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It will be observed that numerous specimens of Protocalliphora were caught from the middle
of June until mid-September. Both species, P. groenlandica and P. azurea were apparently represented,
though Mr. Bates thinks the latter was more frequently met with after the beginning of
August, nearly all the earlier specimens being Protocalliiphora groenlandica. Whilst Musca, Homalomyia,
and Muscina showed a steady increase, as the season progressed, Protocalliphora varied in an erratic
manner, the numbers rising and falling from week to week without apparent reason. Thus in the
week ending 21st August, when Calliphora and Lucilia showed marked increase, following a spell of
thirteen days warm dry weather, Protocalliphora began to decrease, but rose again at the end of
the month. The Rev. W. J. Wingate, in his book on " Durham Diptera," has remarked on the
presence of Protocalliphora in considerable numbers during the time that a small manure factory
was in existence in a particular neighbourhood, followed by their complete disappearance when the
business was discontinued.
Of the genera Muscina and Lucilia there is not much to say except that in London they
appear to be most numerous in the neighbourhood of street markets where meat and fish are sold.
A remarkable point noticed with regard to Muscina was the frequency with which it was infested
with parasites. This was especially the case in early June, and of 300 flies of this genus then
examined 40 were thus affected. The parasites were very small, and in some instances were aggregated
so closely together that no part of the fly's abdomen was visible. In fact, the vermin formed
a sort of chain mail, and must have been numbered by hundreds. Mr. E. E. Austen, to whom some
specimens were sent, pronounced them to be Gamasid mites, allied to those found on dung beetles.
Another parasite generally occurring singly, and as a rule on Musca domestica, was occasionally
observed ; in size it is comparable with one of the compound eyes of this species.
. In addition to flies belonging to genera named in Diagrams II. and III., others were occasionally
met with—Hydrotcea deutipes (some 900), Sarcophaga hcemorrlwidalis (some 340), Scatophaga
stercoraria (some 170), Spilogaster uliginosa (some 100), and nearly 100 further specimens belonging to
other species.
4. The distribution of flies in sexes.
In the inset chart of Diagram I. the percentages of male and female flies caught in balloons
are shown. This determination of the sexes is based upon the examination of many thousands of
specimens. In the Diagram, opposite each generic name is shown a series of columns, corresponding
to successive weeks, each series having an irregular line upon it. The portion of each column above
this line represents the percentage of male flies, the portion beneath that of female flies. The curve
for all flies shows that the two sexes are fairly equally distributed throughout the summer, and that
for Calliphora also shows nearly equal distribution of males and females. The curves for Protocalliphora
and Lucilia are more irregular. In the case of Musca there is some slight tendency for
males to predominate early in the season, and females later. Homalomyia proves quite exceptional,
for the males distinctly predominate throughout, constituting between 75 and 85 per cent of all
the flies of this species caught. A similar distribution so far as Homalomyia is concerned was found
to hold good last August in a house in the country. Specimens of Homalomyia caught indoors were
generally males, but out of doors small swarms of Homalomyia were occasionally encountered, and
examination showed that these were females. The latter persistently settled upon human beings,,
and were a source of considerable annoyance, while the flies of this genus met with in living rooms
gave little or no trouble of this sort. It may be noted as a point of interest, that in the house in the
country just referred to Musca domestica was not met with; Stomoxys calcitrans was fairly plentiful,
but the large majority of the flies indoors belonged to the species Homalomyia canicnlaris, and were
male flies.
Further Observations on the Relation of the Fly to Food Contamination and to Spread of Disease.
(a) Correspondences in the fly and diarrheal curves.—This question has already been discussed
in the reports of 1907 and 1908, and it possesses interest, inasmuch as the observed correspondences
have been cited as a proof that flies cause diarrhoea. The phenomena might be explained, however,
as has been pointed out, on the assumption that the same causes (temperature, rainfall, etc.) which
operate upon the curve of a particular year in the one case (diarrhoea), also produce the corresponding
variations observed in the other case (number of flies). If, moreover, the curves of one year be compared
with those of another, discrepancies are found to present themselves. Thus in the Report
(October, 1908) on Nuisance from Plies, certain features of the 1907 and 1908 fly curves were commented
upon, as telling in favour of an explanation by a common cause, rather than by the hypothesis
that flies cause diarrhoea. In a similar way, appeal may be made to Diagram 1. of the present report.
The ascending portion of the fly curves of 1907 and 1909 almost exactly correspond, and thus it
may be said that, so far as prevalence of flies is concerned, the two years stood throughout the
months of July and August almost exactly on a par; yet the figures showing the rise of diarrhoea
mortality in those years reveal a marked contrast, for in 1909 a sharp rise reaching a maximum in
August was observed; while in 1907 the rise was very slow, and the maximum was not attained until
the end of September or early October. Considerations of this sort make it clear that, if flies play any
considerable part in causing diarrhoea, the effect produced by them is, at any rate, much obscured
by other influences, and too great importance must not be attached to correspondences in the fly
and diarrhoea curves of any one year. On the other hand, as has before been stated, even granting