"Does clearing the jungle do any good?"
"Oh, yes. It clears the flies out of that particular spot. But it scatters them abroad. It doesn't destroy them. It doesn't destroy the pupæ, which are buried under the roots in the ground. Burning is better, perhaps. Burning may do for the pupæ, but then it doesn't affect the grown flies."
"Tell me," said Roger, "is blood necessary to the tsetse?"
"I wish I knew."
"I've been thinking about the spread of the disease. Is it caused by game, by slave-raiders, or by ivory-hunters? How is it spread?"
"We don't know. It seems to have followed the opening up of the Congo basin to trade. The game are reservoirs, of course."
"Have the natives any cure?"
"None. They have a disinfectant for their cattle. They boil up some bitter bark with one dead tsetse and make the cattle drink the brew. Then they fumigate the cattle with bitter smoke. They go through this business when they are about to trek cattle through fly-country. They travel at night, because the flies don't bite after dark. But the fumigation business is really useless."
"The tsetse is useless, I suppose?"
"All flies are useless."