744 MEDICAL ENTOMOLOGY 



species of Crithidia found in blood-sucking insects are in reality develop- 

 mental stages of vertebrate trypanosomes. It must be admitted that 

 there is much to be said for the contrary opinion, that parasites found in 

 insects are primarily their parasites, and that the development of a 

 phase passed in the blood or tissues of the vertebrate or even in the 

 juices of a plant (Herpetomonas davidi) is a secondary phenomenon. 

 A study of the comparative anatomy shows clearly enough that the forms 

 with sucking mouths are more recent than those with the parts adapted 

 for cutting and tearing, and in particular that the blood-sucking forms in 

 all orders are more recent than those which are vegetable feeders ; the 

 contrary would have to be assumed if the parasites which they contain 

 are to be regarded as derived originally from the blood of vertebrates. 

 To take a precise instance, if the flagellate found in Musca domestica is 

 to be regarded as descended from a trypanosome, then Musca domestica, 

 its host, must be considered as evolved from a blood-sucking fly, an 

 assumption which is directly contrary to the anatomy of the parts. 

 Further, the occurrence of flagellates which are morphologically true 

 Crithidia in flies and other insects which are purely either dung or 

 vegetable feeders can no longer be denied. Another argument is to be 

 found in the effects produced in the two hosts. A large number of 

 trypanosomes are pathogenic to their vertebrate hosts, and as Minchin 

 himself points out, the pathogenicity is a ' disharmony ', indicating 

 imperfect adaptation ; as no such intolerance is to be found in the rela- 

 tions between parasite and insect host it is probable that the association 

 between the two is of much earlier date. The truth may well lie between 

 these two views, neither of which necessarily puts the other out of court. 

 Before this interesting if somewhat academic question can be definitely 

 settled, very much more will have to be found out with regard to the life 

 histories of both the natural !and alternating parasites. 



The practical importance of these natural parasites lies, as has already 



been stated, in the possibility that they may be confused with stages of 



pathogenic organisms. Their frequency is so great 



that it is a sound rule in practice never to assum e 

 Natural Parasites . . r .... 



in Experiments that a parasite found in an invertebrate is derived from 



a vertebrate until the natural parasites have been ex- 

 cluded often a matter of considerable difficulty. The methods to be 

 adopted vary according to circumstances. Caught specimens, about the 

 history of which nothing is known, should never be used for experiments 

 if it can be avoided. The suspected hosts should be bred from the 

 egg under defined and carefully controlled laboratory conditions, and a 



