41 
apparently complete recovery of the patient. By the agency of flies 
which visit such excrement the bacilli may be carried far and wide to 
food supplies, and by their consumption may enter the digestive tract 
of many healthy individuals. 
An investigation has been carried on in this office for the purpose 
of ascertaining just what flies breed in human excrement or are in 
the habit of visiting such substances, and, conversely, just what flies 
are found in dining rooms and kitchens where food is being served 
and prepared. These investigations have been conducted with the 
utmost care and in many different parts of the country. <A very 
large amount of material was studied, and the detailed results were 
published in the proceedings of the Washington Academy of Sciences 
(Vol. II, pp. 451-604). Briefly summarized, it was found that the 
number of species of insects which breed in or frequent human excre- 
ment is very large. There are many beetles (44 species, and many 
hymenopterous parasites); none of these, however, are especially 
significant in this connection. Flies are the important creatures, and 
Fic. 20.—Musca domestica: Puparium at left; adult next, with enlarged antenna; larya and enlarged 
parts at right—enlarged (original). 
of these 77 species were studied. Thirty-six of them were found to 
breed in human feces, while 41 were simply captured while visiting 
this substance or feeding upon it. Some, of course, were scarce and 
others were very abundant. 
Now, in order to ascertain exactly which ones of these-are important 
in the disease-bearing function more than 2,300 flies were caught in 
kitchens and dining rooms in different parts of the country from Mas- 
sachusetts to California and from New York to Louisiana, and were 
all carefully examined. It was proven that of the excrement flies six 
species are found in houses in sufficient numbers to constitute them 
dangerous species. The most abundant species found in or on excre- 
ment do not occur in kitchens and dining rooms, but, as just stated, 
these six species are sufficiently abundant in both relations to become 
very dangerous. 
At the head of these six species must stand the common house fly, 
Musca domestica (fig. 20). This insect constituted over 98 per cent of 
