Ill 



INSECTS AND HUMAN DISEASE 



ONLY insects that are closely associated with man, and 

 normally have every opportunity to suck his blood 

 repeatedly, can be considered as potential disease-carriers. 

 The fact that a species occasionally bites man is not suffi- 

 cient. Take the case of forest mosquitoes ; they may bite 

 an infected person, but the chances are remote that they 

 will survive to develop the blood parasites and then have 

 an opportunity of biting and infecting another person. 

 By the same line of argument, most mosquitoes, also the 

 gadflies, or Tabanidce, and the buffalo gnats, or Simulidce, 

 and many other blood-sucking insects may, probably, be 

 dismissed from consideration as disease-carriers. Most of 

 these insects, too, appear only for a time each year, so 

 they would have to survive long intervals, when there 

 could be no propagation of disease. 



All insects that have been found to transmit human 

 blood parasites are closely associated with man, and 

 habitually suck his blood. Two tropical mosquitoes, 

 Stegomyia fasciata and Culex quinquefasciatus, have only 

 become effective transmitters of yellow fever and filariasis 

 respectively by a combination of circumstances. They are 

 always closely associated with man, and prefer his blood 

 over other food ; they are abundant and comparatively long- 

 lived ; their meals are taken at frequent intervals, and, as 

 breeding is practically continuous, individuals are always 

 present to act as intermediary hosts for the blood parasites. 

 Under these conditions, the chain in the life-cycle of the 



