TRUE INSECT FLAGELLATES 47 



The amphibian trypanosomes are of interest, for 

 one of the earliest trypanosomes ever noted was 

 T. rotatorium from the frog, which was first seen by 

 Gruby in 1843. 



The method of transmission of the trypanosomes of 

 terrestrial vertebrates is mainly by the bite of insects. 

 Now insects, like the higher animals, have parasites 

 peculiar to themselves, and these parasites have a 

 superficial resemblance to trypanosomes. Unfortu- 

 nately, some investigators have ignored the existence 

 of these true insect flagellates and also that there are 

 three definite phases to their life-history. They have 

 jumped to the conclusion that the flagellates of 

 insects must be developmental forms of trypano- 

 somes, because cultures of certain trypanosomes on 

 artificial media (and so under somewhat unnatural 

 conditions) give forms resembling these parasites of 

 insects. There is at present no evidence whatever 

 that such is the case ; but, on the other hand, there 

 is a continuous accumulation of evidence that very 

 many flagellates occurring in insects are parasites of 

 the insect alone, and have no connexion with the 

 trypanosomes of vertebrates. 



These flagellates, belonging to the genera Crithidia 

 and Herpetomonas, occur in the alimentary canals 

 of various flies and bugs, some of which insects are 

 feeders on vertebrate blood, some on excrement of 

 man and beast, while others feed on the juices of 

 plants. In the case of Crithidia and Herpetomonas 

 a definite phase of the parasite is peculiarly adapted 

 for life outside the body of the host. This form of 

 the parasite, which is compact and oval, is known as 



