Lesson XCI

The House-fly

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.

Observations—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.