260 ARID AGRICULTURE. 



that almost entirely destroy certain scale insects 

 or the eggs of potato beetles, and other detri- 

 mental forms. Many insects serve a useful pur- 

 pose in their relation to plants. They carry pol- 

 len, securing cross-fertilization and are indispen- 

 sable to the perpetuation of some plants. Of the 

 insects that lap their food there are none which 

 are directly injurious to crops. It has been 

 shown that such insects may carry disease from 

 one plant, to another or from many sources of in- 

 fection to animals and man. The house fly 

 seems to serve no useful purpose and does untold 

 harm in spreading infectious diseases. The war 

 on house flies should never cease. The butter- 

 flies and moths are not directly injurious, but 

 their larva, the caterpillars, have biting mouths 

 and are among the most destructive forms. 



INSECTS The mosfc destructive insects that suck their 



THAT SUCK food are plant lice, scale insects, squash bugs, 

 mosquitoes and biting flies. We have a great 

 variety of plant lice often called the "green fly" 

 or "brown fly" or the "green bug." We cannot 

 feed these bugs anything that will kill them, be- 

 cause they stick their bills into their food and 

 suck the juices. We must hit them from the 

 outside. It may be done by killing the bug or 

 its eggs in the winter stage, as by cleaning up all 

 refuse like old cabbage stumps, etc., in the field 

 or garden, or we may kill them by contact pois- 

 ons. Insects breathe by means of small pores 



