4IO 



PHYLUM ARTHROPODA 



Leishmania tropica, the parasite of " Oriental sore," is 

 carried by sand flies (Phlebotomus). Sand flies also spread 

 sand-fly fever in Mediterranean countries. A number 

 of biting flies belonging to the genus Tahanus transmit 

 Trypanosoma evansi, causing surra, a now widespread 

 tropical disease of horses, camels, and other animals. 



A Tabanid fly, Chrysops, carries the West African 

 nematode parasite Filaria loa {F. ditirna). In contrast to 

 F. hancrofti, the parasites are found in the peripheral blood 

 vessels of the infected person by day, not by night, in 

 accordance with the difterence in habit of the carrier insect. 



Several species of tsetse flies (Glossina) are now known 

 to carry trypanosomes. Glossina palpalis transmits T. 

 gambiense (see Fig. 69), causing African sleeping sickness. 

 This disease occurs over an area stretching from the west 

 coast of Africa to Lakes Victoria and Tanganyika, its 

 width varving from 15° to 10° on either side of the Equator. 

 Glossina morsitans transmits T. brucei, to which man in 

 general is immune, but which causes tsetse-fly disease or 

 Nagana in horses, cattle, goats, sheep, and other domesti- 

 cated animals. It is widely distributed in Africa, infected 

 areas being called "fly-belts." An interesting point is 

 that, in the case of T. brucei especially, the native wild 

 fauna form a reservoir for the parasite, which exists in 

 them, causing little or no inconvenience, but when trans- 

 mitted by the tsetse fly to a domesticated animal may be 

 deadly. When the blood of an infected subject is sucked 

 by a Glossina (see Fig. 67), trypanosomes from the vicinity 

 of the mouth may be handed on if the fly bites a second 

 time within forty-eight hours. Ingested trypanosomes 

 undergo a developmental cycle lasting eighteen to twenty 

 days within the fly. By that time they are, however, found 

 in the fly's salivary glands and thereafter are readily trans- 

 mitted in the salivary juice. 



The role of the house-fly, Miisca domestica, as a carrier of 

 disease is now well known. Germs of typhoid, summer 

 diarrhoea, and other diseases are distributed mechanically 

 by being caught up on the hairs of the body, legs, or feet 

 and deposited on food-stuffs left open to flies. Horse 

 manure, human and other excrement, ash heaps, etc., are 

 the favourite breeding-places. From 100 to 150 eggs are 



