CYCLE OF DEVELOPMENt 733 



interval elapses between the ingestion of the infecting meal and the time 

 at which the transmitter becomes capable of infecting a vertebrate host. 

 During this interval or latent period, which may be only a few days, 

 as in the case of Malaria and Yellow Fever, or may extend to many weeks 

 as in Sleeping Sickness, the parasite is undergoing a cycle of changes 

 which result in the production of the infective form. While it would be 

 out of place to discuss these changes here, it will be of considerable 

 interest to consider the sites at which they may take place, and the 

 means which the parasite may eventually adopt to leave the body and to 

 enter a fresh vertebrate. But before going on to this subject the case of 

 ' accidental ' or ' mechanical ' transmission must be referred to. 



This term is applied to those cases in which there is neither any 

 specific relation between the parasite and its insect host nor any mul- 

 tiplication or cycle of development undergone during 



the passage from one vertebrate to the next. It is in ' M 6chanical or 

 . . . 'Accidental' Trans- 



fact mechanical infection by the proboscis, or perhaps mission 



by the legs or body, such as might be produced experi- 

 mentally in the laboratory, the organisms adhering to the proboscis and 

 passing to the next animal on which the insect feeds. Several trypano- 

 some diseases, among which may be mentioned ' debab ', a disease of 

 camels in the Soudan, in which various species of Tabanidae are 

 suspected as transmitting agents by the Sergents, are believed to be 

 spread in this way. The haematophagous Muscids, on account of their 

 habit of feeding on the juices which exude from sores on the host, and 

 on the blood exuding from the punctures made by other flies, may also 

 act as mechanical carriers of infection, as appears to have been the case 

 in the epidemic of ' murrina ' recorded by Darling from the Panama 

 Canal Zone. The possibilities of this kind of transmission are necessarily 

 limited. Most biting flies take a full meal at each feed, and seldom feed 

 more than once a day, so that the parasite cannot be passed on to a 

 second host unless it is capable of living during this period in what would 

 appear, judging from the behaviour of such organisms under laboratory 

 conditions, to be a very unfavourable environment; or if it happened 

 that the fly was interrupted in its meal, and settled on another individual 

 to complete it. The intermittent habit of feeding of the non-biting 

 muscids suggests that they are more likely to spread infection in this way 

 than are true biting flies. (See page 348.) 



The precise locality selected for the site of the cycle of development 

 varies with the different classes of parasitic protozoa, and it will serve 

 our purpose best to take a general case, and to follow the movements 



