137 



A FISH THAT PREYS ON MOSQUITO LARV.15 IN SOUTHERN 



NIGERIA. 



By Dr. W. M. Graham, 

 Director of tlie Medical Research Institute, Lagos. 



During a seach for mosquito larviiJ at Yaba, near Lagos, my attention was 

 drawn to their complete absence from a swamp about a mile from the Medical 

 Research Institute. The pools in the swamp appeared to be well suited for the 

 growth of mosquito larva?, aud I caught female mosquitos full of eggs in the 

 adjoining fringe of bush. The absence of larvte was therefore evidently not due 

 to any lack of fertile female mosquitos. An examination of the pools showed 

 that they all contained small, active, surface-feeding fish,* belonging to the 

 family Cyprinodontid.e, and it seemed probable that it was by their agency 

 that the pools were kept free from larva;. 



To test this hypothesis, a number of the fish were caught and taken to the 

 Laboratoiy in a large jug of swamp water, and the contents of the jug were then 

 emptied into a basin and put aside, as it was getting dark. The following 

 morning a number of the fish were found dead on the floor, owing to their having 

 jumped out of the basin diu-ing the night. As these fish can leap a distance of 

 from one to two feet, in all future experiments it was found necessary to keep 

 them in covered vessels. 



On watching the fish in their native haunts, the importance of this faculty of 

 leaping becomes plain, for it is by this means that they are enabled to pass from 

 pool to pool. Thus every pool in a swamp is likely to be visited, and the whole 

 food supply will be better distributed among the fish. No other species of fish 

 were found in the swamp, and the Haplochilus itself was absent from a dug-out 

 water-hole near the swamp, in which I found tadpoles. 



At the Laboratory, the Haplochilus were found to eat mosquito larvse greedily. 

 Even when a hundred larvae were introduced into a vessel with a dozen fish, all 

 the larvas had disappeared in an hour. I was unable to discover that preference 

 was shown for any particular species of larva;, though I tried the fish with one 

 Auopheline, and five species of Culicine larva;. They declined, however, to eat 

 pupa; of anj' species, though they nibbled at them occasionally. Probably these 

 fish are unaccustomed to see pupa;, for in the pools they inhabit the larvas 

 would be very unlikely ever to reach that stage of development. 



** Mr. Boulenger has supplied the following notes on the structural characters of the species, 

 for which he proposes the name HajAochihis r/rahai)ii, sp. n. : — 



" Allied to //. senegaleiisis, Steiudachner. Depth of body, 4 to 44 times in total length, length 

 of head, 3 to 3;^ times. Snout a little shorter than eye, which is 3 to 3 j times in length of head, 

 and IJ times in iuterorbital width ; lower jaw projecting. Dorsal with 7 rays, originating about 

 twice as far from occiput as from root of caudal, above posterior fourth of anal. Anal with 15 

 or 16 rays. Pectoral, § length of head, extending beyond base of ventral ; latter small, equally 

 distant from end of snout, and from root of caudal. Caudal rounded-acuminate, longer than 

 head. Caudal peduncle slightly longer than deep. 28 or 29 scales in a longitudinal series, 22 

 round body in front of ventrals ; lateral line indicated by a series of pits." 



