Dr. Koch appears to have been questioned on his return to Europe by some journalists as to the results of his study of sleeping sickness during the past year and a half in Uganda. It was already known (three years ago), from the observations of Professor Minchin, Dr. Gray, and Dr. Tulloch (the Royal Society’s observers in Uganda), that the tsetse-fly in Uganda sucks the blood of crocodiles, also of fishes and of hippopotami. Dr. Koch confirms this observation. Minchin also observed a trypanosome in the blood of the crocodile differing from that of sleeping sickness. Whether crocodiles help, in an important degree, to keep tsetse-flies alive in the regions where they occur, by offering them a ready meal of blood, is uncertain. So far as the facts are known, they do not lead to the belief that the crocodile is a “reservoir host” for the trypanosome of sleeping sickness.

“Reservoir-host” is a very useful and expressive name for animals which can tolerate or support a parasite in their blood which is deadly to other animals. The parasite flourishes in abundance in the reservoir-host with entire satisfaction to both host and guest. But a blood-sucking fly or gnat, of promiscuous tastes in the matter of blood, comes along, sucks the reservoir-host a bit, and then goes off for another meal to a susceptible animal, into which it introduces the parasite now adhering to its already blood-smeared proboscis or beak. Such a history was first established by Bruce in regard to the trypanosome parasite which causes the deadly nagana disease in the “fly-belts” of South Africa. The big game animals are reservoir-hosts to this parasite, from which they are carried by the tsetse-fly to horses, mules, and dogs, which, being of foreign origin, are not tolerant of it, but are killed by the poison to which its multiplication in their blood gives rise. Thus, too, native children, both in Africa and the East Indies, appear to be tolerant of the malaria parasite, and act as reservoir-hosts from which the spot-winged gnats suck and distribute the parasite to the non-tolerant, susceptible adult natives and white men.

The tsetse-flies are little bigger than the common house-fly, and bite, or rather stab, very rapidly after alighting on the skin. The study of flies and gnats, and other blood-sucking insects, has become extremely important, and has been carried on with great energy by many specialists since it became known that these insects play such a terribly important part in the causation of disease. At the Natural History Museum I received (in response to a circular issued at my request by H.M. Government) thousands of specimens of gnats (mosquitoes) from all parts of the world, and some hundreds of new species have been described in a series of volumes by Professor F. V. Theobald, published by the trustees. Other volumes are in preparation illustrating the blood-sucking flies of various regions of the world, and one concerning those of the British Islands has already appeared. The common gnat, the spot-winged gnat, and the tsetse-fly—as well as the microscopic parasites causing malaria and sleeping sickness—are illustrated by greatly enlarged models—very carefully executed under my direction, which are exhibited in the central hall of the museum.

It is a curious fact that the coloured races of men—especially those of Africa—have little or no objection to being bitten by flies. They seem to accept the attention of flies and ticks with indifference. The men sleep in the day under trees, and are willing food-supply to the insects. The eyelids of children are literally inhabited by flies in some countries, and the folds of the skin of fat adults hide whole rows of fast-holding ticks. But the white man does not willingly permit either fly, flea, or gnat to settle on him. He is (or has been), nevertheless, unwisely tolerant of house-flies in his habitations, and the poorer and less cleanly population are in large proportion infested with wingless insects. The newly established knowledge that certain flies (glossina or tsetse-fly) are the carriers of sleeping sickness, that gnats are the carriers of malaria and of yellow fever, that fleas are the carriers of the plague, and that certain kinds of ticks are the carriers of cattle-fevers and dog-fevers, and probably of some obscure fevers of man, must make us all more anxious than we were about contact with insect life. For ages popular tradition has ascribed diseases of one kind and another in various parts of the world to the bites of flies. But actually it is little more than fifty years ago since it was really shown that deadly germs or parasites existed which could be, and actually are, carried by flies from one animal to another, and introduced into the blood by the flies’ stab. This was first shown in regard to the bacterium of splenic fever (or anthrax, or wool-sorters’ disease), a blood-disease of cattle which is transferred by the big, fiercely-biting “horse-flies” (tabanus), from animals to man, and is invariably fatal. Another bacterial disease, “pernicious oedema,” is inflicted on man in the same way. These cases were exceptional, and it is only quite recently that the agency of flies and fleas in great epidemics, and in diseases causing thousands of deaths every year in well-known regions, has been discovered.