INSECTS AND DISEASE 385 



the filaria that cause elephantiasis. Mosquitoes are also prob- 

 ably responsible for the spread of dengue or break-bone fever. 

 We do not yet know the organism that causes the disease, but 

 experiments have shown that it can be spread by Culexfatigans, 

 and possibly by other species of mosquitoes. 



For a long time the theory that pellagra is in some way asso- 

 ciated with the eating of maize has been generally accepted. 

 Recently Dr. Sambon has suggested that pellagra may be 

 caused by some organism that is transmitted by certain small 

 black flies belonging to the family Simulida. This theory is 

 not yet firmly established and there are many who do not 

 accept it at all. The investigations that are now being carried 

 on will doubtless soon settle the question. The black-flies are 

 very serious pests of live stock and their habits are discussed 

 in Chapter XXX. The stable-fly has also been considered 

 as a possible agent in the transmission of pellegra. 



Certain little moth-like flies, Phlebotomus pappataci, which 

 on account of their habits are sometimes called "sand-flies," 

 carry an unknown germ that causes a very infectious disease 

 known as three-day fever, or sand-fly fever. But as this and 

 several other diseases of lesser importance spread by insects, do 

 not occur in our country we need not discuss them. 



The relation of ticks to several important diseases has already 

 been discussed in Chapters XIX and XXVIII. 



