INSECTS AND HUMAN DISEASE 119 



species of Stegomyia, are also known to transmit the 

 disease, as do other mosquitoes. This is in marked contrast 

 to the behaviour of the blood parasites of malaria and 

 yellow fever, which are only transmitted by Anopheles and 

 Stegomyia respectively; it resembles these parasites, 

 however, in that the mosquito is absolutely essential for 

 its complete development. 



" A most interesting fact with regard to this association 

 between Filaria bancrofti and its mosquito intermediary 

 host is that the larval filarise are found only at night in 

 the general or peripheral circulation, hiding during the 

 day in the vessels of the lungs and other internal organs. 

 If the blood of the patient is examined during the day, 

 the young parasites are rarely seen, or not at all. As 

 evening approaches, the tiny larvas begin to enter the 

 peripheral circulation in greatly increasing numbers. The 

 swarm goes on increasing until about midnight, at which 

 time it is not unusual to find as many as three hundred, or 

 even six hundred, in every drop of blood. This uncanny 

 swarming of the larvae of Filaria bancrofti in the 

 peripheral circulation becomes at once comprehensible 

 when connected with the nocturnal habits of their 

 liberating agent, the mosquito. The young filarise may 

 persist for months in the blood of man, and no further 

 development is possible, unless they reach their necessary 

 intermediary host, and this is why they swarm into the 

 peripheral circulation at the time when mosquitoes bite. 

 There are a large number of similar correlations in 

 Nature." 



HOUSE FLIES AND DISEASE 



As disease-carriers, the house flies, Musca domestica, 

 stand in a class by themselves. The belief that they dis- 

 seminate infective diseases is not new. " Mercurialis (1577) 

 considered that they carried the virus of plague from those 

 ill or dead of plague to the food of the healthy. Sydenham 



