NON-BACTERIAL DISEASES 195 



immediatel}' return. If undisturbed, they engorge themselves 

 with pus, blood, serum or sebaceous secretion, until their 

 abdomens are greatly distended." 



Summary. 



All observers agree that flies swarm round the ulcers of yaws, 

 and some consider them to be important agents in spreading the 

 disease. It is remarkable, however, that Castellani's experiments, 

 which appear to have been carried out under ideal conditions 

 for fly infection, only yielded a small proportion of positive 

 results. Further careful investigations and experiments are 

 required. 



CHAPTER XX 



ON THE PART PLAYED BY FLIES IN THE DISPERSAL 

 OF THE EGGS OF PARASITIC WORMS 



Experiments. 



Grassi (1883) was the first to demonstrate that flies could 

 ingest the eggs of parasitic worms ( Tcenia solium, and Oxyuris), 

 and that these eggs could pass through their intestines, and be 

 deposited in the faeces apparently unaltered and undamaged. 

 By placing sheets of white paper on the floor on which the flies 

 defsecated he showed that the flies could deposit the eggs of 

 TricJiocepJialns 10 metres from the place at which they had fed. 

 Stiles (1889) "placed the larva; of Musca with female Ascaris 

 liiinbricoides, which they devoured, together with the eggs they 

 contained. The larvae, grubs, as also the adult flies, contained 

 the eggs of Ascaris. The experiment being made in very hot 

 weather, the Ascaris eggs developed rapidly, and were found in 

 different stages of development in the insects, thus proving that 

 the latter may serve as disseminators of the parasite" (Nuttall 

 and Jepson, 1909, p. 28). Galli-Valerio (1905) found that flies 

 could carry on the surface of their bodies, not only the eggs 



