246 THE CARRIAGE OF TYPHOID FEVER BY FLIES 



flies frequenting decaying animal matter and the next highest on 

 those caught near manure. 



Torrey (1912), in the United States, has also made a study of 

 the numbers and types of bacteria carried by flies under city con- 

 ditions. Flies examined up to the latter part of June were found 

 to be free from faecal bacteria and carried a homogeneous flora of 

 coccal forms. During July and August there occurred periods in 

 which the flies examined carried several millions of bacteria, 

 alternating with periods in which the number of bacteria was 

 reduced to hundreds. The scanty flora, he considered, probably 

 indicated the advent of large numbers of recently-emerged flies. 

 Faecal bacteria of the colon type were first encountered in abund- 

 ance during the early part of July. The bacteria in the intestines 

 of the fly were 8'6 times as numerous as those occurring on the 

 external surface of the insects ; an important fact to note. On 

 the surface of the flies bacteria of the colon group constituted 

 13'1 per cent, of the total and within the intestine they constituted 

 37 "5 per cent, of the total. Of the lactose-fermenting bacteria 

 which were i.solated and identified 79"5 per cent, belonged to the 

 colon-aerogenes group and 20"5 per cent, belonged to the acidi- 

 lactici group. Fifteen cultures of Sti'eptococci, isolated and identi- 

 fied, were distributed among the equinus, faecalis and salivariiis 

 groups. He found none of the pyogenes type. The most im- 

 portant isolations were three cultures of Bacillus paratyphosus, 

 Type A. Bacteria of the paracolon type, causing a final intense 

 alkaline reaction in litmus milk and fermenting only certain mono- 

 saccharides, were frequently found during August. 



Further references to studies of this nature are given in a later 

 section (p. 288). The foregoing experiments, apart from the indi- 

 cations which they give as to the nature of the flora of what one 

 might call a normal fly, demonstrate how necessary it is to exercise 

 great caution in making deductions from the results of isolated 

 examinations and cultures of flies both externally and internally. 



Reference has already been made to the fact that Jackson gave 

 bacteriological evidence as to the ability of house-flies to carry 

 B. typ)hosus. There are, however, a number of workers who have 

 carried out experiments along bacteriological lines and their results 

 will be given. 



