This species is usually regarded as non-pathogenic, but in experimental work, especially with white rats, heavy infestations often result fatally, and naturally infested specimens sometimes show evidence of injury. Rats which have been infested exhibit at least temporary immunity against new infection. _Trypanosoma lewisi_ is transmitted from rat to rat by fleas and by lice. Rabinowitsch and Kempner (1899) first found that healthy rats which were kept with infested rats, showed trypanosomes in their blood after about two weeks. They found the trypanosomes in the alimentary canal of fleas which had fed on the diseased rats. On teasing such fleas in physiological salt solution and inoculating them into fresh rats they were able to produce the infection. Finally, they showed that the fleas which had fed upon infested rats were able to carry the parasites to healthy rats. Corresponding experiments with lice were not successful. Prowazek (1905) found in the rat louse (_Hæmatopinus spinulosus_) organisms which he regarded as developmental stages of the _Trypanosoma lewisi_. He believed that the sexual cycle was undergone in this insect. Nuttall (1908) readily transmitted the trypanosomes through the agency of fleas, (_Ceratophyllus fasciatus_ and _Ctenopthalmus agyrtes_). He believes that these insects are probably the chief transmitters of the parasite. He was also able to transmit it from diseased to healthy rats through the agency of the rat louse. He was unable to trace any developmental stages in the louse and inclined to the opinion that Prowazek was deceived by the presence of extraneous flagellates such as are known to exist in a number of blood-sucking arthropods. Nuttall concludes that since three distinct kinds of blood-sucking insects are capable of transmitting _Trypanosoma lewisi_ it appears doubtful that this flagellate is a parasite of the invertebrate "host" in the sense claimed by Prowazek and other investigators. TSETSE-FLIES AND NAGANA--One of the greatest factors in retarding the development of certain regions of Africa has been the presence of a small fly, little larger than the common house-fly. This is the tsetse-fly, _Glossina morsitans_ (fig. 165) renowned on account of the supposed virulence of its bite for cattle, horses and other domestic mammals. The technical characteristics of the tsetse-flies, or Glossinas, and their several species, will be found in a later chapter. We need emphasize only that they are blood-sucking Muscidæ and that, unlike the mosquitoes, the sexes resemble each other closely in structure of the mouth-parts, and in feeding habits.