"Is the tsetse migrating, then, or can the thing be conveyed by contagion?"
"No. I don't think it's a contagious thing. I should say it almost certainly isn't. It needs direct inoculation. And as far as we know the tsetse keeps pretty near to one place all through its life."
"I know a writer who claims that we are spreading it. Is that so?"
"Indirectly. You see, East Africa is not like America or any other horse country. You haven't got much means of transport, except bearers, unless you go by river, and even then you may have to make portages. Going with natives from one district to another is sure to spread the infection. When infected people come to a healthy district, their germs are sure to be inoculated into the healthy by some tick or bug, even if there are no tsetses to do it. I believe there are trypanosomes in the hut-bugs. I don't know, though, that hut-bugs are guiltier than any other kind. It's impossible to say. From the hour you land until the hour you sail, you are always being bitten or stung by something. Bugs, ticks, fleas, lice, mosquitoes, tsetses, ants, jiggers, gads, hippos, sandflies, wasps. You put on oil of lavender, if you have any. But even with that you are always being bitten."
"And what is the tsetse bite like?"
"You've been to Portobe, haven't you? I remember Ottalie Fawcett speaking of you, years ago, before I went out. You had that cottage at the very end of the loaning, just above the sea? Well. Did you ever go on along the cliff from there to a place where you have to climb over a very difficult barbed-wire fence just under an ash-tree? I mean just before you come to a nunnery ruin, where there is a little waterfall?"
"Yes," said Roger. "I know the exact spot. There used to be a hawk's nest in the cliff just below the barbed wire."
"Well, just there, there are a lot of those reddy-grey flies called clegs. You get them going up to Essna-Lara. That's another place. They bite the horses. You must have been bitten by them. Well, a tsetse is not much like a cleg to look at. It's duller and smaller. It's likest to a house-fly, except for the wings, which are unlike any other kind of insect wings. It comes at you not unlike a cleg. You know how savage a cleg is? He dashes at you without any pretence. He only feints when he is just going to land. And he follows you until you kill him. A tsetse is like that. He'll follow you for half a mile, giving you no peace. Like a cleg, he settles down on you very gently, so that you don't notice him. You'll remember the mosquitoes at Belize. Mosquitoes are like that. Then, when he has sucked his fill and unscrewed his gimlet, you feel a smarting itch, and see your hand swollen. If you are not very well at the time a tsetse bite can be pretty bad. If you'll come to my rooms some time I'll show you some tsetse. They're nothing to look at. They're very like common house-flies."
"And you have been studying all this on the spot? Will you tell me what made you take to it?"
"Oh, I was always interested in that kind of thing. I've always liked hot climates, and being in wild, lonely places. And then my old chief was a splendid fellow. He made me interested. I got awfully keen on it. I want to go out again. You know, I want to get at the bottom of the trypanosome. His life-history isn't known yet, as we know the cycle of the malaria parasite. We don't even know what it is in him which causes the disease. And we don't know very much really about the tsetse, nor what part the tsetse plays in the organism's life. There's a lot which I should like to find out, or try to find out. It's the trying which gives one the pleasure."