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2. The evidence as to nuisance at particular premises.
Observation was maintained, as in 1907 and 1908, at centres surrounding a dust depot. During
1907 this depot was used throughout the summer, and great nuisance from flies resulted. In 1908
operations, resumed during cold weather, were discontinued at the end of June, and a marked falling
off in the number of flies followed; indeed, it was estimated that there were some 6,000,000 fewer
flies in houses within a radius of two hundred yards of the depot during the summer of 1908 than in
that of the previous year. In 1909 work at the depot was continued until the first week in July,
but the meteorological conditions seem to have afforded opportunity for many flies to hatch out during
July, inasmuch as early in August there was a smart rise in the numbers. While there was no such
extreme development of nuisance as in 1907, it became clear that the use of the depot, up to the time
named, caused serious fly nuisance, despite the fact that operations were entirely discontinued after
that date. It should be noted that there was again, as in previous years, complaint of nuisance from
smoke from the destructor chimney of the depot; this formed the subject of correspondence between
the London County Council and the Borough Council owning the depot, and also of several reports
by H.M. Inspector in connection with Pritchard's-road School. The Board of Education, indeed,
intimated that, unless some steps were taken to abate the nuisance, it would have to consider the
suitability of the school buildings for use as an elementary school.
The chief special point of interest at the two other centres around which places of observation
were chosen, in 1909, arises in connection with the prevalence of the particular species of fly,
Protocalliphora groenlandica, to which reference was made in last year's report. The centres in
question are near one another, one of them is a glue and size maker's premises, on which bones are
received, the other is a railway-siding, to which, in addition to much stable manure and refuse material,
the bones just referred to are brought on their way to the glue and size works. The sacks of bones
contain vast numbers of larvae of the "special green fly," indeed the ground beneath some vans
containing these sacks was found one day last summer to be covered by larvae, so that from a little
distance this portion of the yard surface had the appearance of snow. On this occasion the unloading
of the vans had been delayed, and the weather was warm. Late in the summer a system of destroying
the larvae was adopted at the glue works, and it was found that this had some influence in lessening
fly nuisance in surrounding houses.
It was at first thought that observations on the occurrence of the special green fly at varying
distances from the glue-works would enable a judgment to be formed as to the distance to which
flies wander in their peregrinations. It was soon found, however, that this fly was caught not only
around the glue works but also at houses surrounding the railway siding more than half-a-mile away.
The suggestion was then made that the occasional presence of a railway truck containing bones,
which would ultimately be carted away to the glue works, determined this distribution of the special
fly near the railway siding. The observations of 1909, however, show that the special green fly was
occasionally met with, not only near the glue works, but also in other parts of the town, so that the
distribution of this fly in London is clearly not so limited as to enable it to be used as an index of the
wandering capacities of flies.
3. Differences observed in the yield of flies obtained by using different forms of fly traps.
The criticism has been made upon the methods employed in 1907 and 1903 that the exposure
of sticky papers in living rooms does not give a complete record of the distribution of the various
species of flies which enter dwellings. In order to obtain further light on this question in 1909, in
addition to the use of sticky papers, balloons appropriately baited were exposed in the open air, but
in close proximity to houses. Diagrams II. and III. exhibit the results obtained. The flies were
sorted, by Mr. Bates, as far as practicable, into species (or at least the genus was determined), and
in the diagrams the numbers of the six principal genera of flies are separately charted. The lowest
eurves in Diagram II. relate, in each instance, to the common house fly, Musca domestica. The next
curve in order gives the total for Musca and Homalomyia (the lesser house fly, Homalomyia
canicularis being, no doubt, the species captured in the large majority of instances). Calliphora
erythrocephala and Calliphora vomitoria (the "Blue-bottle Flies") were then added in to the totals,
and the curve thus obtained follows next in order. Next the number of Protocalliphora groenlandica
(the "special green fly" of former reports) was added. Then follows the curve obtained by adding
in the numbers of Muscina stabulans, and last of all comes the genus Lucilia (Lucilia ccesar being
the common species).
Comparison of the flies caught in balloons with those caught on papers shows that while,
among the latter, Musca greatly predominates, and other species (save Homalomyia) are very rarely
met with, in the balloons all six genera are well represented, and, indeed, Calliphora bulks even more
largely than Musca itself. The curves possess interest, but the results do not appear to affect the
main conclusions drawn, in former reports, from use of sticky papers only. It seems clear, in view
of the great predominance of Musca domestica inside houses, that any prejudicial influence exerted
in relation to contamination of food supplies is more likely to be due to this species than any other.
In Diagram III. the results are given in a different form—each genus of flies being shown in
a separate chart—all the curves are drawn to a common scale. This diagram shows that, in the case
of Homalomyia, the two methods of trapping the flies do not give greatly divergent results; in
that of all the other flies (save Musca) balloons yield far more flies than papers. Stomoxys calcitrans,a,
biting fly met with in a cowshed in 1908, has only rarely been captured in 1909. In the latter
year no cowshed was kept under observation, but occasional visits, by one of the Council's inspectors,
to the cowshed centre of last year showed that the fly was still present there in large numbers.
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