HE house-fly is surely an up-to-date member of that select class which evolutionists call the "fit." It flourishes in every land, plumping itself down in front of us at table, whether we be eating rice in Hong Kong, dhura in Egypt, macaroni in Italy, pie in America, or tamales in Mexico. There it sits, impertinent and imperturbable, taking its toll, letting down its long elephant-trunk tongue, rasping and sucking up such of our meal as fits its needs. As long as we simply knew it as a thief we, during untold ages, merely slapped it and shooed it, which effort on our part apparently gave it exhilarating exercise. But during recent years we have begun trapping and poisoning, trying to match our brains against its agility; although we slay it by thousands, we seem only to make more room for its well-fed progeny of the future, and in the end we seem to have gained nothing. But the most recent discoveries of science have revealed to us, that what the house-fly takes of our food, is of little consequence to what it leaves behind. Because of this, we have girded up our loins and gone into battle in earnest.
I have always held that nature-study should follow its own peaceful path and not be the slave of economic science. But occasionally it seems necessary, when it is a question of creating public sentiment, and of cultivating public intelligence in combating a great peril, to make nature-study a handmaiden, if not a slave, in this work. If our woods were filled with wolves and bears, as they were in the days of my grandfather, I should give nature-study lessons on these animals, which would lead to their subjugation. Bears and wolves trouble us no more; but now we have enemies far more subtle, in the ever-present microbes, which we may never hope to conquer but which, with proper precautions, we may render comparatively harmless. Thus, our nature-study with insects which carry disease, like the mosquitoes, flies and fleas, must be a reconnaissance for a war of extermination; the fighting tactics may be given in lessons on health and hygiene.
Perhaps if a fly were less wonderfully made, it would be a less convenient vehicle for microbes. Its eyes are two great, brown spheres on either side of the head, and are composed of thousands of tiny six-sided eyes that give information of what is coming in any direction; in addition, it has on top of the head, looking straight up, three tiny, shining, simple eyes, which cannot be seen without a lens. Its antennæ are peculiar in shape, but are evidently sense organs; it is attracted from afar by certain odors, and so far as we can discover, its antennæ are all the nose it has. Its mouth-parts are all combined to make a most amazing and efficient organ for getting food; at the tip are two flaps, which can rasp a substance so as to set free the juices, and above this is a tube, through which the juices may be drawn to the stomach. This tube is extensible, being conveniently jointed so that it can be folded under the "chin" when not in use. This is usually called the fly's tongue, but it is really all the mouth parts combined, as if a boy had his lips, teeth and tongue, standing out from his face, at the end of a tube a foot long.
The thorax can be easily studied; it is striped black and white above and bears the two wings, and the two little flaps that are called balancers and which are probably remnants of hind wings which the remote ancestors of flies flew with. The fly's wing is a transparent but strong membrane strengthened by veins, and is prettily iridescent. The thorax bears on its lower side the three pairs of legs. The abdomen consists of five segments and is covered with stiff hairs. The parts of the leg, seen when the fly is walking, consists of three segments, the last segment or tarsus being more slender, and if looked at with a lens, is seen to be composed of five segments, the last of which bears the claws; it is with these claws that the fly walks, although all of the five segments really form the foot; in other words, it walks on its tip-toes. But it clings to ceilings by means of the two little pads below the claws, which are covered with hairs that excrete at the tips, a sticky fluid. Because of the hairs on its feet, the fly becomes a carrier of microbes and a menace to health.
The greatest grudge I have against this little, persistent companion of our household is the way it has misled us by appearing to be so fastidious in its personal habits. We have all of us seen, with curiosity and admiration, its complex ablutions and brushings. It usually begins, logically, with its front feet, the hands; these it cleans by rubbing them against each other lengthwise. The hairs and spines on one leg act as a brush for the other, and then lest they be not clean, it nibbles them with its rasping disc, which is all the teeth it has. It then cleans its head with these clean hands, rubbing them over its big eyes with a vigor that makes us wink simply to contemplate; then bobbing its head down so as to reach what is literally its back hair, it brushes valiantly. After this is done, it reaches forward first one and then the other foot of the middle pair of legs, and taking each in turn between the front feet, brushes it vigorously, and maybe nibbles it. But as a pair of military brushes, its hind feet are conspicuously efficient; they clean each other by being rubbed together and then they work simultaneously on each side in cleaning the wings, first the under side and then the upper side. Then over they come and comb the top of the thorax; then they brush the sides, top and under sides of the abdomen, cleaning each other between the acts. Who, after witnessing all this, could believe that the fly could leave any tracks on our food, which would lead to our undoing! But the house-fly, like many housekeepers with the best intentions in the matter of keeping clean, has not mastered the art of getting rid of the microbes. Although it has so many little eyes, none of them can magnify a germ so as to make it visible; and thus it is that, when feeding around where there have been cases of typhoid and other enteric diseases, the house-fly's little claws become infested with disease germs; and when it stops some day to clean up on our table, it leaves the germs with us. Thus our only safety lies in the final extermination of this little nuisance.
It is astonishing how few people know about the growth of flies. People of the highest intelligence in other matters, think that a small fly can grow into a large one. A fly, when it comes from the pupa stage, is as large as it will ever be, the young stages of flies being maggots. The house-fly's eggs are little, white, elongated bodies about as large as the point of a pin. These are laid preferably in horse manure. After a few hours, they hatch into slender, pointed, white maggots which feed upon the excrement. After five or six days, the larval skin thickens, turns brown, making the insect look like a small grain of wheat. This is the pupal stage, which lasts about five days, and then the skin bursts open and the full-grown fly appears. Of course, not all the flies multiply according to the example given to the children. The house-fly has many enemies and, therefore, probably no one hibernating mother fly is the ancestress of billions by September; however, despite enemies, flies multiply with great rapidity.
I know of no more convincing experiment as an example of the dangerous trail of the fly, than that of letting a house-fly walk over a saucer of nutrient gelatin. After three or four days, each track is plainly visible as a little white growth of bacteria.
Much is being done now to eradicate the house-fly, and undoubtedly there will be new methods of fighting it devised every year. The teacher should keep in touch with the bulletins on this subject published by the United States Department of Agriculture, and should give the pupils instructions according to the latest ideas. At present the following are the methods of fighting this pest: Keep the stable clean and place the manure under cover. All of the windows of the house should be well screened. All the flies which get into the house should be killed by using the commercial fly papers.
Leading thought—The house-fly has conquered the world and is found everywhere. It breeds in filth and especially in horse manure. It is very prolific; the few flies that manage to pass the winter in this northern climate, are ancestors of the millions which attack us and our food later in the season. These are a menace to health because they carry germs of disease from sputa and excrementitious matter to our tables, leaving them upon our food.
Method—Give out the questions for observation and let the pupils answer them either orally or in their note-books. If possible, every pupil should look at a house-fly through a three-quarters objective. If this is not possible, pictures should be shown to demonstrate its appearance.
1. Look at a fly, using a lens if you have one. Describe its eyes. Do you see that they have a honeycomb arrangement of little eyes? Can you see, on top of the head between the big eyes, a dot? A microscope reveals this dot to be made of three tiny eyes, huddled together. After seeing a fly's eyes, do you wonder that you have so much difficulty in hitting it or catching it?
2. Can you see the fly's antennæ? Do you think that it has a keen sense of smell? Why?
3. How many wings has the fly? How does it differ from the bee in this respect? Can you see two little white objects, one just behind the base of each wing? These are called poisers, or balancers, and all flies have them in some form. What is the color of the wings? Are they transparent? Can you see the veins in them? On what part of the body do the wings grow?
4. Look at the fly from below. How many legs has it? From what part of the body do the legs come? What is that part of the insect's body called, to which the legs and wings are attached?
5. How does the fly's abdomen look? What is its color and its covering?
6. Look at the fly's legs. How many segments can you see in a leg? Can you see that the segment on which the fly walks has several joints? Does it walk on all of these segments or on the one at the tip?
7. When the fly eats, can you see its tongue? Can you feel its tongue when it rasps your hand? Where does it keep its tongue usually?
8. Describe how a fly makes its toilet as follows: How does it clean its front feet? Its head? Its middle feet? Its hind feet? Its wings?
9. Do you know how flies carry disease? Did you ever see them making their toilet on your food at the table? Do you know what diseases are carried by flies? What must you do to prevent flies from bringing disease to your family?
10. Do you think that a small fly ever grows to be a large fly? How do the young of all kinds of flies look? Do you know where the house-fly lays its eggs? On what do the maggots feed? How long before they change to pupæ? How long does it take them to grow from eggs to flies? How do the house-flies in our northern climate pass the winter?
11. Lesson in Arithmetic—It requires perhaps twenty days to span the time from the eggs of one generation of the house-fly to the eggs of the next, and thus there might easily be five generations in one summer. Supposing the fly which wintered behind the window curtain in your home last winter, flew out to the stables about May 1st and laid 120 eggs in the sweepings from the horse stable, all of which hatched and matured. Supposing one-half of these were mother flies and each of them, in turn, laid 120 eggs, and so on for five generations, all eggs laid developing into flies, and one-half of the flies of each generation being mother flies. How many flies would the fly that wintered behind your curtain have produced by September?
12. Pour some gelatin unsweetened, on a clean plate. Let a house-fly walk around on the gelatin as soon as it is cool; cover the plate to keep out the dust and leave it for two or three days. Examine it then and see if you can tell where the fly walked. What did it leave in its tracks?
13. Write an essay on the house-fly, its dangers and how to combat it, basing the essay on Bulletins of the U. S. Department of Agriculture.