THE STABLE-FLY (_Stomoxys_)
Fly! Thy brisk unmeaning buzz
Would have roused the man of Uz;
And, besides thy buzzing, I
Fancy thou’rt a stinging-fly.
Fly—who’rt peering, I am certain,
At me now from yonder curtain:
Busy, curious, thirsty fly
(As thou’rt clept, I well know why)—
Cease, if only for a single
Hour, to make my being tingle!
Flee to some loved haunt of thine;
To the valleys where the kine,
Udder-deep in grasses cool,
Or the rushy margined pool,
Strive to lash thy murmurous kin
(Vainly) from their dappled skin!
(CALVERLEY; _The Poet and the Fly_.)
The common names for common insects in English are confusing. Not
only are the same insects frequently known by different names on
different sides of the Atlantic, but in many cases quite different
insects—insects even belonging to different genera—are connoted
by the same common name. In this respect matters are different in
Germany: partly, perhaps, because the Germans on the whole are more
scientifically inclined than we are, but partly, I suspect, because the
German language lends itself more easily to express in one word—however
long—the characteristics of any given insect.
[Illustration: FIG. 37.—The Stable-fly (_Stomoxys calcitrans_).]
[Illustration: FIG. 38.—_Stomoxys calcitrans_ × 5. Left antenna right
× 1, resting position. (From Graham Smith.)]
The genus _Stomoxys_ is generally called in Great Britain the
‘stable-fly,’ but there are other ‘stable-flies.’ One of the commonest
species of the genus is _S. calcitrans_, a two-winged muscid fly, not
at all unlike the common domestic fly, _Musca domestica_; but there
are one or two points which readily distinguish it from the commoner
insect. To begin with: it has a hard, firm, chitinous, piercing
proboscis, which when at rest stretches forward in front of the head,
and when in action is pressed down at right angles to the longitudinal
axis of the body; then, again, when resting, its wings diverge; those
of the house-fly approximate. Like other flies, the _Stomoxys_ varies
somewhat in length, between 5·5-7 mm. The thorax has on its back four
longitudinal, dark stripes, broken by a transverse suture; and, as
the accompanying figure shows, the third of the great, long veins
which traverse the wing is much more slightly bent than is the case
in _Musca domestica_. Further, whereas the hinder edge of the eye in
the house-fly is straight that of the stable-fly is concave, and the
antennae bear hairs on the upper side only and not above and below as
they do in the domestic fly.
As a biting-fly and a blood-sucking fly, the habits of _Stomoxys_
naturally differ from those of _Musca domestica_; but, like the latter,
its distribution is almost world-wide. It is found in all temperate
and tropical countries, and extends as far north as Lapland. But it is
perhaps most abundant (or shall we say it has been most observed?) in
temperate climates and during the summer months.
[Illustration: FIG. 39.—Wing of _Musca domestica_ above, and of
_Stomoxys calcitrans_ below.]
In any farm or country house large numbers of _Stomoxys calcitrans_ are
found in and about the cowsheds and stables, and in warm weather the
same is true wherever cattle are grazing in the field. Later in the
year, at the beginning of autumn, they are frequently found indoors,
and in some ‘fly counts’ they have furnished quite 50 per cent. of the
flies of a country house, the remaining 50 per cent. being made up
of many other species and genera. When resting on a vertical surface
_Stomoxys_ generally has its head pointing upwards, whereas, as a rule,
the house-fly rests upside down. The adult fly feeds upon any decaying
matter; but whenever it can, it sucks the blood of vertebrates, and
at times is a real nuisance to animals as well as human beings. So
voracious are they that should a well-fed one be injured, the others
immediately attack it and suck up every drop of blood which it had
secured for its own food.
[Illustration: FIG. 40.—Side view of head of _Stomoxys calcitrans_. A,
Proboscis in resting position; B, proboscis extended. (After Graham
Smith.)]
It has often been disputed whether a meal of blood is essential to the
female mosquito before oviposition, but it seems perfectly clear that
the female _Stomoxys_ can produce fertilised eggs without having had a
meal of blood.
The female lays a number of white, banana-shaped eggs a few inches
below the surface of any decaying organic matter; fermenting grass from
the lawn, decaying garden stuff, stable manure—each forms a favourable
nidus. The eggs are laid in a heap like those of the house-fly, each
heap containing from fifty to seventy. The egg is 1 mm. in length and
has a grooved side, through the thicker end of which the larva escapes
when the egg-shell splits.
[Illustration: FIG. 41.—_Stomoxys calcitrans._ Eggs. (After Newstead.)]
The issuing larva is almost transparent. It not only has no head,
but the anterior end dwindles almost to a point. When fully grown
it attains a length of 11 mm., and the larval stage usually lasts
from two to three weeks; but development may be retarded by adverse
circumstances up to eleven or twelve weeks, and in such cases the
full-grown larvae are often stunted in size. In these circumstances
the pupae they produce are markedly smaller than those which have
followed a more normal course of development. As is true of the egg
and of the larva, the pupa resembles the pupa of the house-fly, being
barrel-shaped and of a chestnut-brown colour; it is 5 to 5·5 mm. in
length. The pupa stage lasts from nine to thirteen days, but this
period is prolonged by cold.
[Illustration: FIG. 42.—Acephalous larva of _Stomoxys calcitrans_.
(After Newstead.)]
On emerging from the pupa-case the insect has to push its way to the
surface of the rotting vegetation in which it has been produced.
This it does partly by the alternate inflation and deflation of the
so-called ‘frontal sac,’ and by actively pushing forward the body by
means of its legs. Once on the surface the insect begins to clean
itself, pumps air into its body, forces it along the tracheae in
the wings, which expand and ultimately harden. In the processes of
unfolding they are aided by the hind legs. For a time the insect is
immobile, gradually stiffening; but when the integument has hardened it
flies off to explore the outer world. Under normal conditions the whole
life-cycle varies from twenty-seven to thirty-seven days.
The chief interest of _Stomoxys_ to the public, rests upon the fact
that it is a very potent carrier of disease. There are certain forms
of _Trypanosoma_ which, under experimental conditions, are undoubtedly
transferred by this species. But opinion is still unsettled as
to whether the transference of these protozoa occurs in nature.
The _Surra_ diseases of horses and camels is, according to some
authorities, transferred by _Stomoxys_, and so is the _Surra_ disease
of cattle; and there are others, all fully set forth in Mr. Hindle’s
work on ‘Flies and Disease.’
[Illustration: FIG. 43.—Coarctate pupa of _Stomoxys calcitrans_.
(After Newstead.)]
Certain thread-worms—for instance, _Filaria labiato-papillosa_—which
occur in the peritoneal cavity, and sometimes in the eyes of cattle
and deer in India, are undoubtedly conveyed by _Stomoxys calcitrans_.
The superficial vessels of the cattle swarm with the larvae of these
thread-worms, which readily pass through the proboscis of the insect
into its stomach. They then wriggle through the walls of the stomach
and make their way into the thoracic muscles; here they undergo a
‘rest-cure,’ and after a time they are readily transferred to a new and
possibly uninfected host.
But by far the worst infection which is attributed to this fly is acute
epidemic poliomyelitis, or infantile paralysis. That this disease
occurs in epidemics has been known—especially in Scandinavia—for some
time; and eight years ago it attracted serious attention in North
America and in our country. In 1907 there were many local outbreaks in
the United States and Canada, and it is thought that the infection was
first introduced from Scandinavia along the Atlantic coast, and later,
inland, as far as the State of Minnesota, by the numerous Scandinavian
immigrants that settle there.
The disease is one of those which are apparently due to a protozoon too
small to be visible under the highest power of the microscope, and so
small as to be able to pass through a Berkefeld filter. It can readily
be artificially transmitted to monkeys. It is thought that the disease
is by no means transmitted only by means of the biting _Stomoxys_, and
that it may be directly transmitted from one person to another without
the aid of any intermediate host. But there seems little doubt that it
can be, and is, transmitted by _Stomoxys_, and therefore it is of the
highest importance to reduce the number of these insects.
The most efficient way of controlling this pest is to destroy or put
out of action its breeding-places. All decaying vegetable matter should
be either removed or burnt or buried, or covered with some agent which
will prevent the larvae living. In fact, the methods that have been
advocated for the common house-fly are applicable to _Stomoxys_. If
stable manure were carefully removed, from May to October, at least
every seven days, the number of flies would be materially reduced.
Where this is impracticable, manure-heaps should be covered with
some insecticide, so as to destroy the eggs and larvae. Experiments
are still being made with the view of finding a substance capable
of killing the eggs, larvae, and pupae, which will be at once cheap
and unharmful to the fertilising value of the manure. The American
experts recommend borax or colemanite (crude calcium borate), calcined,
powdered, and applied by a flour-dredger. The proportions which seem
most effective are 0·62 lb. of borax and 0·75 lb. of colemanite to
10 cubic feet, or 8 bushels of manure. Two or three gallons of water
should then be sprinkled over the manure-heap.