AQUATIC AND MARGINAL INSECTS. 



By N. AnnandaeE and Stanley Kemp. 



In the case of the insects it is particularly difficult to draw a line between aqua- 

 tic and terrestrial species. Strictly speaking, indeed, all those that occur in water or 

 on its surface should be called amphibious rather than either aquatic or terrestrial, 

 for all insects, at any rate in adult life, are air-breathing animals. Moreover, though 

 many species and larger groups are specialized for peculiar modes of life, the class as 

 a whole is remarkably tolerant and not easily deterred from occupying all situations 

 available. In considering the fauna of any body of water, the doubtful status of a 

 number of insects that can hardly be rejected as terrestrial animals or claimed as true 

 aquatic species must, therefore, be decided — the status, that is to say, of forms 

 that frequent the damp margins, concealing themselves under stones and jetsam, 

 burrowing in sand or mud, or crawling on damp rocks. Some of these species are 

 essentially moisture-loving terrestrial forms, found also in other situations, while a 

 few occur only at the edges of rivers, lakes, ponds or lagoons. 



In discussing the insect fauna of the Chilka Lake we have found it convenient 

 for this reason to devote a section of our paper to " marginal " species, in addition 

 to annotating the insects that may legitimately be called aquatic. We are indebted 

 to Mr. F. F. Laidlaw for an account of the only dragon-fly that breeds habitually l 

 in the lake. 



I. AQUATIC INSECTS. 



Apart from Coleoptera, 2 which we are unable to consider at present, the aquatic 

 insects of the Chilka Lake include at least twenty species, the majority of which (15 

 species) belong to the order Rhynchota. Only a very small minority of these insects 

 can be regarded as anything but casual visitors. Except for a moth, a dragon-fly 

 and three Diptera, the only species that we know to complete its metamorphosis in 

 the lake is the Hydrometrid bug Euratas formidabilis, and it is quite clear that all 

 the former deposit their eggs indifferently either on the surface of the lake or on any 

 other body of water they may chance to encounter in their flight. 



There are thus only six forms that we know to breed in the Chilka Lake ; parti- 

 culars of these species are given in the following table : — 



1 We found cast nymphal skins of two other species, an Aeschnid, probably Anax guttatus (Burm.), 

 and a L,ibellulid, adhering to rocks at the edge of the lake, but the species of these families that are 

 often seen flying over its surface usually breed elsewhere. 



2 About six species of Dytiscidae, ten of Hydrophilidae and one of Gyrinidae are found in the lake. 



