290 
NATURE 
[May 13, 1915 

applied to the surface of whatever it rests upon, 
excrement or food-stuff, as opportunity offers. 
The alimentary canal comprises a_ gullet, 
stomach, crop, intestine, and rectum (see Fig. 2). 
The gullet is prolonged forwards to a minute 
opening between the flaps of the proboscis, half- 
way down which it is joined by the salivary duct 
(S D). At the entrance to the stomach (S) it is 
bifurcated, and one limb of the bifurcation is ex- 
tended backwards to the bilobed crop (C). By a 
valvular apparatus at the entrance to the stomach, 
the insect can direct the liquid driven by the pump 
in its trunk into either the stomach or crop. The 
proboscis is a highly elastic muscular organ with 
movement. 
universal At the end are two flaps 

Fic. 1.—Leg of a house-fly. 
or labella (only one of which is shown) , which it 
can open out like the leaves of a book, and apply 
the medial surfaces to the material it feeds upon. 
From the middle line or hinge, minute chitinous 
channels pass outwards to the margin. At the 
base of the trunk a number of muscle fibres are 
attached to the gullet by the peristaltic contraction 
of which fluid is pumped up from the mouth and 
propelled into the stomach or crop. The struc- 
tural arrangement of the channelled flaps of the 
trunk acts as a filter, through which solid objects 
larger than 1/4oooth in. seldom pass. When 
feeding on a liquid, the fly applies the labella to the 
surface, and sucks the liquid through the 

Fic. 2.—Alimentary system of a house-fly. 
“strainer ” first of all into the crop. When this is 
full, a further quantity is admitted into the 
stomach. In the case of solid material, such as 
sugar, the insect must first dissolve the material. 
This is done by pouring saliva upon it, or by re- 
gurgitating some of the contents of the crop. 
A well-fed fly deposits feeces abundantly, and 
also the contents of its crop upon sugar and other 
solid objects. 
It is clear, therefore, that there are a priori 
reasons for suspecting the fly of carrying bacterial 
infection, Born in a dunghill, it spends its days 
flitting between the sugar basin, milk pan, and 
any fecal matter available. Its hairy, probably 
sticky, feet and the habit of regurgitating the 
NO. 2376, VOL. 95] 

| by them, if cholera or typhoid appear. 
| contents of the crop and defecating at frequent 
intervals, suggest it as an excellent inoculating 
agent for any bacteria it may pick up in the 
satisfaction of its catholic tastes. That it does, 
indeed, operate in this way has been abundantly 
demonstrated. Flies which have wandered over 
cultures of organisms and afterwards been allowed 
to walk upon gelatin plates leave a rich crop of 
germs in their footprints, which can be demon- 
strated by subsequent incubation. 
Flies fed in the laboratory upon material con- 
taining easily identifiable pathogenic microbes 
have been shown to harbour them in their crops 
for days, and to deposit them in their faeces and 
the regurgitations from their crops. Internal car- 
riage is probably more important than soiling of 
the exterior of the insect, as many pathogenic 
bacteria soon die from desiccation on the appen- 
dages of the insect. 
In addition to these laboratory experiments, 
there are numerous recorded instances in which 
the pathogenic organisms of cholera, typhoid, 
phthisis, anthrax, and plague have been recovered 
from the interior or dejections of flies which have 
been captured in the immediate neighbourhood of 
cases of the disease, or, in the last two cases, of 
carcases of animals dead of the disease. 
Although, however, flies may be discovered with 
the infection of a number of diseases in or upon 
them, and by their habits may not unlikely serve 
as agents in transferring infection, it by no means 
follows that they are the determining factor of 
epidemicity in the case of cholera, typhoid, dysen- 
tery, etc. In the case of fulminating epidemics 
of typhoid and cholera associated with an infected 
water supply, this is obviously not so. 
It is in temporary encampments of troops or 
pilgrims, when the disposal of excreta must neces- 
sarily be of a primitive character, that the con- 
ditions obtain which are most favourable to the 
breeding of flies and the distribution of infection 
Even 
in these circumstances it is difficult to assess the 
relative importance of fly carriage and other means 
of spread, but the conclusion that fly transmission 
is the principal means of spread of typhoid in 
military encampments and stations has been 
arrived at by a number of competent observers, 
amongst them the commission to inquire into the 
origin and spread of typhoid fever in the United 
States military camps during the Spanish war of 
1898, and by a number of medical officers con- 
cerned with the severe outbreaks of enteric which 
occurred during the Boer war. 
The sanitary arrangements of a military camp 
are not exactly those of the Ritz Hotel, and the 
prevalence of flies in late summer can scarcely be 
appreciated by those who have not had camp ex- 
perience. The conditions are most favourable for 
transmission of disease by flies, and the circum- 
stantial evidence against them is so strong as to 
have left.no doubt in the minds of the American 
Commission that these insects play a large part 
in disseminating infection, for on page 28 of their 
general statement and conclusions we read = 


