Handbook of Nature-Study · Lesson XCI · Topic 14: Mosquitoes, flies and beetles
91. The house-fly
Goal: Your child can describe a fly's eyes, wings, feet and tongue, explain how flies carry germs, and work Comstock's arithmetic problem about how fast flies multiply.
You'll need
- A fly resting on a window pane (or a dead fly from a windowsill and tweezers)
- A hand lens (magnifying glass)
- Paper and pencil for the arithmetic
- A nature notebook and a pencil (colored pencils if you have them)
What's in the book
Comstock's story of the house-fly as a thief at every table on earth and a carrier of disease: its breeding in manure, its eyes, tongue and feet, and how it passes the winter. Then Lesson XCI's leading thought, method and twelve questions, including an arithmetic problem and an experiment with a plate of gelatin.
The lesson, step by step
When and where
Summer and early fall, at a window, porch or barn. A fly sitting on the inside of a window pane can be studied with the lens from very close.
Safety first
Don't touch live flies. If you use a dead fly, pick it up with tweezers. Wash hands with soap and water afterward.
Comstock's question 12 has children let a fly walk on a plate of gelatin and then watch germs grow. Growing germs at home isn't safe, so skip it, or look for pictures of that experiment online.
Keep the big idea in mind
Comstock's "leading thought" is the one idea the lesson is meant to lead to. Read it to yourself now, and save it to share at the end, after your child has looked and answered:
The house-fly has conquered the world and is found everywhere. It breeds in filth and especially in horse manure.
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.
In plain words: flies walk on dirt and droppings, then on our food.
Eyes
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?
After seeing a fly's eyes, do you wonder that you have so much difficulty in hitting it or catching it?
Wings and balancers
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.
Fly toilet
Watch a live fly on the window for a while.
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?
What must you do to prevent flies from bringing disease to your family?
Lesson in arithmetic
Read Comstock's problem aloud:
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?
Work it together, one generation at a time: 120 flies; half of them, 60 mothers, lay 120 eggs each; and so on. A calculator is fine for the big steps.
Nature notebook
Draw a fly from above, large, with its big eyes, two wings and the two little balancers. Write the answer to the arithmetic problem underneath, and read the leading thought aloud.
Answers to the book's problems
Generation 1: 120 flies. Generation 2: 60 mothers x 120 = 7,200. Generation 3: 3,600 x 120 = 432,000. Generation 4: 216,000 x 120 = 25,920,000. Generation 5: 12,960,000 x 120 = 1,555,200,000. All five generations together: 1,581,559,320 flies (about a billion and a half). If your child counts only the fifth generation, 1,555,200,000 is right too.
Worked out for this site from the scan. If a number in your copy differs, trust the book.