192 METHODS IN TEACHING 



be provided for them to climb upon when they wish to leave 

 the water. The insects mentioned, if kept in reasonable 

 numbers, will find food for themselves. Such insects are 

 found in ponds and watering troughs. The eggs of the 

 mosquito will be found in little rafts, which look like flakes 

 of soot floating on the surface of the water. Placed in the 

 aquarium these eggs hatch in a few days. Pupils should 

 see the insects emerge from the eggs ; they should continue 

 their observations until the final change into the perfect 

 insect takes place. This will occur in from ten to fourteen 

 days after hatching. For close examination place a Half 

 dozen of the wrigglers in a glass of water and use a hand 

 lens of moderate power. Soon after hatching mount a 

 very small one in a drop of water. Place cover-slip pn it 

 very lightly and examine under the low power of a com- 

 pound microscope, as a transparent object. Make out the 

 digestive canal, mouthparts, eyes, and the air tubes (tra- 

 chea) where they connect with the breathing pore at the 

 caudal end of the insect. Aside from their nature study 

 value, mosquito larvae furnish food to other animals which 

 may be kept in the same vessel. 



The dragon flies are also very interesting creatures for 

 the aquarium. In autumn, along the reedy margins of 

 permanent ponds, several species of dragon 

 fly may be found depositing their eggs. 



Certain of the larger species pierce the 

 reed stems just below the surface of the water with the 

 ovipositor, and deposit the eggs in the slit-like openings 

 thus made. Watch the female at work, and when she has 

 finished cut off the piece of reed containing the eggs and 

 transfer it to a small clear-sided jar of water. When the 



