Page:Popular Science Monthly Volume 81.djvu/552

546 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 Psychodidæ 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 probable. 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