164 



FIRST /.FSSONS IN ZOOLOGY 



found in ponds or (ithcr quiet water may be kept alive 

 Tor some time in tlic school aquarium (see p. 332). Ex- 

 amine a live specimen in water in a watch-glass with a 

 magnilier. The body-wall is so transparent that many of 

 the internal organs can be seen. Note especially the beat- 



FiG. 120. — A Mayfly (adult). (Natural size; photograpli by the author.) 



ing of the heart, a slender tube running along the middle 

 of the back. See the dark air-tubes (tracheae) running out 

 into the thin gills, and note the rapid vibration of these 

 gills to keep in contact ^\'ith fresh ^\•ater. The }-oung May- 

 flies feed on minute organisms such as diatoms and other 

 alg.e. The)' live as n)-mphs for a year, or e\'en two or three 

 years in some sjiccics, and then crawl out of the water 

 on a stone or plant-stem, or come simpl\- to the surface 



