MOSQUITO LARV^ AND THEIR NATURAL ENEMIES. 217 



'' The ' Millions ' fish are of considerable value for larger barrels and tanks, 

 and, as far as I have been able to estimate, have no deleterious or unpleasant 

 effect upon the water. The analysis of the water from a 5000 gallon tank, 

 in which four to thirty fish had lived for one month, would pass as 

 sufficiently pure for drinking purposes. In fact I have been totally unable 

 to detect any effect whatever caused by the fish. The tanks from which I 

 obtain drinking water have now been stocked with fish for several months." 



The value of the fish, Girardinus poeciloides, as a destroyer of mosquitos 

 seems now well established, and attempts have already been made to intro- 

 duce it into the West Coast of Africa. The fish were brought over to the 

 Zoological Gardens in London, and from there several consignments were 

 despatched to West Africa, though in no case did the fish survive for long. 

 But as there were certain unavoidable conditions in these experiments which 

 were unfavourable, the results cannot be taken as final. We understand 

 that not long ago a batch of Glrai'dinus was imported into Mauritius, though 

 no information with regard to the success or otherwise of the experiment 

 appears to have yet been published. If successful, this island would be a 

 very favourable point from which to attempt the establishment of the species 

 in Africa. 



On general grounds it would seem likely that a species belonging to a 

 limited insular fauna would have considerable difficulty in maintaining itself 

 in a continental area like Africa, where the struggle for existence would 

 probably be far more severe than in its proper habitat. On the other hand, 

 the mutual inter-relationship of living organisms is so complex that the 

 wholesale introduction of a new animal might have deleterious results which 

 at present it is quite impossible to foresee, and thus should not be too lightly 

 undertaken. 



It seems desirable therefore that before any serious attempt is made to 

 colonise Girai'dinus in Africa, adequate investigations should be carried out 

 with reference to the local enemies of mosquito larvse, which might possibly 

 be utilised in a similar manner. Indeed, there seems reason to believe that 

 there already exist in Africa small fish which are capable of doing work 

 .similar to that done by Girardinus in the West Indies. For in West Africa, 

 Dr. W. M. Graham has noticed the complete absence of mosquito larva3 

 from apparently suitable pools, which however contained numerous small fish 

 (Bull. Ent. Research, i. p. .52). And the writer is informed by Mr. G. A. 

 Boulenger, F.R.S., that there are various African species of small Cyprinodont 

 fishes, belonging to such genera as Fundidas etc., which are nearly allied to 

 Girardinus, and doubtless have very similar habits. The same authority 

 .suggests that the great abundance of '' Millions ^^ in Barbados is due to the 



