546 TEE POPULAR SCIENCE MONTHLY 



The tsetse-flies develop in a very different way from most insects. 

 The female does not deposit her eggs, but a single one develops to the 

 fully grown larval condition before being deposited. This larva soon 

 pupates in the shade beneath the brush bordering the water where it has 

 been dropped by the parent fly, and later emerges in the winged adult 

 condition. The pupa? require such moist shade, and it is apparently for 

 this reason alone that the flies never occur away from the immediate 

 vicinity of the water. As a result of its 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 Glossina 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 moving all the natives back from the edges of the water 

 the flies thus left without opportunities for re-infection, 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 expectations, 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 animals may act as reservoirs 

 for the virus of sleeping sickness which may thus persist in the complete 

 absence of any human subjects. As a result of this discovery the great 

 difficulties of combatting the disease among the ignorant African natives 

 have been vastly increased. 



The regions surrounding the Mediterranean Sea are the centers of 

 distribution for a very interesting, but far less dangerous insect-borne 

 disease known as phlebotomus fever. In this case the carriers are minute 

 gnat-like flies of the family Psychodidas known as Phlebotomus papatasii. 

 These insects are semi-aquatic in the larval condition, occurring in 

 damp situations, drains, cellars, etc., where they feed on plant matter. 

 The adult is a vicious biter in spite of the fact that it is scarcely over 

 one millimeter in length. It rarely bites except at night, following the 

 habits of certain mosquitoes in this respect. The specific cause of 

 phlebotomus fever is not known, but it has been shown to be an invisible 

 virus. At the present time it is impossible to state whether other insects 

 may play a part in its transmission, although such does not seem prob- 

 able. We have at least one species of Phlebotomus in the United 

 States and it is possible that it might act as a vector should the disease 

 be introduced into our country, although it would seem that such a 



