içiS- j Fauna of the Chilka Lake : Aquatic Insects. 181 



Order RHYNCHOTA. 



The aquatic Rhynchotal fauna of the lake comprises fifteen species, representing 

 eleven genera and six families. The majority (8 species, 5 genera) belong to the 

 Hydrometridae ; there are three species (2 genera) of Corixidae, while the Hebridae, 

 Nepidae, Belostomatidae and Notonectidae have each a single species. 



The aquatic species of this order, to judge from the large number described in 

 Distant' s supplement (vol. V, 1910) to his account of the Rhynchota in the Fauna of 

 British India, are still imperfectly known so far as the Oriental region is concerned. 

 It is therefore noteworthy that there is only one species in our collection from the 

 Chilka Lake that we have not been able to identify. It is a small apterous Hydro- 

 metrid belonging to the subfamily Veliinae and bearing some resemblance to Rhago- 

 velia nigricans (Burmeister) ; but as we have only a single specimen, which is prob- 

 ably immature, we refrain from discussing the species further. 



Euratas formidabilis is the only species which we believe to undergo its full 

 metamorphosis in, or rather on, the lake ; the only other form of which we found 

 an immature stage was a Gerris, probably G. spinolae, of which a single larva was 

 obtained. 



Descriptions of all the species here discussed will be found in Distant' s volumes 

 in the Fauna of British India and Ceylon, the volumes in which aquatic families are 

 described being II (1904), III (1906) and V (1910); our references are to this work. 

 The only form on which we have any remarks to offer as to structure or systematic 

 position is Euratas formidabilis (see pi 183). 



Family Hebridae. 

 Hebrus bengalensis , Distant, vol. V, p. 132, fig. 70. 



Mr. Distant has been kind enough to identify specimens of this species. It is 

 not uncommon among rocks and on wet sand at the edge of the lake, occurring both 

 in the main area and in the outer channel at all times of the year. Its original loca- 

 lity is recorded as 1,0 wer Bengal. 



Family Hydrometridae. 



Hydrometra vittata, Stahl, Distant, vol. II, p. 170, fig. 23 and vol. V, p. 137. 



H. vittata is common on the surface of the main area of the lake in winter 

 months, occurring chiefly on thickets of Potamogeton. It is a common species all over 

 India and has also been found in the Malay Archipelago and Japan. In the Gangetic 

 delta it often occurs on pools of brackish water. 



Mesovelia mulsanti, Buch. White, Distant, vol. II, p. 169, fig. 122. 



Another common species found over weeds and also among rocks in the main 

 area of the lake, chiefly in the winter months. It probably occurs all over the Orien- 

 tal region and has been found also in North and Central America and in the Antilles. 



