THE STRUCTURE OF THE HOUSE-FLY
The house-fly has quite the typical insect form, inasmuch as there are
three well defined sections of body—the head, the chest or thorax, and
the abdomen; also it has three pairs of legs, each with nine joints, of
which five joints constitute what may be called the foot. The twelve
segments of the maggot are observable as twelve rings in the puparium,
but in the fly the three which form the thorax look like one, whilst the
eight which should theoretically exist in the abdomen look like four or
five, until the rings of the ovipositor are counted.
The illustration on page 39 will make plain how the permanence of the
twelve-segment structure (conspicuous in the larval stage) has been
thought to persist throughout the life-cycle, but at the same time will
disclose how great is the change in the relative proportions of these
segments.
The prominent features of the hemispherical head are the two large
compound eyes and the proboscis or trunk-like mouth. The antennæ or
horns are very short appendages with three joints; small plume-like
projections, called arista, are attached to the third segment; the horns
hang down over a hollow in the middle of the face, and are insignificant
in size when compared with those of other kinds of insects; but their
structure viewed under the microscope is intricate, and they may be
efficient organs of sense perception, probably in part auditory. The
really unique feature is the retractile and suctorial proboscis, which
is often incorrectly regarded as the tongue; it is normally held doubled
up and withdrawn towards a hollow under the head, whence it is from time
to time extruded. The structure of this member is characteristic of the
entire tribe to which the house-fly belongs; it is a fusion or
combination of mouth parts, which in other insects are used more or less
separately for the various functions of inspecting, biting, masticating,
drinking and swallowing. In the house-fly the proboscis is absolutely
suctorial, and is not provided with the lancets used by the
blood-sucking flies for piercing the skin. Two maxillary palpi are
attached to the upper or basal part of the proboscis, which is called
the rostrum (a snout); the lower part is called the haustellum (a pump),
and it has at the end a pair of soft cushion-like lobes or lips, which,
when spread apart, form a heart-shaped pad with an opening in the
centre. The maxillary palpi are used for feeling and probably smelling.
Each mouth-lobe has a main collecting central channel and thirty
subsidiary cross channels of a wonderfully complex character. Imbibed
fluids pass from the mouth-lobes to the gullet along a passage in the
haustellum and the rostrum.
As with many other flies and other insects, there are on the top of the
head very small simple and rather inconspicuous eyes called ocelli,
three in number, between the large and prominent compound eyes, which
latter are said to possess each four thousand facets. The compound eyes
of the male fly are proportionately larger than those of the female; it
is quite observable that they approach each other more closely at the
top of the head, a feature of sex differentiation which is shared with
bees, wasps, and many other insect creatures. It is thought that a
single brain image arises from the combined views of the four thousand
facets of the compound eyes blending with the view conveyed through the
"ocelli." However, it is a most curious fact that it is the
inconspicuous ocelli which are of supreme importance visually. The
compound eyes have doubtless some special function, but throughout the
insect world the size of compound eyes is not a certain indication of
keenness of sight. The vision of the fly is good for distinguishing the
movement of any broad mass, but it is rather ineffective for observing a
thin line, as may be proved by slowly lowering a knife blade, with a
steady hand, when its body may be severed before the fly takes alarm. It
is a remarkable fact that the family of _Tabanida_ (blood-sucking breeze
flies), which are destitute of "ocelli," are the dullest sighted of all
flies; in fact, at least semi-blind. Moses Harris observed that a
blue-bottle became practically blindfolded when its ocelli were covered
with an opaque pigment. Probably this is the case with other insects.
Bees, which require long distance sight for home returning, are well
provided with ocelli. Butterflies, however, without the use of ocelli
have a distinct faculty of daylight vision for a moderate distance. The
investigation of sight by blindfolding is very difficult in flies.
There are two cephalic ganglions, which are regarded as the brain; these
are situated in the upper part of the head close to the neck. There are
also ganglions in the thorax with connections extending into the
abdomen.
The thorax is mainly occupied with the powerful muscles which actuate
the attached wings, the legs, and the small appendages called halteres
or balancers, which are supposed to be obsolete hind wings. There are
three unequal segments in the thorax; the pair of front legs belong to
the first segment, the wings and the pair of middle legs are attached to
the second larger segment, whilst the third is connected with the hind
legs and the halteres.
The breathing apparatus of the fly is distributed in portions over the
head, thorax, and abdomen; it consists of a number of internal air-sacks
with membranous ducts ramifying everywhere; the largest air-sacks are in
the abdomen near the waist. There is a pair of external spiracles to
each segment of the body, and these lead to the air-sacks.
The lines on the wings of the house-fly called nervures have already
been alluded to in Chapter II. These nervures are strengthening ribs to
the transparent tissues of the wings. The tissues are double (top and
bottom) enclosing the nervures, which are so united to the connections
called trachæ of the air-sacks, that the newly emerged fly helps to
extend its limp and crumpled wings by a process of inflating the
nervures.
The stomach is located partly in the thorax and partly in the abdomen. A
passage from the gullet passes through the neck into the lower part of
the thorax, where are the entrances to two long capacious chambers, of
which the upper one is the true stomach and the lower one a store pouch,
which latter may be likened to the honey bag of the bee. The fly
habitually regurgitates liquid food stored in this pouch, and, somewhat
after the manner of the cow chewing the cud, passes the same back into
the true stomach, whence it proceeds onwards through the digestive
track.
The abdomen holds all the other ordinary internal organs including that
which may be called the heart, and which lies above the stomach; it
consists of a long muscular tubular vessel with four contractile
chambers.
Although the organ called the brain is located in the head, and although
that called the heart is in the abdomen, yet some sense of control over
bodily motions curiously exists separately in the ganglions of different
parts of the body. This fact seems to make it possible for one extremity
of the body to continue performing a pleasurable action (say, the head
drinking honey) after the other extremity has endured a painful
catastrophe (say, amputation of the abdomen). However, it may be fairly
surmised that no creatures of a lower grade than warm-blooded vertebrate
animals feel pleasure and pain in any way at least after the manner of
mankind.
The most vital part of the fly is not the head but the thorax. A severe
squeeze on the thorax will effectually paralyse and kill the creature.
Muscular movements of different parts of the fly's body, which continue
after severance or other fatal injury, cannot be regarded as visible
proof of a slow death and prolonged sensibility.
Possessed of six legs, each with nine joints, the fly exercises a unique
capability of walking; the legs are moved three at a time, a front and a
hind leg on one side advancing simultaneously with the middle leg on the
other side; thus the fly proceeds most securely always poised on three
feet, which are so well furnished with pads, claw-like hooks, and hairs,
that it can walk over polished glass and can even walk upside-down along
comparatively smooth surfaces.
In comparison with the more heavily constructed wasp, with its four
wings, the house-fly, with its two wings, is the more alert and active
flier. The wasp is more robust than the fly and will be active in
weather too inclement for the latter; however, some of the frail and
slender gnats will brave cold temperatures impossible for the wasp.