INSECTS AND HUMAN DISEASE 139 



relatives and often sleep on the mainland. It explains the 

 immunity of the men permanently employed at the Venice 

 arsenal. It explains the immunity of the Burano women 

 and girls who remain continually on the island to work at 

 that wonderful needle-made lace, the ' punto in Aria,' for 

 which the island is famous. It explains the exceptional 

 occurrence in the very few women who go to work in the 

 fields of the neighbouring mainland." The conclusions from 

 these investigations alone seem reasonably obvious, but, 

 taken in conjunction with Dr Sambon's later work in the 

 Americas and the West Indies, his theory appears incontro- 

 vertible. Pellagra occurs in districts where maize is never 

 eaten, but, with one exception, the Nile delta, it is always 

 found in association with buffalo gnats. In the delta 

 pellagrins are found, but despite the most diligent search 

 no species of Simulium has been discovered, though 

 another fly, Leptoconops, is much in evidence. Perhaps it 

 too is a vector of pellagra. 1 



The Simulium (fig. 37) is a very small fly, one to four 

 and a half millimetres long, usually black or dark brown, 

 its long legs often banded with white, its transparent wings 

 relatively large and broad ; whilst its arched, hairy thorax, 

 and short, stout antennae have earned it the name of 

 buffalo gnat, from a supposed resemblance to the buffalo. 

 It is cosmopolitan, and the common British species are 

 Simulium lineatum and Simulium reptans. Like many 

 other biting flies, the female alone is the culprit, the male 

 being inoffensive. Unlike other flies, however, the so-called 

 bite is not brought about by piercing the skin but by a con- 

 tinued scraping till blood flows natural vaccination, in fact. 

 All the stages, except the adult, are passed under running 

 water. The female fly oviposits in the evening ; flitting 

 along the water with a peculiar darting flight, she holds on 



1 Since the above went to press, Dr Hunter has announced, in an 

 American journal, that he has succeeded in infecting monkeys with 

 pellagra by means of infective Simulidce. 



