| dc.description.abstract |
Heen Ganga is a major tributary of river Mahaweli which drains through the Knuckles forest
region in Sri Lanka. The community composition of benthic insects in four tributaries of Heen
Ganga was investigated from June to September 2004 and in April 2005. Two of the tributaries
were situated closer to the human settlements and the other two were relatively less disturbed.
Another tributary in Galmal Oya was selected which is located at higher altitudes and with minimum
disturbances, for the comparison. Two replicate samples were taken from each site using the
Surber sampler (25 cm x 25 cm metal frame) while lifting the stones and manually stirring the
bottom substrate demarcated by the frame for five minutes. Larvae and adult insects in the samples
were sorted and examined under a stereo – microscope at suitable magnifications and the specimens
were preserved in 70% alcohol and identified to the furthest possible taxonomic levels
Insects belonging to seven orders were observed in the overall collection and Diptera (larvae only,
30 %), Coleoptera (larvae & adults , 28%), Ephemeroptera (larvae only , 21%) and Trichoptera
(larvae only , 18%) dominated the orders, Odonata (larvae only, 1.8%), Plecoptera (larvae only,
1.06%) and Hemiptera (larvae only, 0.23%) observed at the Heen Ganga study sites. Also, among
the six families of dipterans observed chironomids apparently dominated the other families, Tipulidae,
Ceratopogonidae, Simuliidae, Athericidae and Psychodidae. Members of six families of Coleopterans,
Psephenidae, Elmidae, Scirtidae, Gyrinidae , Lampyridae and Hydrophilidae and six families
of ephemeropterans, Baetidae, Leptophlebiidae, Ephemeridae, Ephemerilidae , Caenidae &
Prosopistomatidae were also identified. Trichopterans belonging to three suborders and eleven
families, namely Glossosomatidae, Hydroptilidae , Dipseudopsidae, Ecnomidae , Hydropsychidae,
Goeridae ,Calamoceratidae, Lepidostomatidae, Leptoceridae, Sericostomatidae and
Helicopsychidae were observed while the members of Goeridae and Glossosomatidae were
apparently dominant among them. At the Galmal Oya study site, coleopterans (33%) and dipterans
(30%) were dominant while the trichopteran (26%) and ephemeropterans (8%) were observed
in lower proportions. In addition, Hemiptera (1.6%) Plecoptera (1.2%) and Odonata
(0.33%) were also observed. The values of diversity indices for morphospecies of common five
orders indicated that there was not much difference between the diversity of benthic insect community
in Heen Ganga tributaries (H’= - å pi ln pi =3.3) and the Galmal Oya study site. (H’=3.6). |
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