tion IS not 

 dangerous 



110 REPORT OF THE ENTOMOLOGICAL SECTION 



stirred from the time the resiii was added imtil the end." Mr. le Prince states that as 

 the composition of crude carbolic acid varies greatly, the proportion of ingredients of 

 the larvicide will vary, and it is necessary to have small experimental lots made in the 

 laboratory and tested before the batch of larvicide for use in the field is manufactured. 

 One part of this mixture placed in five thousand parts of water containing mosquito larvae 

 is said to kill them all within five minutes. If it is used in the proportion of one to 

 eight thousand the larvae are killed in thirty minutes. 

 Basin irriga- A method of irrigation, practised in some districts, notably in the province of Dongola, 



is that known as " basin irrigation." Where the land-levels admit of its being done, a 

 canal is dug, along which water flows from the river when it is in flood on to land situated 

 generally behind the sakia-lands. There is, I think, little fear of mosquitoes breeding out in 

 the pools left by these basins when the Nile falls, as the myriads of fish, which are 

 brought in from the river with the water, are constrained by hunger to devour any 

 mosquito larvae they can find, even though under more normal conditions, such larvae 

 would not be very palatable to them. 



During some three months spent on the White Nile in 1909, I endeavoured to 

 find a species of fish which would emulate the example set by Girardinus pxcilloides. 

 This Barbados fish has gained for itself a world-wide reputation as a mosquito- 

 destroyer, under the name of " Millions," and it was suggested by the Director of 

 the Laboratories that possibly some species of fish existed in this country which would 

 be of like value. Such a fish, to be capable of yielding the best results, must possess several 

 characteristics. It must be capable of living and breeding in stagnant as well as in 

 running water, it must be small, preferably not more than 10 cm. in length, a prolific 

 breeder,! hardy enough to bear transportation and, last but not least, so fond of mosquito 

 larvae that it will take them in preference to anything else. It is sometimes said that any 

 fresh water fish will eat mosquito larvae, and this may possibly be the case when no 

 other food is available, but it certainly is not so with Nile fish living in their natural 

 habitat. In some of the marshes bordering the White Nile, shoals of tiny fish can be 

 seen in water literally alive with mosquito larvffi and yet making no effort to catch 

 them. Many species of fish exist in these marshes and all which could be caught 

 were given a trial. Some would not live in glass jars — no other and larger vessels were 

 Fish as available — while others, though they throve in the jars, refused to eat mosquito larvse 



larvicides . ' & J J ' i 



until they had been without food for periods varying from one to three or four days. 

 A single species, — later determined by Mr. G. A. Boulenger of the British Museum 

 (Natural Hist.) to be Ophiocephalits obscurus — taken in a khor situated between Gebel 

 Ahmed Aga and the river, appeared to look on mosquito larvtB as its natural and only food. 

 The specimens caught were small fry — in which stage they rather resembled tadpoles — 

 and though only 1-5 — 2 cm. in length would tackle large Stegomyia and Scufomi/ia 

 larvEB with the keenness of terriers catching rats. They remained healthy although 

 the water in their jar was sometimes unchanged for several days and allowed to become 

 green and slimy. Later, when mosquito larvae could not be obtained, they were fed on 

 earthworms. This fish, therefore, possesses all the characteristics needed for a mosquito 

 controller, except one, or perhaps two — when full grown U. ohscunis attains a length 

 of 35 cm. and possibly it does not breed freely. In swamps and similar places, where the 

 fry after feeding in the shallows — and it is in shallow water that mosquitoes prefer to 

 breed — could as they increased in size regain the deeper parts, I believe this fish, wlien 

 once established, would yield the verj' best results, but its size unfits it for use on 

 artificially irrigated farms. On a farm with a system of water channels as described 



