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2
Dr. Hamer's Report.
Part I.—Nuisance from Flies.
In continuation of the observations of previous years, study has been made of the question of
fly nuisance in certain selected localities. Instead, however, of dealing with twelve centres around
which 163 places of observation were chosen as in 1907, or with nine centres surrounded by 141
places of observation as in 1908, only three centres were, in 1909, selected from among those studied
in 1907 and 1908, and around them 35 places of observation in all were chosen. The three centres
were—
(1) A depot used for the manipulation of house refuse.
(2) A glue and size manufacturers' premises. (See report of 1908, p. 2.)
(3) A railway siding at which trucks were loaded with stable manure and other refuse
materials.
While the inquiry was thus limited in its geographical scope, it was considerably extended
in respect of points of detail. Thus the work of previous years was undertaken only during the
period of special fly prevalence, but in 1909 it was started in the middle of May and continued until
the end of October. Again, experience had shown that species not commonly found on sticky papers
could be trapped in balloons. The attempt was, therefore, made in 1909, to institute comparison
of the results obtained, first by use of gummed papers exposed in 35 places of observation as in previous
years, and second by use of fly balloons (25 in number) exposed in the open air, but in close proximity
to the doors or windows of living rooms. The flies caught were classified into genera, and where
practicable into species, and curves have been constructed giving the seasonal distribution of the
various sorts of flies caught by one or the other method. This work was undertaken by Mr. E. P.
Bates and Mr. A. C. Palmer (both of the Public Health Department), and their familiarity with
the process of classifying flies, proved of great service. In several instances, in which some doubt
arose, appeal was made to Mr. E. E. Austen of the British Museum, who has kindly named the specimens
for us. All the flies caught in balloons, numbering from 4,000 to 20,000 a week, according to the time
of the year, were sent to 8, St. Martin's-place for examination.
Some difficulty was experienced early in the season, when the method adopted (at the place of
capture) for killing the flies was to pour boiling water over the balloon containing them. This plan
proved unsatisfactory, the flies were rendered moist and prone to decompose. Mr. Bates, therefore,
suggested that a wooden box should be constructed, large enough to hold a balloon, and that it should
be made as air-tight as possible by means of a close-fitting lid, with hinges and a clamping screw. The
balloon containing the flies was placed in the box, sprayed with benzene, and the box was then closed
for three or four minutes. Sorting was in this way rendered much less difficult, as the flies were kept
dry and their natural colours were maintained. A further advantage was that the benzene killed
not only the flies but their parasites, which the hot water sometimes failed to do. Larvae, however,
were unaffected save when exposure to benzene was prolonged. The method of killing by hot water
employed early in the year afforded striking instances of the vitality of flies; the use of benzene
proved far more effective.
Last year many flies were found mutilated, the contents of the abdomen or eye being eaten
out, so that only a shell remained; and there was much speculation as to the cause. Early this year
there were found, in a particular catch containing many flies thus mutilated, two specimens of Rove
Beetle (Pterostichus madidus and Calathus cisteloides).* Later, more specimens of these two beetles
were frequently found, and always in conjunction with mutilated flies. Earwigs also have a marked
partiality for flies.
The results of the year's work may be considered under the following heads:—
1. The total yield of flies on fly papers.
The observations made with sticky papers have now been continued for three years, and it is
possible to make a comparison of the results obtained on a uniform system. The curves in Diagram
I. are drawn to a common scale, so as to admit of such comparison. For 1907 two sets of figures
are charted on the curve, and the area between the upper and lower boundary lines thus obtained
has been shaded. The upper boundary line represents the actual total numbers obtained at the 163
centres of observation dealt with. The lower boundary line gives the comparative figures obtained
by reducing these totals in the proportion so as to make the 1907 figures comparable with
those of 1908. The line which gives the figures for 1908 is based on the results obtained at the 141
centres in that year. The yield of flies in the two years may, therefore, be compared by examining
the 1908 line and the lower boundary of the shaded area between the two 1907 lines. In 1909 there
were only 35 centres, the figures therefore have been multiplied by four so as to obtain results for that
year determined on the same scale as those of 1907 and 1908. All three lines thus represent the yield
of flies obtained by use of fly papers at approximately the same number of centres of observation.
Again, for purposes of comparison, the figures giving the diarrhœal mortality, week by week,
have also been plotted out in Diagram I. It will be seen that the diarrhœal curves of the three years
exhibit features which reflect, as it were, peculiarities of the corresponding fly curves—thus, as regards
diarrhœal mortality, there was in 1907 a wholly exceptional delay in the rise to a maximum, and
the same phenomenon is conspicuous also in the fly curve of that year. Close study of the three pairs
of curves (fly curves and diarrhœal mortality curves) reveals many similar correspondences. (See
P. 4.)
* Kindly identified by Mr. E. E. Austen,