156 Arthropods as Simple Carriers of Disease 
tions, they are important factors. Howard has emphasized that in 
such cities there are still many uncared-for box privies and that, in 
addition, the deposition of feces overnight in uncared-for waste lots 
and alleys is common. 
Not only unicellular organisms, such as bacteria and protozoa, 
but also the eggs, embryos and larvae of parasitic worms have been 
found to be transported by house-flies. Ransom (1911) has found 
that Habronema muscce, a nematode worm often found in adult flies, 
is the immature stage of a parasite occurring in the stomach of the 
horse. The eggs or embryos passing out with the feces of the horse, 
are taken up by fly larvae and carried over to the imago stage. 
Grassi (1883), Stiles (1889), Calandruccio (1906), and especially 
Nicoll (1911), have been the chief investigators of the ability of 
house-flies to carry the ova and embryos of human intestinal parasites. 
Graham-Smith (1913) summarizes the work along this line as follows: 
“It is evident from the investigations that have been quoted that 
house-flies and other species are greatly attracted to the ova of 
parasitic worms contained in feces and other materials, and make 
great efforts to ingest them. Unless the ova are too large they often 
succeed, and the eggs are deposited uninjured in their feces, in some 
cases up to the third day at least. The eggs may also be carried on 
their legs or bodies. Under suitable conditions, food and fluids 
may be contaminated with the eggs of various parasitic worms by flies, 
and in one case infection of the human subject has been observed. 
Feces containing tape-worm segments may continue to be a source of 
infection for as long as a fortnight. Up to the present, however, 
there is no evidence to show what part flies play in the dissemination 
of parasitic worms under natural conditions.” 
Enough has been said to show that the house-fly must be dealt 
with as a direct menace to public health. Control measures are 
not merely matters of convenience but are of vital importance. 
Under present conditions the speedy elimination of the house-fly 
is impossible and the first thing to be considered is methods of pro¬ 
tecting food and drink from contamination. The first of these 
methods is the thorough screening of doors and windows to prevent 
the entrance of flies. In the case of kitchen doors, the flies, attracted 
by odors, are likely to swarm onto the screen and improve the first 
opportunity for gaining an entrance. This difficulty can be largely 
avoided by screening-in the back porch and placing the screen door 
at one end rather than directly before the door. 
