536 The Outline of Science 



locusts descend on a plantation of forty thousand young plants. 

 Twenty seconds later not a leaf remained! The Old Testament 

 speaks of the locust as one of the plagues of Egypt. "They 

 covered the face of the whole earth, so that the land was 

 darkened; and they did eat every herb of the land, and all the 

 fruit of the trees which the hail had left: and there remained 

 not any green thing in the trees, or in the herbs of the field, 

 through all the land of Egypt." 



In addition to the formidable list of insects, larvae and adults, 

 injurious to plants, another list must be added of those which 

 affect the health of man and of his stock. There are a number 

 of ways in which insects may affect the health of man. They may 

 have poisonous bites or stings, as in the case of certain Bugs, 

 Bees, Wasps, etc., which cause inflammation and sometimes 

 feverishness ; or they may be parasitic, either true parasites such 

 as fleas and lice, or accidental parasites, such as fly-maggots, 

 which sometimes reach the stomach and cause great pain ; again, 

 they may carry disease germs. Most important of all are the 

 cases in which an insect is an essential host in the development of 

 a disease-producing organism, without which the life-history of 

 the organism cannot be completed. 



For example, the Mosquito is not only the means of in- 

 troducing into the blood of man the ProtozoSn which causes 

 malaria, but the life-history of the malaria organism cannot 

 proceed without the insect; the different stages can only be 

 reached within the bodies of man and mosquito respectively, so 

 that the extermination of mosquitoes would wipe out malarial 

 fever. In other cases, the insect is not necessary to the life of the 

 disease-producer, but acts as a transmitter, as in the case of 

 plague, where the bacillus is conveyed from rats to man by means 

 of ratfleas, which inoculate the victims while biting. Further 

 cases of disease-carrying form another list those of the simple 

 carriers, such as the common House-Fly : it is not a blood-sucking 

 insect, but it has a body and legs thickly covered with hairs 



