100 INSECTS AND DISEASE 
So that birds migrating or blown from a continent and 
alighting on an oceanic island might carry with them the 
germs of plants. The case of the house-fly is analogous. 
Its tastes are unfortunately diverse. It appears equally 
fond of filth and fecal matter on the one hand, and of food 
prepared for mankind on the other. That a fly alighting 
on any substance containing bacteria, and subsequently 
alighting on a suitable culture medium, will inoculate the 
latter with bacteria is easily understood, and has been 
frequently verified by experiment. The disease-organisms 
most likely to be conveyed by flies are those of typhoid 
fever, and of dysentery and other diarrhceal disorders. 
If a fly, which has visited material containing these organisms 
subsequently alights on some food material, such as milk, 
which is a suitable culture medium for the disease organism, 
this food will within a few hours, in warm weather, teem 
with the organism, and any susceptible person who imbibes 
it will probably contract the disease. Though this is not 
of course the only way in which typhoid and dysentery 
bacilli may reach food material, it is, I think, one of the 
commonest and most dangerous. Again, a fly alighting 
on an inflamed eye, will carry away with it pus organisms, 
which are implanted on the next eye which the fly visits, 
and so the fly may convey ophthalmia. This disease may, 
of course, be conveyed in other ways, but the fly is a most 
persistent and dangerous carrier. 
What practical lessons may we draw from these facts 
in the way of prevention? The house-fly, we are informed 
by the entomologist, ‘“‘ runs through its life-history in a very 
short time. It lays about 150 very small eggs on dung, 
or any kind of soft damp filth ; the larvae hatch in a day 
or two, and feed on the refuse ; they may be full-grown 
in five or six days, and then pupating may in another week 
emerge as perfect flies.’ Consider what this means. 
Assume that % of the progeny of a female fly survive. In 
two weeks the solitary fly has become 100, of which we will 
suppose half are of each sex. In four weeks they are 5,000. 
In six weeks 250,000. In eight weeks, 12,500,000. We 
need not carry our calculation any further. Evidently 
a plague of flies is no miracle, and any direct assault on the 
insect by flypapers or otherwise in a very feeble palliative. 
