4o8 Handbook of Nature-Study 



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 — i. 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 to- 

 gether. 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 antennae? 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 trans- 

 parent? 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 pupae? 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. 



