28 INSECTS AND HUMAN WELFARE 



immediate vicinity of the water. As a result of their method 

 of development, the tsetse-flies do not multiply rapidly, and 

 under favorable conditions only one larva is produced in a 

 ten-day period. 



The trypanosome of sleeping sickness was discovered by 

 Bruce in 1902 and a year later the role of (llossiua palpalis in 

 its transmission was proved. Since then much energy has 

 been expended in attempting to stamp out the disease by 

 every possible means. It was thought at first, that by mov- 

 ing all the natives back from the edges of the water the flies 

 thus left without opportunities for reinfection, would become 

 free from trypanosomes, and that by isolating and treating 

 cases of the disease in fly -free areas it would be possible to 

 eliminate it entirely. In conjunction with this, the cutting of 

 brush, especially about boat landings and watering places, 

 has been practised as far as possible. Contrary to expecta- 

 tions, it has been found that even after three or four years, 

 infected flies still occur along the uninhabited shores. This 

 led to experimentation upon animals and it is now known 

 that various wild antelopes as well as certain domestic ani- 

 mals may act as reservoirs for the virus of sleeping sickness 

 which may thus persist in the complete absence of any hu- 

 man subjects. As a result of this discovery the great difficul- 

 ties of combating the disease among the ignorant African 

 natives have been vastly increased. 



Louse-borne diseases have come into prominence during 

 the recent war in Europe on account of the prevalence of 

 typhus fever in Serbia and certain parts of the eastern war- 

 front. Typhus fever has been known to be conveyed by lice 

 for a number of years, but the association of these insects 

 with trench fever was proved only during the latter part of 

 the war, and their role as carriers of a certain type of relaps- 

 ing fever is also a quite recent discovery. 



Human lice, or cooties, to use their most recent nickname, 

 belong to three species, two known respectively as the head 

 and body louse (Fig. 12) {Pediculus humanus and P. corporis), 

 and the third as the crab louse {Phthiriiis pubis). All three 



