270 
tliidia. Sucli a mistalve can hovvever be easily avoided by studying’ 
tlie parasite in the way i bave describecl above. d iiese small 
round or oval llagellates pass down the gut and encyst in the 
colon or rectum of the fly. It niay now be asked, what is the use 
ot the C}'sts, because Hcrpetoinonus nuiscae domesticae passes 
Irom one fly to another without ever becoming a cyst; yet in 
quite a large percentage of bazaar flies very large numbers of 
these forms are found. A nuinber of experiments liave been car- 
ried out to answer this question. It was first found that if hatched 
flies are well infected, and are tlien lea continaously on spleen 
juice, the flagellâtes persist for at least fourteen days and then 
begin to encyst ; this is long after the female flies hâve laid their 
first batch of eggs. If, however, well infected flies are given 
only jam and water after the first three leeds of spleen the para¬ 
sites begin to encyst on the sixth dav. These observations shew 
that the food of the fly lias a great deal to do with the after his- 
tory of the flagellâtes of Herpetumonas muscac domcslicae, and 
that the cysts are a spécial provision in order to enable the pa¬ 
rasite to escape total extinction. These encysted stages are nearly 
always found in enormous numbers, and may be recovered from 
the stomachs of clean flies placed in a cage with flies known to 
oontain cysts. The further development of these parasites in the 
stomach (mid-gut) of a clean lly is exceedingly rapid. In less 
than eight hours they had ail llagellated, and this accounts for 
the great difficulty in finding these stages in bazaar flies; I 
hâve however seen them in a small percentage. These young 
flagellâtes, which are either round or oval, are very character- 
istic, and when stained hâve the same granular appearance as 
the cysts. 
By exact feeding experiments with bred flies 1 hâve now con- 
clusively proved the following points: 
1. Herpetomonas muscac domesticae may be transmitted from 
one fly to another in three ways : 
a) By a fly ingesting the long flagellâtes ; 
h) By a fly ingesting the short encysting forms; 
c) ] 3 y a fly ingesting the cysts. 
2. If the food is suitable the long flagellâtes multiply contin- 
uoLisly for al least fourteen days and then begin to encyst. 
3. d'he short encysting forms if ingested pass down the alimen- 
