GENERAL LIFE HISTORY
Whereas the blue-bottle rarely enters the dwellings of mankind, except
gravid females led by the sense of smell in search of fish, or flesh
meat, and (less eagerly) sweets, both species of house-fly and _both
sexes_ seem to delight in the mere odour of humanity; breeding females
will seek the larder and the dust-bin, but others will very provokingly
pervade all quarters. Although avoiding a dark or deeply shaded room,
the house-fly seems to like partial shade; it will be content to remain
indoors and to rejoice in a warm kitchen, even on a hot summer's day,
whilst all the other kinds of flies are enjoying the outdoor sunshine.
It may be said of nearly a dozen other species, occasionally observable
crawling on window panes, that they are "outdoor" flies, and that their
occurrence indoors is accidental. In fact, they are mostly observed when
trying to escape.
Next after human habitations, stables, cow-sheds and pig-sties are the
delight of the breeding female house-fly. Round about and in these
latter resorts she associates with an immense host of rather small sized
flies, and amongst a few others of equal size with the skin-piercing and
blood-sucking stable-fly; but many stablemen are ignorant of the
difference of the two kinds of flies and of the serious suffering of
their horses from the bites of the stable-fly. This lamentable ignorance
was shared by the joint authors of "Humble Creatures," published in
1858, when Neo-Darwinism was in vogue, and many books were published for
popularising a knowledge of common things and spreading an interest in
nature-study; this publication, which is still (1914) in print and very
little revised, has probably led some later would-be nature-study
teachers to follow suit in confusing the characteristics of the two
species. Very often the fly most numerously breeding in the manure heaps
of the mews will be _Borborus equinus_, or some other of the same
family, which are characterised by a very simple pattern of wing
nervures and by the absence of squamæ or scales behind the wings; also
the ankle joints of the feet are most peculiarly short and broad. _B.
equinus_, and a great host of other dung breeding flies of a still
smaller size, may be considered beneficial insects; they do not pester
cattle, and their larvæ make food more scarce for injurious flies.
The breeding habits of the blue-bottle are very conspicuous by reason of
its haste and boldness in taking possession of dead animals. It is
incapable of breeding in horse or cow dung, to which latter the
green-bottle fly often resorts.
The blue-bottle deposits her eggs, 500 or 600, preferably on dead fish,
or flesh, and sometimes on the sores or the flesh of wounded animals,
but both the house-flies preferably affect dung, carrion, garbage, and
all kinds of fermenting vegetable matter. It has been commonly but not
truly said that the principal breeding places of the house-fly are the
mews and the farmyards where manure is allowed to accumulate; the
house-fly has a preference for horse dung before cow dung, which is
preferred by some other kinds of flies; however, near towns, the
domestic dust-bins, heaps of market garbage, and deposits of town refuse
give rise to a worse plague of house-flies than stables. All these flies
deposit batches of white eggs, and are careful to place them as much as
possible in crevices and shielded from exposure to strong light, or from
draughts.
The two house-flies and the blue-bottle have similar larval stages, but
their larvæ, called maggots, differ. The larvæ avoid daylight and cannot
withstand dryness. As the larvæ feed, they have the power of ejecting or
excreting a juice, which dissolves the food before they imbibe the
material; their mouths are suctorial and are destitute of teeth or
biting jaws.
The larva of the house-fly is an eyeless and legless maggot, one half
inch long when full grown and extended; twelve cylindrical segments may
be counted in its body, or even thirteen if we separately distinguish
the small head segment, which may be withdrawn, and but little
observable; five or six rear segments are of nearly equal stoutness when
only half grown; afterwards counting from the three stoutish rear
segments, the others taper towards the very small head. The middle and
rear segments have pad-like bristly processes underneath, which aid the
maggots in creeping, in which action they also make much use of the head
segment's grappling hook. The maggots feed voraciously, but they seem,
like the larvæ of the honey bee, to pass out very little anal excreta;
some have thought that, like what is said of bee larvæ, no excrement is
discharged until after the imago has emerged from the puparium; but such
conduct seems altogether incredible. In the bee-hive doubtless the
assiduous workers ever wash their babies clean and lick up all matter,
just like domestic cats and dogs, when nursing their young.
The larva of the blue-bottle, called a gentle, is proportionately larger
but very similar, except that the rear segment possesses a ring of
tubercles, which may have some useful function in connection with two
breathing tracks, which have their orifices at that part of the body.
The larva of the lesser house-fly is very peculiar; all its segments
have projecting tubercles; its whole body is rather louse-shaped, having
not cylindrical but somewhat flattened segments, of which the middle are
the broader, and those near the head and tail the narrower.
The transformations in the case of the blue-bottle are typical of the
house-fly and others of closely related families and genera which are
many-brooded within the year; these creatures develop very rapidly
immediately after emerging from the egg. Some other kinds of dipterid
maggots, which are single-brooded, pass a very prolonged and obscure
early period of skin-shedding and non-feeding, a preparatory sort of
baby-hood metamorphosis; then at last they begin to feed voraciously and
to follow the general habits of other maggots. Some maggots curiously
refuse to feed except in company; probably some are unable to feed on
dung except where other species are providing the necessary dissolving
juice.
When the common maggots or gentles have ceased feeding, they burrow into
the ground or crawl away, often to a considerable distance, apparently
seeking a secluded, a more wholesomely clean, and a dryer spot. During
this migrating time, they are palatable food for many birds, which would
not eat them in their former food-loaded or unscoured state. Indeed, it
is doubtful whether either a vulture or a raven could eat a fly-blown
carcase without danger of myiasic punishment. The skin of the larva
whilst growing is transparent, but, when about to pupate, it thickens
and becomes an opaque creamy white.
The most marvellous part of the metamorphosis of the blue-bottle is
concealed, when the gentle becomes the pupa; according to Réaumur the
embryonic fly develops most curiously inside the puparium by a procedure
not exactly like the change from the caterpillar to the chrysalid in the
case of the butterfly. After a pause of a day or two, the front segments
of the fully fed maggot contract, so that the body assumes a barrel-like
shape; the skin then hardens, and turning a reddish brown it becomes a
much contracted shell or case called the puparium. However, the long
slender maggot has done something more than merely shrink and shape
itself conformably to the case; it has withdrawn its embryonic head, so
small as to be hardly distinguishable microscopically, together with its
embryonic legs, wings and thorax into its embryonic abdomen! As the
development proceeds, and the embryonic members of the future perfect
insect acquire their destined shape, the immensely increased head and
the thorax with its appendage members slowly emerge, and the partly
inverted integument of the abdomen rolls back, disclosing the shape of a
fly not before recognisable.
Other naturalists would have it believed that the true account of the
transformation is as follows,—when the maggot has shrunk and freed its
body inside its skin which forms the case or puparium, all its
pre-existing internal organs become absolutely dissolved; then out of
the fluid mass a new growth ensues, constituting the pupa with its
recognised shape. This account is the one represented in most modern
entomological books, and is based partly upon B. T. Lowne's monographic
work on the blow-fly.
The comparative embryologist of our day is inclined to be a
hyper-theorist, and so it seems that some have not remained content with
either of the above accounts; to them, apparently, the production of the
large and complex head of the imago out of a single small anterior
segment of the maggot requires a more recondite explanation, and must be
brought into harmony with analogous facts. To this end some degree of
linked support is found by the investigations of microscopic anatomy,
and it has been conjectured that not one or two head segments, but five
are lying blended and embryonically hidden in the larvæ, all ready to
bud forth. However, for fear of wearying too much with the theories of
advanced erudite scientists, the following _jeu d'esprit_ is presented,
instead of a more elaborate and sober attempt, to lure the unscientific
lay reader to an extreme hypothetical conception of the "essential
unity" underlying the apparent diversities of Nature within that vast
domain of the Kingdom of Fauna, which is obviously outside the later
creation of a vertebrate Animalia.
[Illustration:
Fig. 5. Illustrating the debatable continuity of a
12-segmented structure throughout the metamorphosis.
]
The futurist's dogmatic CREDO of creative progress, "For him who would
meritoriously pass his histological examinations, and qualify as a
Professor and Doctor of Science, above all it is necessary that he
should acknowledge the unicellularity of the primæval OVUM (or egg),
whence proceeds the seventeen-segmented boneless ANNELID (or worm), out
of which there develops the quadrangular articulated crustacean INSTAR
(or shell-encased aurelian), which metamorphosises into the winged IMAGO
(the angelic? or diabolic? fly); in the contemplation of this knowledge
alone is there supreme Darwinian Modernismal salvation and felicity."
Amen.
In view of the prosaic illustration of transmutation, figure 5 above,
the futurist disciple will have to accept the seventeenness of
segmentation by something like faith without sight.
The quadrangularity of the crustacean stage is based upon the idea that
the wings bud out from the two upper corners, whilst the legs develop
from the lower corners of the transmuting instar.
Perchance the reader will desire information about the use of this
curious word "instar," which has not the honour of notice in Dr. Sir J.
Murray's New English Dictionary. One might well feel proud of the
opportunity of adding the smallest item to such a stupendous and
monumental work, but I fear I am only qualified to venture a fair guess.
Virgil, I believe, used this term in allusion to the legendary wooden
horse of the Greeks at their siege of Troy. Some time less than one
hundred years ago entomologists recognised that the words aurelian,
chrysalis, and pupa were none of them an inherently fit term of general
application to the stage of insect life to be indicated. After many
attempts, this latest proposed substitute seems to be gaining favour.
The fly emerges after bursting apart the first four segments of the
puparium; this it does by a curious provision, whereby it can inflate a
chamber in its head in a queer, balloon-like fashion, making a bag-like
extrusion, which it uses as a punching and pushing machine.
After emerging from the ground, the fly withdraws the bag-like extrusion
and cleans itself. Its body soon grows fit and it becomes very active,
as long as daylight and warm weather favour it; otherwise it seeks
shelter and becomes quiescent; however, artificial light and heat will
awaken it to nocturnal activity. Sweets, carrion, and filth are all
attractive to the blue-bottle, but the house-fly and the lesser
house-fly also find extraordinary attraction in both man and his
dwelling.
Considering the superfluity of other flies, and the multitude of other
insects ever ready to do duty as devourers of carrion, garbage, and
filth, the scavenging services of the larvæ of the house-fly can be well
dispensed with.
In civilised communities cremation in a refuse destructor is the only
sanitary method of treating town refuse. The economic value of the fly
is very little, and consists merely in its food value for certain birds.
In warm weather the scavenging capabilities of all the carrion and filth
feeding maggots are very remarkable, and there appears no exaggeration
in the statement by Linnæus, that the progeny of three flesh flies can
eat up the carcase of a horse sooner than it could be devoured by a
lion.
When a batch of eggs has matured in the abdomen of the female, she is
most careful in the location and manner of their disposal. Guided by the
sense of smell, she will not lay her eggs except in contact with food,
or in places securing her progeny access to their intended food. By the
use of her soft, slender ovipositor, which is telescopically extensile
and flexible, the eggs are deposited in shaded and concealed situations.
The house-fly is credited with laying batches of eggs at intervals,
perhaps four or more times, and about 150 on the first occasion, then
100, and less on subsequent occasions. Under favourable circumstances
the eggs may hatch within a few hours of their being laid. The maggots
of midsummer broods may be full grown and pupate in six days, and the
perfect insect may emerge from the puparium in another ten days of warm
weather, but in cold weather the pupæ of autumnal broods may remain
dormant for several weeks, or even months. When nine or ten days old the
mature fly may begin to lay eggs; hence, with such a life-cycle, in a
month of very favourable weather the progeny of a single pair may
number, say, 500; in two months' time the number may become 250 times
500; and in three months' time many millions!