and climbs back after a short while. If swept to rock climbs about 1-2 inches above water level and remains above water, flattening itself against rock. Very common, females more numerous than males."

Kanneliya Forest Reserve, Galle District. 14 September 1985: " stream - _Scelimena gavialis_ very easy to capture, in flight shows bluish wings. Red ornamentation on body (spikes and thin lines)... _Paranemobius pictus_ very difficult to catch, many on rocks especially close to water, on shaded sides. Best method of catching: quickly lowering pill box over insect - 3 collected. _Euphaea splendens_ numerous - many captured. _Orsinome_ common, adult male on rock watched long."

Starmuhlner (1984: 240, fig 8) gives an interesting diagram indicating various microhabitats in Sri Lankan hill streams and the species found in them; he does not mention the spider _Orsinome_ sp., however. For the distribution of some of the species mentioned here see the following:

_Euphaea splendens_: Leiftinck (1971).
_Scelimena gavialis_: Blackith (1988).
_Paranemobius pictus_: Sandarasagara (1954), Chopard (1969).

Since streams very similar to those that occur in Sri Lanka's hills are found in the Western Ghats I am very curious to know if associations of rock-inhabiting animals similar to the small community I have tried to describe above have been observed there. Because of the refugial/ vicariant nature of the distribution of rainforests in South Asia one may reasonably expect to see similar associations and related species also in Assam and the North-east, even extending into Malaysia (the famous 'Indo-Malayan horseshoe pattern').


Blackith, R. E. 1988. The Tetrigidae (Orthoptera) of Sri Lanka. Entomologica Scandinavica Supplement 30: 91-107.

Chopard, L. 1969. The Fauna of India and Adjacent Countries: Orthoptera. Vol. 2. Grylloidea. Zoological Survey of India, Calcutta.

Leiftinck, M. A. 1971. Odonata from Ceylon. Entomologica Scandinavica Supplement 1: 188-207.

Sandarasagara, T. R. 1954. Checklist of the Tridactylidae and Gryllidae (Insecta, Orthoptera) of Ceylon, with records of distribution. Journal of the Bombay Natural History Society 52(2-3): 50-562.

Starmuhlner, F. 1984. Mountain stream fauna, with special reference to Mollusca. In: Fernando, C. H. (ed.) Ecology and Biogeography in Sri Lanka: 215-255. Dr. W. Junk, The Hague.



Dilrukshan Priyantha Wijesinghe
Department of Entomology              Fax: 212-769 5277
American Museum of Natural History
Central Park West at 79th Street
New York, NY 10024

BACK TO *********************************************************************