DIVERSITY OF GERROMORPHA ( HETEROPTERA : HEMIPTERA : INSECTA ) IN THE WESTERN GHATS STATES , INDIA

The infraoder Gerromorpha comprises of semi-aquatic bugs characterised by long conspicuous antennae, longer than head and inserted in front of eyes, distributed in all kinds of climatic zones of the world, except the coldest and driest parts of the world (Andersen, 1982). These bugs are the most successful group living on the surface of water, occupying almost every kind of aquatic habitat. This infraoder contains 'water striders' or 'pond skaters' (Family Gerridae); 'small water striders' or 'water measurers' (Family Veliidae); 'marsh treaders' or 'water measurers' (Family Hydrometridae); 'water treaders' or 'pond weed bugs' (Family Mesoveliidae); 'velvet water bugs' (Family Hebridae) and bugs pertaining to the families Macroveliidae, Paraphrynoveliidae and Hermatobatidae. The last three families do not occur in India.


INTRODUCTION
The infraoder Gerromorpha comprises of semi-aquatic bugs characterised by long conspicuous antennae, longer than head and inserted in front of eyes, distributed in all kinds of climatic zones of the world, except the coldest and driest parts of the world (Andersen, 1982).These bugs are the most successful group living on the surface of water, occupying almost every kind of aquatic habitat.This infraoder contains 'water striders' or 'pond skaters' (Family Gerridae); 'small water striders' or 'water measurers' (Family Veliidae); 'marsh treaders' or 'water measurers' (Family Hydrometridae); 'water treaders' or 'pond weed bugs' (Family Mesoveliidae); 'velvet water bugs' (Family Hebridae) and bugs pertaining to the families Macroveliidae, Paraphrynoveliidae and Hermatobatidae.The last three families do not occur in India.
Aquatic and semi-aquatic bugs play an important role in fresh water ecosystems, as polyphagous and carnivorous insects in different trophic levels.These water bugs serve as zoogeographical indicators for diverse habitats due to their poor dispersal capability.Besides, the role of these bugs as predator in biological control of mosquito is well known (Jenkins, 1964).Knowledge on the ecological aspects of the bugs is essential to study (a) fish biology, due to reversals in preypredator relationship and (b) food web as prey to higher vertebrates like newts, salamanders, water snakes and birds.Further, studie~ indicate that the quality of aquatic environment is partially dependent on aquatic bug population dynamics.(Thirumalai & Raghunathan, 1988).
The political states of Karnataka, Kerala, Maharashtra and Tamilnadu falling within 8° & 22° Nand 73° & 81 ° E have a geographical feature in common, the Western Ghats traversing them at one part or the other (Goa not included).Since taxonomy holds the key to biodiversity studies, this work has been undertaken to document diversity of semi-aquatic bugs of the four states.The similarity and biotal dispersity of these insects are taxonomically analysed by applying Sorensen and Koch's Indices.Distant, 1915 Halobates Buchanan-White, 1883
To study the similarity and dispersity of the semi .. aquatic bugs of the four states.Sorensen Index of similarity (1948) and Index on Biotal Dispersity (IBD) of Koch (1957) have been applied on the formulae : Sorensen Index (S) = ~ Cfa+b X 100 where 'C' is the number of species common to both the associations; 'a' the number-of species in one association and 'b' the number of species in the other association.
Index of Biotal Dispersity (IBD) = T .. S X 100fS (n-l) where 'T' is the arithmetical sum of species livin'g in each of 'n' compared association and'S' is the total list of species in 'n' compared association.If the Sorensen Index is 100, it indicates absolute similarity and if the IBD is '0', 'T' will be equivalent to'S', indicating a completely different set.t.
Sorensen Indices and Koch's Indices of Biotal Dispersity are presented in table 2 and 3.The highest of Sorensen Indices is found for Kamataka-Kerala (65) and the lowest for .The highest of IBD is. for  where as the lowest for Karnataka-Tamil Nadu (21).The total, common and percentage of common occurence of species in all the combination of states are projected in Fig. 1.To render the data more useful, comparison has been made betwe~n the geographical and wetland areas of the four states with the total number of species in each state (Fig. 2.).

DISCUSSION
The aquatic and semi-aquatic groups of insects represent a significant level of diversity (Ghosh, 1996).In India, 123 species of Gerromorpha constituting 8.3% of global occurence command taxonomic work.It is further felt that every effort should be made to have data bas~s of local and provincial biodiversity especially of globally important areas like Western Ghats (Daniels, 1997).In this context, the present work signifies the necessity to a~thentical1y record 64 species from the province consisting of Karnataka, Kerala, Maharastra and Tamil Nadu to dispel taxonomic uncertaint~es existing at various levels (Daniels, 1997) in Western Ghats biodiversity.It is interesting to note that more than 50% of the known Gerromorpha species of India are found, in this province where Western Ghats run through.There is . .little doubt that the Ghats states have an array of various forest types and-complex assemblages •of ecosystems providing home to diverse gerromorphan faunistic constituents.Besides, the original fauna of peninsula constitute the true Indian elements derived from the Indian part of Gondwana land (Mani, 1974).~ence, inventorisation of aquatic insect resources becomes a high priority area to study the functional aspects of community structure incorporated in food webs that provide information on energyflow (Ananthakrishnan, 1999).
The highest number of species in common is in Karnataka-Tamil Nadu sector (24 species) where as the Sorensen Index is the nighest for the Kamataka-Kerala sector (65).On the contrary, the lowest number of species in ~ommon is in the Karnataka-Maharashtra sector (10 species) and Sorensen Index is also the lowest for the same area (38).Even though Karnataka-Kerala and Kerala-Tamil Nadu have a similar numerical strength (21 species), the Sorensen ind~ces differ.The same trend is seen in Kerala-Maharashtra and Maharashtra-Tamil Nadu sectors also (Table 2).The highest of IBD for Kerala-Maharashtra sector (54) indicates the possibilities of less exploration, diverse habitats and different biogeographic conditions (Table 3).It is evident from Fig. 2 that, though wetland area is more in Kamataka and Maharashtra states, the number of species reported is less than Tamil Nadu which ranks third in wetland availability.Hence, it invites more efforts for systematic exploration of this group in Maharashtra intensively.
As has been aptly pointed out by Ganeshaiah et al (1997), variety of indices quantifying biodiversity treat all species as equivalent, ignoring taxonomy, morphological and biological differences among species of community.Thus, one will have to rely on hard core taxonomy for a meaningful approach to understand issues concerning bio-diversity.