May 13, 1915] 
NAT ORE 
289 

HOUSE-FLIES AS CARRIERS OF DISEASE. 
‘Vysie discovery of the réle of insects in the 
transmission of human and animal diseases 
is one of the most striking achievements of medical 
science during the last twenty-five years.  Filari- 
asis, Texas fever, nagana, malaria, sleeping 
sickness, yellow fever, dengue, sandfly fever, re- 
lapsing fever, plague, typhus, and many other 
diseases of the lower animals, have been shown 
to be transmissible by blood-sucking insects— 
mosquitoes, ticks, tsetse flies, fleas, or lice, as 
the case may be. The pioneers in this line of 
inquiry were Manson, Smith and Kilborne, Bruce 
and Ross. 
In a number of cases the necessity of interven- 
tion by an insect has been established by the dis- 
covery that a portion of the life cycle of the 
parasite is passed in mosquito, tick, or tsetse fly 
respectively. In other cases, the evidence rests 
upon the correspondence in time and space of the 
incidence of the disease with the presence of some 
particular insect which has been experimentally 
shown capable of transmitting the infection. In 
yet other cases, such as plague, the microbe can 
also pass directly from patient to patient, as 
happens in the pneumonic variety of the disease, 
but the paramount importance of flea transmission 
in bubonic plague gains in recognition daily. 
The rich harvest of discovery reaped by the 
investigations into the part played by _ blood- 
sucking insects in the spread of the above-men- 
tioned diseases. naturally stimulated inquiry into 
the possibilities of insect carriage as a factor in 
outbreaks of cholera, typhoid, dysentery, and 
epidemic diarrhoea. These are not diseases in the 
transmission of which a blood-sucking insect is 
likely to play a part, for in none of them is the 
infecting microbe present in the blood-stream in 
sufficient quantity, but the dejecta, faces, and 
often urine, contain the bacilli in countless num- 
bers. A small proportion of convalescents con- 
tinue to excrete them for weeks, months, and, in 
the case of typhoid, for years afterwards, although 
enjoying perfect health. These people are par- 
ticularly dangerous to the community as they form 
an unsuspected reservoir of infection. 
To produce an epidemic of typhoid, cholera, or 
dysentery, the bacilli dejected by persons sick or 
convalescent from the disease must find access 
to the alimentary tract of others. There are, 
however, ways in which this may happen inde- 
pendent of the agency of insects. A water supply 
may become contaminated with infected material; 
the dejecta may dry up and be distributed as 
dust, and fall upon food materials (a method, 
the importance of which may easily be exag- 
gerated, as these bacilli are readily killed by 
desiccation), or, owing to bacteriologically inade- 
quate attention to cleanliness, food-stuffs, in which 
the microbes can multiply, may be infected with 
bacilli from patients or convalescents. Typhoid 
and cholera bacilli are small objects, less than one- 
thousandth of an inch in length, so that fingers 
may be easily soiled by considerable numbers with- 
NO. 2376, VOL. 95] 

out this being obvious, and the microbes are not 
removed by perfunctory washing. 
Although these three means of spread do pro- 
duce and maintain epidemics, one has but to con- 
sider the habits of the house-fly to realise that 
this insect may be an able and willing assistant 
in the distribution of the bacilli which are the 
cause of cholera, typhoid, dysentery, and diar- 
rhoea, and that flies, if in sufficient numbers, and 
under conditions favourable for their operations, 
may constitute the principal way in which infection 
is distributed. In order to appreciate how this 
may happen it is necessary to be in possession of 
some few points in the life-history, and structure 
of the fly. : 
These subjects have been submitted to careful 
inquiry during the last few years, particularly in 
America and this country, by Newstead, Howard, 
Griffith, Hewitt, and Graham Smith, and we are 
now well acquainted with this insect, intimate 
knowledge of which was, until recently, curiously 
lacking. 
The female fly lays about 120 eggs at each 
laying, and may produce four broods. The eggs 
are mostly laid on horse manure or other fer- 
menting refuse; they are about 1°5 mm. in length 
and o°3 in their greatest diameter, and hatch in 
from three days to eight hours, according as the 
temperature ranges from 50°F. to 80°F. The 
larva is a little active grub 2 mm. long; and on 
hatching out burrows into the manure or other 
material on which the eggs are laid. The larval 
stage lasts five days to three weeks, and pupation 
five days to a month, according to temperature. 
Thus the whole cycle from laying of the egg to 
emergence of the fly occupies ten days to two 
months, according as the weather be warm or 
cold. The young female is ready to lay its first 
batch of eggs in about ten days, or even sooner 
in warm weather. Owing to this influence of tem- 
perature upon the rate of development of egg, 
larva, pupa, and imago, the number of flies in 
August depends on the temperature during June 
and July. 
During winter a few flies survive in warm and 
secluded places. In the spring these start the next 
year’s supply. Dr. Howard, of the United States 
Department of Agriculture, estimates that in forty 
days the descendants of one fly might number 
twelve million, or 800 lb. weight. 
It will therefore be obvious that any attempt to 
overcome the nuisance from flies must, if success 
is to be achieved, be directed to their breeding 
haunts, and as early in the season as possible. 
The points in the anatomy of the fly of import- 
ance for our present object are the legs and feet 
and the alimentary apparatus. These will be suffi- 
ciently obvious from the diagrams (Figs. 1 and 2). 
The feet are covered with minute hairs, which are 
more numerous and finer than in the diagram, 
and extremely fine hairs are also placed upon the 
pads. A sticky substance is secreted by the sur- 
face of the pads, by means of which the fly grips. 
Each leg is like a minute paint brush, which is 
