DISTRIBUTION AND CONCENTRATION OF FLIES
It might be supposed that a strongly developed house haunting proclivity
would not be consistent with a disposition to roam far afield from the
locality of birth. Many clever experiments have been made with marked
flies released and recaptured within measured distances and times. After
an immensity of pains taken, very little profitable knowledge has been
arrived at thereby. Little of what we really want to know is indicated
by such a fact as that, out of hundreds or of thousands of marked flies
released, one per cent was recaptured at spots as remote as a mile
within two or three days, or by such a fact as that a large percentage
should be observed to remain within a more limited home circuit. The
variable factors of temperature, wind, sunshine and rain inevitably tend
to discredit the reliability of the observed results following any such
experiments.
Close observations of the habits of the house-fly reveal the very
appurtenant fact that the movements of newly-hatched flies, for their
first six or seven days' active life, differ from those of a more mature
age, when the breeding instinct has grown strong. The latter are
disposed to locate themselves for the rest of their lives in and about
one attractive spot, and they are indisposed to fly high above ground or
to travel far, unless it be with the object of leaving an unsatisfying
locality and discovering a better place. However, the younger flies seem
to feel no such restrictive influence, for, as soon as they have become
fit and the weather suits, they show an inclination to fly high and thus
may travel to very remote places. It is just the same with peacock, red
admiral, and tortoise-shell butterflies, which I have often reared and
released for adding to the interest and beauty of a flower garden. In
sunny weather many or most will soon wander never to return; those which
have remained a few days continue residence close round about,
especially if nettles, the food plant, grow in the neighbourhood.
It would be of great interest if we could discover how far a plague of
flies arising from unsanitory surroundings in one locality is liable to
spread to the injury of other localities.
On this subject nothing useful can be said other than can be safely
surmised from the known habits of the fly. The female has none of the
attachment of the honey-bee to its hive and community; she is not moved
by an instinct like that of the wandering bumble-bee in spring to found
a colony; she is indeed very solicitous about the disposal of her eggs,
but she is not impelled by any desire to place successive deposits in
one locality.
The lesser house-fly has proclivities similar to those of the common
house-fly, but probably she travels less far afield although a little
more inclined to outdoor life.
Very little is known about most of the common outdoor sweat-flies. Some
breed in dung, and may be many-brooded and otherwise resemble the
house-fly in prolific increase; others are more consistently vegetarian
in the larval stage and slower in development; and some are possibly
even single-brooded, like certain foreign large sized flies which
fortunately appear only for a few weeks of summer weather, for they have
a curious semi-blood-sucking habit of feeding after or alongside the
skin-piercing flies, and their suctorial mouths are capable of further
inflaming wounds and carrying infection from one animal to another.
The robust blue-bottle very closely resembles the house-fly in an
inclination to spread the brood. Mature females, however, do sometimes
show a slight temporary kind of "homing" instinct; having secured a cosy
corner and a well sheltered retreat in a sunny wall, the occupant will
battle for its possession, buffeting new comers.
Some of the smaller filth flies and many of the fungus flies have their
lives, in the imago stage, influenced and shortened by their extra early
sexual maturity; the females are fertilised whilst newly hatched and
their wings limp and unfolded. This fact accounts for our seldom seeing
some kinds of these flies abroad except females; and these are never
seen to indulge in dances, flirtations, and games of chasing and
buffeting each other, after the manner of so many kinds of flies. They
habitually fly low; nevertheless they travel very great distances, for,
though short, their flights are incessant when searching for their
special kind of food.
The most disinclined to roam of all common flies is the stable-fly. None
other is a more eager seeker of sunshine, but when basking on a sunny
wall it seems unwary and sluggish; it is seldom to be seen far from
where horses or cattle are stationed or stabled; however, it will make
very long journeys hovering about a driven horse or reposing on the car.
A plague of flies of local origin will not take many weeks of summer
weather to spread, but it is generally observable that plagues of flies,
like many other occurrences, are simultaneous co-incidences distributed
over wide areas and at places remote from each other.