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on the paper, and of then releasing the portion folded back and allowing it to hang as before at the
lower margin of the paper. It was upon the slightly gummed surface of the marginal strip that Homalomyia
specimens were particularly apt to be found (see Diagram IV. The specimen of Homalomyia
seen at the upper part of Fig. 1 in Diagram IV. was, no doubt, transferred from the lower strip, for
the paper had apparently been folded over.) It was thought that possibly Homalomyia by preference
alighted on a surface (the marginal strip) upon which there was but little gum ; and again, the
suggestion was made that possibly, while, Musca and Homalomyia both alighted upon the strip,
Homalomyia alone was held there, but this, it will be noted, fails to explain the absence of Homalomyia
from the rest of the fly paper. Yet again it was conjectured that Homalomyia had a habit of
approaching from below upwards, and thus of alighting on the lowest part of the paper ; and
this contention was supported by the distribution of the two species of flies on a paper (see
Diagram III., Fig. 3) having its gummed surface nearly horizontal, but with one half of that
surface at a somewhat lower level than the other half. This last hypothesis was further
supported by the fact that, in instances in which the special phenomemon was observed, the
place in which the paper was exhibited was at a somewhat high level (in first-floor rooms, and
in one case in a third-floor room). Mr. Powell, the inspector, who supplied the very remarkable
paper shown in Diagram IV., Fig. 2, informs me that the room in question invariably supplied a
large percentage of Homalomyia ; that it was a dormitory occupied by 87 men ; and that no
food was prepared or consumed in it. The paper was not only at a high level (third floor),
but it was further suspended 10 feet above the floor of the dormitory, and these facts help to explain
the extraordinary distribution of flies upon it. Whatever the explanation of this curious
phenomenon may be, it is clear that Homalomyia tends to alight on certain portions of the gummed
papers which are comparatively free from Musca; and in corroboration of this, the fact may be
noted that even in the earlier part of the year, when Homalomyia was specially in evidence, papers
which yielded large numbers of Musca supplied comparatively small percentages of Homalomyia.
More striking even than the behaviour of Homalomyia was that of Stomoxys, in relation to
presence of Musca. Stomoxys calcitrans was found in large numbers at centre 7 and in smaller
numbers at two other centres. But despite the vast numbers of this fly in the cowshed itself, at centre
7, no specimens of Stomoxys were secured at surrounding stations of observation in living rooms of
houses. It was indeed remarkable to observe that although, during July and August for example,
there were at times as many as 100,000 specimens of Stomoxys (this is, of course, an estimated number
based upon the actual number found on a measured surface) in the cowshed, the papers exhibited
in three living rooms situated within 10, 10 and 20 yards of the shed yielded not a single specimen. The
flies caught in these living rooms were almost exclusively Musca domestica.
The sharpness of the line of demarcation between Stomoxys and Musca was observed on more
than one occasion, when it was noted that up to, and even at the door of, the cowshed, Musca was
plentiful and Stomoxys quite uncommon, or entirely absent; while, on the cows themselves, and
within the shed generally, Stomoxys was the only fly found. A few specimens of Stomoxys were, however,
seen in a shed used for dairying purposes, which adjoined the cowshed. The workers in this
shed and in the adjoining house and those in the cowshed itself said they were never bitten by
Stomoxys. This immunity was very striking, having in view the extent to which the flies collected
on the fly papers were gorged with cow's blood.
Although in the neighbourhood of these London cowsheds, Stomoxys did not enter houses, a
different experience is obtained elsewhere. Thus, during August, I found Stomoxys calcitrans was
plentiful in an isolated house in the country where there was no Musca. Stomoxys under these conditions
entered living rooms freely, but did not alight on food or on human beings with the persistency
of Musca ; the fly showed a preference for sunny patches on the floor or on the window panes. The
entire absence of Musca from the neighbourhood of this house seems worthy of note. I attempted to
find specimens in the village half a mile away, but though Homalomyia was common enough there,
I did not, in the course of a somewhat cursory examination, succeed in finding a specimen of Musca.
4. As to the mutilation of flies caught on the papers.
During July and August many papers from various parts of London were found to present
mutilated flies. In some instances the fly was decapitated, or the abdominal contents had been
removed, a shell only, as it were, of abdomen remaining. On some of the papers a track of mutilated
flies could be traced, so that clearly some fly-eating creature was responsible for the damage caused.
Various suggestions were forthcoming in explanation of this phenomenon. First, that spiders were
concerned. It was found by experiment that some spiders, and more particularly large spiders
with strong legs, were able to walk without difficulty over "honey gum" papers on which there were
many flies. If there were but few flies the spider could not as a rule traverse the paper. As a matter
of fact, four spiders were trapped on fly papers exposed in the ordinary way. Other suggestions made
were that various insects might have caused the mischief. This year, as last year, several bugs were
found upon fly papers. It may be noted, moreover, that two wasps were caught.
Further observations bearing upon the relation of flies to diarrhoea.
At the outset, attention may be directed to two considerations, of which mention was not
made in the report of 1907, and which, it may be thought, favour an hypothesis that flies serve as carriers
of infection. It might in the first place be suggested, in view of the occurrence of the Homalomyia
season at a period slightly earlier than the time of special prevalence of Musca, that if the seasonal curve