FISHES AND MANKIND 



421 



broken. Many fish that have had their tails bitten off will 

 survdve the injury, the wound will heal, and rudimentary 

 fin-rays may be developed in the region of the scars. 



There is another important relationship between fishes and 

 mankind which has not yet been mentioned, namely, the use 

 of certain species in the control of disease. During the last 

 twenty years great advances have been made in the study of 

 this subject, and it has been found that a number of fishes are 

 valuable allies to man in his war against malaria, yellow fever, 

 and other dread diseases of tropical countries. These maladies 

 are spread through the agency of mosquitoes, and it has been 

 shown that the most effective method of reducing the numbers 

 of these insect pests is to destroy them while still in the larval 

 stage. The absence of yellow fever in Barbados, although 

 prevalent in the neighbouring islands, had long been noticed, 

 and it w^as suggested that this might 

 be due to the presence of a Top 

 Minnow or Cyprinodont known as the 

 "Millions" {Lebistes), so called be- 

 cause of its great abundance. This 

 little fish (Fig. 144) inhabits the pools 

 and marshes in which the mosquitoes 

 habitually breed, and the theory was 

 that the fish fed on the larvae in 

 sufficient quantities to make the num- 

 bers of fever-bearing adult insects 

 negligible. The theory has recently 

 been put into actual practice with excellent results, 

 introduction of the "MilUons" into fever-stricken 



Fig. 144. 

 Male and female " 

 (Lebistes reticulatus). 



Millions " 

 Nat, size. 



and the 

 districts of 



tropical America has proved most efficacious in checking the 

 scourge. Attempts were made to introduce the same species 

 into Africa and Asia without much success, but it was soon 

 found that these countries also possessed their larvicidal fishes, 

 and it was only necessary to find out which species included 

 mosquitoes in their normal diet, or could be persuaded to feed 

 on them if required. These include not only Cyprinodonts 

 but Carp, Gold-fish, Barbels, Eels, Cichlids, Gobies, and many 

 other forms, and all have been used with some success in 

 controlling human disease. In most cases the assistance of man 

 is required in order to bring the fish to the breeding-grounds 

 of the mosquitoes, but once established there they will carry 

 out their duties "without further charge"! 



In considering the relations between fishes and mankind, the 



