ARE THE ODONATA OF ECONOMIC VALUE? 



By Stephen Gottheil Rich, M.A., B.Sc. 



(Read July lo, 1918.) 



It is often stated — e.g., in Miall's " Natural History of 

 Aquatic Animals," that the Odonata have economic value as 

 destroyers of mosquitoes, flies, midges, and the like. So far as 

 the northern mosquitoes, which are found on the wing in daytime, 

 are concerned, this is probably correct; at Harpswell, Me., U.S.A., 

 I have seen the dragon-fly Aeshna constricta hawking on a warm 

 morning along a road along which at the same time mosquitoes 

 of the genus Culex were annoying me to an excessive degree. 



My observations of African dragon-flies, mainly in Natal, 

 during the last two and a half years, lead me to assign a much 

 less important role to them. In the matter of mosquitoes, for 

 example, I note that our Natal species are twilight-fliers, or are 

 nocturnal. But our dragon-flies are sun-lovers, and vanish from 

 flight as soon as the sun sets or long shadows of hills cover their 

 haunts. 



At Durban, along Ridge Road, I have several times seen 

 swarms of the dragon-fly Pantala flavescens hawking, but always 

 at high noon. Their prey was entirely small Muscid flies. These 

 small Muscidas do not appear to have any economic importance, 

 however. 



I am, therefore, not inclined to attribute economic value to 

 the dragon-flies of this country as adults. 



In the nymphal stage, however, there is more to be said in 

 favour of the usefulness of dragon-flies. In the Amanzimtoti 

 River, Natal, dragon-fly nymphs are abundant. There are very 

 few mosquitoes found thereabouts, although the river has many 

 small pools which are ideal haunts of the mosquito larva. On 

 the other hand, there are fewer dragon-flies in the Umbilo River 

 and the spruits near Durban, and mosquitoes are plentiful. T 

 notice this same correlation of fewer dragon-flies and many mos- 

 quitoes in the Dorp Spruit at Maritzburg. 



Needham, in 1903, noticed dragon-fly larvae as an occasional 

 constituent of trout-food, in North America. I do not know of 

 any evidence as to the food of fresh-water fish in this country; 

 but I have heard that at Nottingham Road, Natal, where trout- 

 fishing is good, dragon-flies are plentiful. I hope to be in a 

 position to investigate this matter further ere long. 



In "The Life of Inland Waters" (1916), Needham and 

 Lloyd show a photograph of a dragon-fly nymph catching and 

 eating a young sunfish. I am of the opinion that this destruction 

 of fry by dragon-fly larvae, Dytiscid beetles, and other aquatic 

 carnivores, is of importance in preventing an increase of fish in 

 our streams. It is a remarkable coincidence that, except at Not- 



