INTRODUCTION

Freshwater insects have important roles in the ecology of running waters. They are vital  for riparian and flood plain food webs, processing organic matter and transporting energy along stream channels, laterally to the flood plains and even vertically down into the stream bed. In many forest streams aquatic insects break down leaf litter, supplying nutrients, carbon and energy to the stream and associated ecosystems, may be lake, river and estuaries. Their activities can alter water quality and influence energy flow patterns in different trophic levels; their biological interactions often have significant effects on community structure. So ubiquitous and fundamental to riverine processes are aquatic insects that their diversity, distribution and assemblage are routinely assessed as an indicator of the ‘health’ of running waters (Boulton and Lake, 2008). The distribution of aquatic insect functional feeding groups in running waters is supposed to reflect process-level aquatic ecosystem attributes. Specialized feeders, such as shredders and scrapers, are presumed to be more sensitive to perturbation, while generalists, like gatherers and filterers, are more tolerant to pollution as indicated by the availability of certain food (Barbour et al., 1996). Therefore, in recent years, functional feeding groups have been used as bio-indicator and biomonitoring organisms.

The Western Ghats has many large rivers and streams and is one of the world’s 34 global Biodiversity Hotspots rich in endemic floral and faunal endemism. The Central Western Ghats is recognized as a hot speck in the biodiversity hotspot and forms the northern limit of many endangered ecosystems, sheltering rare trees Myristica magnifica, Semecarpus kathlekanensis, Gymnacranthera canarica and primates like the Endangered Macaca silenus (Lion tailed Macaque) etc. Hill stream and river invertebrates, especially insects, which form bulk of the diversity, have not yet been thoroughly documented despite the efforts by earlier workers (Sivaramakrishnan and Job, 1981; Sivaramakrishnan et al., 1996 & 2000; Burton and Sivaramakrishnan, 1993; Martin et al., 2000;  Ramachandra et al., 2002; Anbalagan et al., 2004; Subramaniam and Sivaramakrishnan, 2005;, Subramanian et al., 2005;, Dinakaran and Anabalagan, 2007 a & b, 2008; Dinakaran et al. 2009; Gupta and Paliwal, 2010; Selvakumar et al., 2012 and Balachandran et al., 2012. The aquatic insects of Aghanashini River of Central Western Ghats, the subject matter of this paper, were never studied before.