1918.] N. Annandale : Fauna of the InU Lake. 203 



Geographical relations of the Fauna. 



The lower invertebrates of the lake and its connected waters throw 

 little light on the geographical relations of the fauna, and the aquatic 

 insects (none of which, of course, are completely aquatic in a literal 

 sense) are still, with the exception of the Rhynchota and the Odonata, 

 unknown. I will deal with the Rhynchota separately. Among the 

 strictly aquatic forms we need consider only the Decapod Crustacea, 

 the Mollusca and the fish. The facts about the geographical distri- 

 bution of these groups will be found on pp. 37-38, pp. 81-82 and 

 pp. 145-148. of this volume. They may be summarized as follows : — 



33 genera are represented, of which 2 (fish) are endemic in the 



lake = ca. 6 per cent. 

 2 other genera (molluscs) are practically confined to the Shan 



Plateau, giving a total of four endemic Shan genera =ca. 



12 per cent. 

 67 species and races have been found, of which 30 have been found 



only in the lake and connected waters, giving a percentage 



of ca. 45 per cent. 

 2 other species and two races are practically confined to the 



Shan Plateau, giving a total of endemic Shan forms of 34 



= nearly 51 per cent. 



The fauna is thus mainly a fauna endemic on the Shan Plateau, 

 with a very large percentage of peculiar lake forms probably not existing 

 outside the Inle system. The only other elements that can be detected 

 are (a) one consisting of widely distributed Oriental species, (b) one con- 

 sisting of Indian forms found on both sides of the Bay of Bengal, (c) a 

 very small Indo-Chinese element, represented by general affinities rather 

 than common species, and perhaps faint traces of (d) an Eastern Palae- 

 arctic element. The true aquatic fauna of the Inle system belongs, 

 therefore, to that of the Indo-Burmese area, but represents a distinct 

 ofE-shoot thereof. It will probably be found, when the upper waters 

 of the Salween are investigated, that this offshoot is well established 

 in the watershed of that river, in so far as it is not purely lacus- 

 trine. 



A word may be said here about the aquatic Rhynchota. No less 

 than 33 species, representing 20 genera, are known to occur in the Inle 

 system, and 13 (ca. 39 per cent.) are known only from that system. 

 The rest are species widely distributed in India or the Oriental Region 

 generally, with one Palaearctic species (Gerris 'paludum). No peculiar 

 Indo-Chinese forms are found, and no endemic genera. The only genus 

 not of very wide general distribution is Perittopus, which seems to be 

 Malayan in origin but occurs in Assam as well as in Java, Malaya and 

 Tenasserim. The number of endemic species is surprisingly large in 

 view of the wide range of many aquatic bugs. The facts known about 

 the Inle representatives of this order, therefore, bear out what has 

 been said as to the true aquatic fauna in the preceding paragraph. 

 It is probable, however, that none of the Rhynchota are exclusively 

 lacustrine. 



