128 INSECTS AND MAN 



and reaches its maximum in the height of the summer ; it 

 is also a typically rural or semi-rural disease. All these 

 facts point to a probable insect vector. 



The seasonal distribution of poliomyelitis puts out of 

 count fleas, lice, and bed bugs, for they are nearly as pre- 

 valent in the winter as in the summer. Suspicion can 

 hardly fall upon mosquitoes, for they are so generally 

 abundant and so persistent in their search for human blood, 

 that diseases borne by them usually develop into extensive 

 outbreaks, and this is never the case with infantile paralysis. 

 Ticks and Reduviid bugs, both of which are known to 

 transmit human diseases, are unlikely vectors in this case, 

 because their attacks leave considerable wounds, which 

 are easily observed. Various Diptera have been suspected 

 from time to time, but, with one exception, they may all 

 be dismissed the gadflies are rare in inland rural com- 

 munities ; the closely related Chrysops does not occur in 

 the latter part of the summer, when the disease is most 

 prevalent ; the buffalo gnats, too, are most abundant in the 

 spring. 



The only insect which cannot be dismissed for any 

 reason whatever is the stable fly, Stomoxys calcitrans. 

 This fly has bloodthirsty habits which render it a probable 

 vector of disease ; its seasonal abundance is closely corre- 

 lated with the incidence of the disease ; its geographical 

 distribution agrees with that of poliomyelitis ; it is a rural 

 rather than a town-frequenting insect ; and in a very large 

 number of cases the children had played about stables, etc., 

 where Stomoxys was abundant. Experimentally the bites 

 of infected stable flies have produced poliomyelitis in 

 healthy monkeys, so there is every reason to believe that 

 they may spread the disease naturally, and that probably 

 some domestic animal acts as a reservoir for the virus. 



The stable fly resembles a stoutly built house fly, grey 

 in colour, and with a resting position, with wings somewhat 

 spread, which distinguishes it from the house fly. Its 



