April, '12] KNAB: DISEASE TRANSMISSION 197 



not only the majority of mosquitoes, but many other blood-sucking 

 insects, such as Tabanidse and Simuliidse, may be confidently elim- 

 inated. Moreover these insects are mostly in evidence only during 

 a brief season, so that we have the additional difficulty of a very long 

 interval during which there could be no propagation of the disease in 

 question. 



The truth is that all insects that have been found to be transmitters 

 of human blood-disease are more or less closely associated with man 

 and habitually suck his blood. This relation has long been recognized 

 in the case of the two house-mosquitoes of the tropics, the one {Aecles 

 calopus) being the intermediary host of the yellow fever organism, 

 the other iCulex quinquejasciatus) of those of filariasis and dengue 

 fever, but its significance has not been grasped. It is only through a 

 combination of circumstances that these insects are effective trans- 

 mitters. These conditions are: The association with man and a 

 predilection for his blood, abundance, comparative longevity — which 

 means blood meals repeated at intervals, and practically continuous 

 breeding, so that individuals are always present to act as intermediary 

 hosts of the parasites. When these conditions are fulfilled the chain 

 in the life-cycle of the parasite is continuous and we have an endemic 

 disease. The relations just outlined might be expressed in terms of 

 mathematical formulae, but nothing would be gained thereby. It is 

 the recognition of the principles involved that is important. 



The writer is not at present in a position to review the entire field of 

 disease transmission by biting insects from this viewpoint. However 

 one more striking example in support of his views may be cited. This 

 is the large hemipter Triatoma {Conorhinus) megistus which Carlos 

 Chagas recently has shown to be the transmitter of a dangerous try- 

 panosome disease of man in Brazil. As I have already pointed out 

 in another place (Proc. Ent. Soc. Wash., xiii, 71, 1911) Triatoma megis- 

 tus is remarkable among the American members of the genus in show- 

 ing close adaptation to man. It lives in houses and does not occur 

 naturally apart from man. The eggs are laid in the crevices of walls 

 inside of houses and the young bugs feed on human blood from the 

 start. In spite of its very large size, the bite of the adult occasions 

 so little pain that it does not awaken a sound sleeper. This is clearly 

 adaptational, as the wild species of the genus are known to have a very 

 painful bite. Still another example are the biting flies of the genus 

 Flehotomus, which, in the Mediterranean region transmit the so-called 

 pappataci fever. Here too the species involved are closely associated 

 with man. 



It would seem at first thought that Anopheles, in the transmission 

 of malaria, does not fulfill the conditions above formulated. The 



