88 ANIMAL ECOLOGY 



by day or by night. The habits of these larval worms have 

 a very important bearing upon the means of transmission 

 from one man to another ; for F, hancrofti is transmitted 

 by blood-sucking mosquitoes which fly at night, while Loaloa 

 is now known to be transmitted by Tabanid flies {Chrysops 

 dimidiatus and silaria) which bite by day.^^ 



6. There are a good many other instances of the daj or 

 night habits of blood-sucking insects having an immense 

 influence upon the spread of disease. This has been especially 

 well shown by Carpenter in the course of his studies upon 

 sleeping sickness in the Lake Victoria region in Africa. One 

 big problem was to find out which animal acted as an important 

 reservoir of sleeping sickness from which human beings 

 might become infected. The matter was to some extent 

 simplified by the habits of the tsetse fly (Glossina palpalis)^ 

 which carries the trypanosome of the disease from one host 

 to another. The tsetse is diurnal in habits, and so there are 

 various animals which it never comes across at all in the normal 

 course of events. Certain potential enemies are avoided owing 

 to this ; for it is preyed upon neither by bats which come 

 out at night, nor by tree-frogs, which do not feed except at 

 night .^^ On the islands of Lake Victoria the most important 

 reservoir animals are the tragelaph (a species of marsh-haunt- 

 ing antelope which comes out to feed just when the flies are 

 " on the bite ") and the hippopotamus, which, although mainly 

 nocturnal, comes out about half an hour before sunset and so 

 is just in time to be bitten by the flies. 



7. The most violent fluctuations in light, temperature, and 

 humidity are probably those found in deserts, where a man 

 may be nearly dead with heat in the middle of the day and 

 nearly freezing at night. Often the conditions are so severe 

 that small rodents (which are usually rather sensitive to a dry 

 atmosphere) are able to come out only at night. 



A good account of these changes is given by Buxton in his 

 fascinating and scientific book Animal Life in Deserts? and 

 the subject has been further studied by Williams ^i in a series 

 of papers on the climate of the Egyptian desert. Williams 

 found, Hke most biologists who are engaged upon intensive 



