NATURAL ENEMIES AND PARASITES
Flies, which are such insidious and pertinacious persecutors of man and
beast, are themselves the prey of innumerable enemies; many species are
much sought for by birds, they are devoured by lizards and toads, and
they are equally preyed upon by predaceous insects. Those flies which
have bodies with banded colours, and which otherwise somewhat resemble
bees and wasps, probably escape being the victims of some birds; but the
tribe of flies does not, like the beetles, the lepidopteræ, and some
other insects, furnish instances of other common protective devices,
such as bearing and voiding offensive secretions, or attempted
concealment in repose by mimicry of environment.
All insectivorous birds are fond of a diet of flies, and we may largely
attribute the spring-time immigration of the beautiful swallow tribe to
the fact that in the northern parts of the temperate zone swarms of soft
bodied dipterous insects abound, and there replace the hard cased and
more chitonous insects of hotter countries. The true swallow, the house
martin, and the sand martin, all require a special food for their
nestlings; and they also then require the longer summer days and the
prolonged twilight of our northern clime for the frequent feeding of
their young. The prevalence of flies near houses partly accounts for the
partiality of the swallow for nesting under the eaves of our dwellings,
where unfortunately the aggressive, the pampered, and the demoralised
sparrow in towns generally prevents successful breeding by appropriating
its nest and sometimes by eating its eggs. People who desire to favour
the breeding of the swallow should destroy the nests immediately after
the migratory departure of the builders. Their retention until the
following spring is in no way an enticement for rehabitation; on the
contrary, it favours the objectionable habit of the sparrow to use them
for a night resort, whereby they become very foul with bird vermin. The
cleanly swallow annually desires a newly built habitation, and a bare
peg projecting two inches from the house-wall will much encourage
swallows to start the foundations of a nest thereon.
Ground feeding birds of many kinds eagerly search for flies, their pupæ,
and their larvæ; even some finches will add flies to their diet in the
nesting season. Chaffinches are very fond of the house-fly at all times.
The wasps are assiduous hunters of flies, and, though possessing less
agility than their prey, they manage to pounce upon many victims. Very
common enemies are the predaceous _Empidæ_, of which numerous species
are native to Great Britain. Less common, but very observable when met
with, are the closely related robber-flies, _Asilidæ_, which are hardly
ever to be seen on a hot midsummer day without a captured fly held
between their strong front legs. The largest of the robber-flies, _A.
crabroniformis_, is a conspicuously fine insect. It equals the hornet in
length, but is more slender in body, tapering throughout the abdomen
towards the tail; it resembles the queen hornet in colour. The
dragon-flies also eat flies, but they mostly feed upon winged aphides,
gnats, and the like small game.
More secret destroyers of the fly brood are a few rather obscure
creatures akin in their nature to the ichneumon flies, which are
parasitic mainly but not exclusively on _lepidopteræ_. Likewise, certain
insectivorous beetles share in the good work of fly destruction.
Flies are often observable encumbered with minute vermin; some of these
are true lice, and some are allied parasites called false-scorpions or
chelifers. These are acquired whilst frequenting dung and refuse heaps,
where they abound; thus, probably cheese-mites and the like are conveyed
by flies into our larders and warehouses.
It is not only the web-weaving spiders which prey upon the proverbially
"silly" flies; there are also roving spiders which do not contrive webs,
and some of these are nocturnal feeders; the latter can only be seen in
daylight by looking for them underneath stones and in other hiding
places.
Other fly destroyers are internal parasites, and these include thread
worms (_Nematoidea_), as well as _Protozoa_ of obscure kinds. These are
being scientifically studied by experts, and their life-history is as
curious as that of others of the same order, in that they pass from one
host to another, which fact for long helped to baffle investigation.
Some have now been proved capable and others are suspect of baneful
possibilities.
The house-fly fungus, _Empusa muscæ_, which is prevalent in autumn, has
ever attracted popular wonder and much scientific attention. It has been
much written about and plentifully illustrated, but the complete
life-cycle of this peculiar parasitic growth is not yet understood; much
that has been published as of fact is mere "copy" repeated in one book
after another, originally in fact rather a matter of conjecture, based
upon the idea that fungic propagation must be on exactly parallel lines
with known biologic processes of a botanic order.
The house-fly fungus seems to have a superficial resemblance to some of
the common "moulds," but mycologically examined there is good reason for
classifying it with a family (_Entomophthoreæ_), which may be capable of
an alternative form of fructificative development.
The originating germ somehow at some time must be supposed to have
effected a lodgment in the body of the fly or possibly that of the
maggot. Later on, one cannot say when, fungic spawn (a pulpous mycelium)
starts a course of development, invades every part of the body, quickly
kills the fly, and fixes it to the spot to which in its last moments it
has crawled, often a window-pane. Its corpse is now swollen with the
spawn developed into masses termed hyphal; but these should not be
called hyphal (thread) but quasi-sclerotia, bodies intermediate in a
process of normal development between the mycelium and the fruiting
stage; fungic fructification ensues with great rapidity; and the corpse
becomes suffused with an appearance of white mouldy excrescences,
filiform conidiophores, of which the club-like tips make a copious ærial
discharge of white spores; when these adhere to a glass or window-pane
they imprint thereon a remarkable halo. Attempts to artificially infect
other flies with these spores are common failures or have led to
contrary conclusions.
The period from the first symptoms of distress to the death of the fly,
and from that time to the spore discharge is wonderfully short. The
infectious germs may have been long dormant in the fly, and very likely
may have been acquired in the maggot stage. In the absence of exact
knowledge, we can only make conjectures from observations of some
kindred fungic parasites, which are not very uncommon amongst the
chrysalids of certain moths, beetles, and the pupæ of some annual wild
bees. In these cases it seems very unlikely that the infection was
incurred otherwise than in the caterpillar or larval feeding stage.
Dampness and insanitary conditions seem to favour the spreading of such
disease, especially amongst artificially reared larvæ when crowded
together and closely confined.
Other kinds of flies do not, so often as the house-fly, perish from
_Empusa muscæ_, but I have seen a common yellow cowdung-fly, as early as
June, thus affected.
In spite of all antagonists the brood of the house-fly flourishes and
multiplies, but this is because great opportunity and encouragement is
provided by the neglect of good sanitation by mankind.