212 FIELD ZOOLOGY. 



poke about for water beetles or aquatic bugs. They are 

 thin, translucent cast-offs of various sizes, according to the 

 age of the nymph which crawled out of them. The wall 

 material is chitin, and they will remain in the water a 

 considerable time before they will suffer distintegration ;■ 

 chitin serves its purpose well as a body covering, and' 

 wears long before it suffers injury from any cause. Though 

 it usually suffers decomposition in water after the lapse 

 of a year, it is much less affected by acids than it is by 

 water. 



About three hundred species of dragon flies are known 

 in the United States and adjoining countries ; two thousand 

 in the world. The great majority prefer warm localities, 

 although Kellogg records some as having been found as 

 high as ten thousand feet altitude, and as far north as 

 63° in Siberia, 65° in Alaska, and 76° in Norway. 



