566 



BIOLOGY AND HUMAN LIFE 



stage to wheat straw used as packing for merchandise or as 

 bedding for horses and cattle. It has caused great damage to 

 wheat, and it sometimes attacks barley and rye. 



The San Jose (san ho sa') scale attacks the leaves, twigs, and 

 fruit of many cultivated species of fruit trees. It was intro- 

 duced from China on some nursery stock and was first noticed 



at San Jose, Cali- 

 fornia. In twenty 

 years it had spread 

 to all parts of the 

 United States and 

 also into Canada. 



Many hundreds 

 of other insects at- 

 tack 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. 



407. Insects and 

 useful animals. It 

 has been said that 

 every plant and 

 every animal has 



Fig. 235. The potato beetle (Leptinotarsa 

 decemlmeata) . (Slightly reduced) 



There are two or three broods a year. The full-grown 

 larva crawls into the ground, where pupation takes place. 

 The winter is passed underground in the adult stage. The 

 tachina fly, shown in the center, is one of the most im- 

 portant enemies of the potato beetle. The fly lays eggs in 

 the larva of the beetle and the maggots destroy their host 



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 that are most familiar to us are 

 annoyed by various flies, lice, gnats, and fleas, which sting and 

 suck blood. Numerous parasitic diseases of animals other than 

 man are transmitted by insects either directly or indirectly, the 

 insects acting as intermediate hosts. In addition, some insects 

 attack larger animals more viciously. 



