732 MEDICAL ENTOMOLOGY 



of view of the parasite it is a question of adaptation, and the ideal host 

 would be one in which no effects likely to shorten the life of the host, 

 and thereby diminish the chances of passage to the invertebrate, are 

 produced. Pathogenicity, in fact, is a sign of imperfect adaptation, and 

 if a fatal disease were always produced, the continuance of the race of the 

 parasite would be endangered. Vertebrates in which no disease is 

 produced, such as the antelope and the jackal in the instances given 

 above, are to be regarded as the true hosts of the parasites, rather than 

 man and the dog. 



The importance of this from the entomological point of view lies in the 

 fact that blood-sucking arthropods are not restricted to one species 

 of animal for food, but will feed on many different and often widely 

 separated species. It is not, therefore, sufficient to determine which 

 species of invertebrate can act as a host ; it is of almost equal importance 

 to ascertain on what vertebrates other than the one in which a disease is 

 produced the transmitter will feed in a state of nature, and what are the 

 conditions which influence it in its choice of food. Information on 

 these points is surprisingly scanty even with reference to the commoner 

 blood-sucking insects. 



It should be noted that the protozoa which produce such serious 

 effects on their vertebrate hosts appear to be quite harmless to the 

 invertebrate, and do not shorten its life ; the tolerance appears to be 

 mutual, for the infection is not thrown off once it has become established, 

 and may continue to the end of the life of the insect, a constant supply 

 of the infective stages being kept up by multiplication and development. 

 One transmitter may therefore be capable of infecting a large number of 

 vertebrates. 



THE HABITAT AND MOVEMENTS OF PARASITES 

 WITHIN THE INVERTEBRATE HOST 



In both vertebrate and invertebrate host the cycle of development 

 which the parasite passes through is directed, not only to establish it in 

 its new host, but also to provide forms which are able to pass to the 

 alternate host, and there to start the other phase of its life cycle. The 

 alternation of hosts is evidently the means adopted to secure the 

 continuance of the race of the parasite on the death of the infected 

 individual. 



One of the essential differences between true transmission of parasites 

 and mere mechanical infection is that in the former case a definite 



