ORDER DIPTERA: THE NEMATOCERA, ETC. 57 



by relying on Nature's unassisted processes. However, 

 " Natural Enemies " is now a potent formula, and it behoves 

 the medical officer to consider it, if only in self-defence. 



It is generally known that many small fish eat mosquito- 

 larvae, and among the most useful in this kind are the small 

 fishes of the family Cyprinodontidce, different species of which 

 are found all round the globe in low latitudes ; they thrive 

 in any kind of water, and most of them can live happily 

 wherever a Culex- or StegomyiaA'a.xv^ can. The usefulness 

 of fish is most apparent in small islands, where the conditions 

 of life are comparatively simple. Thus in Barbados small 

 Cyprinodonts, known locally as " Millions," are said to be 

 an effective check on mosquitoes ; and many years ago, 

 when I was in the Laccadive Islands, I noticed that in 

 Minnikoy, where there were no fishes in the wells and tanks, 

 mosquitoes were a terrible pest, whereas in Aucutta, where 

 the tanks and wells contained multitudes of small barbels 

 and perches, no mosquitoes were seen {Naturalist in Indian 

 Seas, 1902, p. 201). 



It is well known that the aquatic larvse of the " Demoiselle " 

 dragon-flies of the Agrion group (p. 259) are very destructive 

 to mosquito-larvae. These dragon-flies are common in the 

 tropics, and their larvae are abundant in stagnant water. 



Among other probable or possible enemies the following 

 common and widely distributed aquatic insects may be 

 mentioned : beetles and their larvae of the families Dytiscidcs 

 (p. 234), GyrinidcB (p. 235), and Hydrophilidce^^. 235) ; larvse of 

 m.3.r\y Epkemerida{^.2iiy); \a.TVS:o{P/ecqptera(p.2S)6); several 

 families of aquatic bugs (p. 210); larv^ of the Chironomid 

 fly, Tanypus (p. 122). The aquatic larvae of the SialidcB 

 (p. 260) are also carnivorous, but they are not found in the 

 tropics. Some aquatic larvae of Caddis-flies (p. 262) are 

 predaceous, though the Caddis-worms of this habit usually 

 live in quick-flowing water. 



Of Culicid larvae themselves there are many species that 

 are predatory and may devour their fellows of other species ; 

 Corethra (p. 60), Mochlonyx (p. 6 1 ), and Megarhinus (p. 113) are 

 good examples. These carnivorous larvae can be recognised 

 by their powerful mandibles, and by having either the antennae 

 or the mouth-brushes specially modified for prehension. 



