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be at work rendering the number of flies unusually large at a particular place of observation, and
this was, it appeared, exerted by some neighbouring premises at which muslin wrappings of
frozen carcases of meat were cleansed ; the operation in question produced a characteristic odour.
From the outset the attempt was made to determine the species of flies caught. More than
nine-tenths were examples of the common-house fly—Musca domestica. Some hundreds were
identified as belonging, and doubtless some thousands actually belonged, to the less common species—
Homolomyia canicularis—which closely resembles Musca domestica, but is rather smaller. A third
species—Stomoxys calcitrans—is said to be not uncommonly met with in houses; the inspectors were
on the look out for this fly, which is a stinging fly and should be readily distinguished by reason of
the appearance of its proboscis, but no examples of this species were detected. A similar experience
is recorded by Mr. Newstead from Liverpool. A considerable number, some 3,000 specimens, of
the common blue-bottle—Calliphora vomitoria—were secured. There was a marked tendency for
blue-bottles to appear in some numbers when they appeared at all; at two places of observation in
the neighbourhood of stables the number was noted as considerable; the centre with the largest
total number of blue-bottles was the depot No. 2, from which over 1,000 were reported; 360 were
captured at the places of observation surrounding one of the offensive trade centres (centre 11), but
only 115 were counted on the papers from houses surrounding the other (centre 10). The observations
with regard to blue-bottles are no doubt less reliable than those relating to smaller flies, for in
numerous, instances blue-bottles were found to be able to effect escape after first alighting and
adhering to the paper. In addition to the insects mentioned, a number of "gnats" and
" midges," and a few "green flies," "moths," "crane flies" and "bed bugs " were enumerated.
While these various subsidiary causes each, no doubt, exert more or less effect, the preponderating
influence is unquestionably shown by the enquiry to be proximity to collections of horse manure and,
in less degree, of house dust and other refuse. In a report by Mr. Robert Newstead, A.L.S., F.E.S.,
prepared by the direction of the Liverpool Corporation, the result is given of enquiry (December, 1906,
to October, 1907) as to the breeding places of house flies, and it is shown that horse manure and spent
hops and ash-pits, containing fermentative material, are especially to be regarded in towns in this country
as "permanent breeding places." In addition to the foregoing, certain other collections of material may
serve as "temporary breeding places," collections of fermenting vegetable refuse, accumulations of
manure at wharves, and bedding in poultry pens are particularly mentioned. The reporter writes,
with reference to stable middens containing horse manure, "The larval stages of the house fly occurred
in countless thousands, revelling in the heat produced by fermentation. The adjacent Walls often
swarmed with newly hatched flies, and occasionally one also found enormous masses of their eggs, while
deep down at the sides, in the cooler portions of the receptacles, the pupa or chrysalis stage occurred in
enormous numbers, looking like small heaps or collections of reddish berries. Middens containing a
mixture of horse and cow dung were also infected, though to a less extent than those receptacles containing
horse manure only."*
The belief that horse manure especially serves as the breeding place for house flies is found
expressed in most books which deal with the house fly and its habits, though a somewhat different
experience Was obtained in India by Major Smith.† This observer studied the ways of house flies
under tropical conditions at Benares, and there found Musca domestica and Musca entaniata mainly
represented in barrack rooms, etc., the former apparently breeding chiefly in cow dung, the latter in
human ordure and in dog excrement. Horse and donkey dung in single deposits only yielded flies
which are not domestic in habit.
In London, horse manure pre-eminently stands out as the chosen breeding ground for house
flies, but it appears clear that the ordinary house refuse deposited in dust-receptacles also possesses great
attractions for them.
An interesting demonstration of the influence of horse manure is afforded if the curve showing the
prevalence of flies throughout the summer, in the places of observation surrounding the four centres
at which horse manure mainly came into question, be contrasted with the corresponding curve for the
remaining eight centres (see Diagram III.). It transpires that in the case of the four centres, following
upon the rise coincident with the development of hot weather in July and August, there ensued a
further and, as it were, super-added wave of prevalence, covering the latter part of September and early
October. This second wave is much less conspicuous in the curve for the other eight centres. At several
of these, however, some influence is known to have been exercised by collections of horse manure, and if
this influence could be eliminated altogether the superadded wave would no doubt disappear from that
curve. On the other hand, the presence of the superadded wave in the general curve of prevalence for
the twelve centres may be safely attributed to the breeding of batches of flies in horse manure during
the close of the summer and early autumn.
A detailed examination of the figures recorded at houses surrounding the several centres confirms
in a very striking manner the conclusion that collections of horse manure exercise an altogether
exceptionally important influence. Thus persistently high figures were recorded at two places of
observation situated some distance from the dust wharf (Centre 3), and this exceptional incidence was
found to be associated with the proximity of the houses in question to some stables, which constituted
a subsidiary disturbing influence. A similar phenomenon was observable in the case of Centre 9, where
* "It is important to note, however," he adds, " that in all cases where fowls (not ducks or geese) were kept
and allowed freedom. in the yards, relatively few of the earlier stages of the house fly were found, and whenever
present were invariably located in places inaccessible to the fowls."
† See Journal of the Royal Army Medical Corps, Vol. ix., No. 2.