— 26— 



uniold, and lying on the surface of the water hold the larva in place. 

 The air enters through this tube and extends through the system of 

 tubes to all parts of the body. 



The larva, after growing considerably, is transformed into a stage 

 quite different in appearance. The head and thorax are very large. 

 Now, instead of one breathing tube at the posterior end of the body, it 

 has two extending from its thorax. This stage is called the pupa. 

 After a few days, the skin of the pupa bursts, and the perfect mosquito 

 comes out. For a while it may float about on the discarded skin of the 

 pupa. It then flies away. 



The mosquito is an interesting object for those who have microscopes. 

 The piercing apparatus, the antennae, the legs, and wings are all easily 

 observed with low powers. 



The children may easily make out the transformations, see the larvae 

 feed, and observe their habits. They may be kept in a glass of water 

 in which is placed a dead leaf or two, oak or maple. For references see: 



Comstock — A manual for the Study of Insects, p. 437. 



Burnet M — School Zoology, p. 95. 



Packard — Zoology. 357 . 



Comstock — Insect Life. p. 131. 



Pressure of Air, Liquids, Etc. 



The previous lessons on air and water will have brought out questions 

 requiring some knowledge of the pressure they exert, and the conse- 

 quent phenomena. It is perhaps better to begin with water. Bodies 

 floating in water may be taken as a starting point. Why do they float? 

 Why do some float more above the water than others, while others sink? 

 What is the effect of the same floating bodies in liquids of different 

 densities, e. g., lighter or heavier liquids. 



These questions may be made the guide to a series of experimental 

 lessons with grades above the fourth. 



The last of the above questions is a good one to begin with. For the 

 lesson there will be necessary: A small wooden rod one-half inch or 

 less in diameter and about five inches in length; some small nails and a 

 piece of copper wire for weighting one end of the rod; two fruit jars, one 

 filled with water, the other with a strong solution of salt; some small 

 blocks of wood; and some corks. The lesson may begin with the ques- 

 tion of how the things will float in two liquids, water and salt solution. 

 If the pupils can be led to invent an apparatus to test the diff"erence, so 

 much the better. 



This may be done by first trying the blocks of wood or the corks. It 

 will soon appear that they, from their awkward shapes and instability 

 in the water, can not give good results. Then some one will be sure to 

 suggest a method good enough. Most likely the rod of wood loaded at 

 one end will be invented by the class. If not it may now be brought out 



