Our Balanced World 541 



butterflies, moths, and certain flies are the chief pollen-carriers 

 of the world. 



In California the growers of figs imported a certain insect in 

 large numbers and placed it among the fig trees in order to insure 

 pollination, or the transfer of pollen from flower to flower. Before 

 its introduction the fig industry had not been profitable. 



Some insects are 'harmful to man because they are greedy 

 eaters, destroying plant life by feeding on leaves and stems; 

 others because they are instrumental in the spread of many dan- 

 gerous diseases. 



How Insects Carry Disease. Insects spread disease in three 

 ways : ( 1 ) they may bring the germs on the surface of their bodies 

 to a wound, to food, or to drink ; (2) they may bear the germs of 

 infection in their digestive tracts ; (3) they may harbor the germs 

 during development and transmit them to human beings by biting 

 them. 



Disease germs are carried not only by mosquitoes and flies, 

 but by roaches, fleas, lice, and bedbugs as well. Experiments have 

 been made in which these insects were caused to walk over cul- 

 tures of various disease germs, and many hours later were made 

 to walk over sterile agar plates, having been kept in the meantime 

 under conditions as natural as possible. Examination of the agar 

 plates afterward showed active cultures of the different kinds of 

 infectious germs. Insects carry germs on their feet, on their 

 wings and on their bodies. It seems that, when this fact is well 

 understood, everyone will make every effort to destroy all disease- 

 carrying insects. Cleanliness and the use of insecticides are the 

 best protection against them. 



Mosquitoes. The relation of mosquitoes to man is especially 

 harmful. They are not merely an annoyance they are dangerous 

 pests. They harbor a parasite, which causes a disease called 

 malaria. In order to understand how these malaria germs get a 

 foothold in the human body, it will be necessary to consider their 

 life history. Their natural home seems to be in the body of a certain 

 kind of mosquito known to scientists as the anopheles. They de- 

 velop in the mosquito's digestive system and make their way into 

 its salivary glands. If the mosquito bites a person after the parasites 



