THE TSETSE-FLIES 39 



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 ob- 

 served a trypanosome in the blood of the crocodile 

 differing from that of sleeping sickness. Whether croco- 

 diles 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, suscep- 

 tible 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 



