The body of the House Fly is divided into head, thorax and abdomen, which are grey in colour marked with streaks of black, and are covered with short bristles. On top of the head are three small simple eyes, arranged in a triangle, and on each side is a large compound eye. The feelers are short and three-jointed, with a hairy spine on the last joint. The hairiness of the thorax masks its division into segments, of which there are three, each bearing a pair of jointed bristly legs which terminate in two small claws. Between the claws are two small pads which secrete a sticky substance, and which help the fly to cling to the surface on which it is walking. It is owing to these pads that the fly can walk upside down on the ceiling. Instead of the two pairs of wings found in a typical insect, the fly has only one pair, fixed to the 2nd thoracic segment. On this account the fly is placed in a large order of insects, called the Diptera or two-winged flies. Each wing is transparent, with a characteristic network of veins, and when the fly is at rest the wings are laid horizontally along the abdomen. The hind pair of wings are represented by a pair of drumstick-shaped projections attached to the 3rd thoracic segment. These appendages are known as balancers, since they are thought to help the fly to maintain its balance. The abdomen of the fly is short and only four segments are visible.

The Mouth-parts and Feeding Habits

The fly feeds on liquid food, which it sucks up by means of a tube or proboscis on the under side of the head. The proboscis represents the much-modified second maxillae of a typical insect, and the mandibles and first maxilla? are absent. At the far end of the proboscis are two flaps or oral lobes, between which lies the mouth. The oral lobes can be folded together or spread out, and have a number of fine, parallel grooves on the under side running from the edge towards the mouth. These grooves run into two main channels situated on the inner edges of the oral lobes, and which open into the mouth. The grooves are strengthened by rings of chitin, and since they look like the breathing tubes or trachea? of insects, they are called pseudo-trachea?. They act as gutters which convey liquid food to the mouth.

The fly is able to feed on solid food, such as sugar, by dissolving it with a drop of saliva. The saliva is produced by the salivary glands, which open by a duct into the mouth. As the solid food is dissolved, the liquid runs back along the pseudo-trachea? to the mouth and is pumped up by the action of the pharynx. When not in use the proboscis is folded beneath the head with the oral lobes closed upon each other.

The Life History of the House Fly

House flies will breed in almost any decomposing matter or filth which is sufficiently moist and warm. Eggs are laid from June to September, and a female fly can lay five or six batches of 100 to 150 eggs each. The white, spindle-shaped eggs hatch in twelve to twenty-four hours into small white larva? or maggots. The fully grown maggot is about £ in. long, and consists of twelve segments, which are broad at the hind end and taper towards the head. The mouth opens between two oral lobes on the head, and projecting from the head is a black hook-shaped spine. This is used by the larva for tearing up food, and it also helps the larva to drag itself along. On the lower side of each segment from the 6th to the 12th there is a small pad covered with short spines. These pads take the place of legs. The larva breathes through two pairs of spiracles, the first pair of which are found on the 2nd segment. The second pair occur in the middle of the posterior surface of the last segment, so that while the rest of the body is buried in the food material, the larva can breathe through these spiracles. The larva avoids the light, and if its food becomes dry it seeks moister regions.

The larva moults its skin twice, and if the temperature is high may become fully grown in five days. Then it migrates to a dry spot preparatory to changing into the pupal state. The larva shortens and the skin becomes a barrel-like case, which in a few hours changes in colour from white to dark brown. Extensive tissue breakdown and reorganization takes place in the pupa, and the fully grown fly emerges about the fourth day after pupation. The fly pushes off the front end of the pupal case in two parts by means of a sac which is protruded from the front of the head. After the emergence of the fly the sac is withdrawn into the head and appears only as a curved slit above the base of the feelers.

During warm weather and with suitable moist food, the whole development of the fly, from egg to imago, may take place in nine or ten days. In about fourteen days from the time of emergence of a fly the new generation produced by the insect may be laying eggs. Most flies die at the end of the summer, but the remainder hibernate through the winter in well-protected cracks and crevices.

Flies and Disease

Since they feed on manure and other decaying material which may harbour disease bacteria, flies are often the cause of the spreading of disease. The body of the fly is thickly covered with hairs, and so is admirably suited for carrying micro-organisms. Its legs are like small brushes which pick up bacteria from contaminated decaying matter and spread them over any surface on which the fly walks. In this way food may be infected with disease bacteria. When the fly feeds on infected matter the bacteria are taken into the gut, and thence may be deposited on the food with the faxes. Frequently the fly regurgitates its food at the mouth in the form of a drop of liquid, which may be sucked up again by the mouth or left behind. A drop so deposited may contain harmful bacteria, so that the fly may also infect food in this way.

Practical Work on the House Fly

Examine a house fly with a hand lens. Also examine the eggs, larva and pupa of the fly.

Remove the proboscis from a fly and mount the oral lobes on a slide : examine the lobes under the microscope and make out the pseudo-trachea;.

Remove a leg from a fly and examine under the low power of the microscope : note the numerous bristles which cover the leg, and also make out the claws and sticky pad at the end of the leg.

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