420 



ELEMENTARY BIOLOGY 



first noticed at San Jose, California. In twenty years it had 

 spread to all parts of the United States and also into Canada. 



. Aside from the 



insects mentioned 

 there are hundreds 

 of others that at- 

 tack all our garden 

 and field crops and 

 orchard and forest 

 trees. It is hardly 

 possible to find a 

 plant that has not 

 one or more serious 

 insect enemies. 

 449. Insects and 

 useful animals. It 

 has been said that 

 every plant and 

 every animal has its 

 parasites and its 

 preying enemies. 

 And it is probably 

 safe to add that 

 every organism of 

 any size has its 

 enemies among the 

 insects. 



The mammals 

 and birds which are 

 most familiar to us are annoyed by various flies, lice, gnats, 

 and fleas, which sting and suck blood. There are a number 

 of parasitic diseases of animals other than man that are trans- 

 mitted by insects either directly or indirectly, the insects acting 

 as intermediate hosts. In addition to these sources of injury 

 there are some insects that attack larger animals more viciously. 



FIG. 221. Gypsy and brown-tail moths (1917) 



The region between the dotted line and the ocean is in- 

 fested with both species. The brown-tail has also invaded 

 the region shown by the shaded surface 



