THE WEB OF LIFE 361 



the wind, or indirectly, owing to the flies being attracted 

 by odours borne by the wind. Fine weather and warmth 

 favour dispersal, and flies travel further in the open country 

 than in towns, probably because the houses offer food and 

 shelter. In thickly housed localities the usual maximum 

 flight is about a quarter of a mile, but in one case a single fly 

 was recovered at a distance of 770 yards (partly over open 

 fen land). When set free in the afternoon, flies do not 

 scatter so well as in the morning. Liberated flies often 

 mount almost vertically to a height of forty-five feet or 

 more. Every detail of this is important because flies are 

 disease-distributors. 



Besides carrying the germs of diseases that affect animals, 

 flies may do something in the way of spreading the diseases 

 of plants. Thus L. Mercier has noticed that a common 

 summer fly, Sciara thomce, carries about the spores of 

 the fungus (Claviceps) which causes ergot on rye-grass. 

 The spores were abundant in the food- canal of the fly and 

 did not seem to be digested ; they also occurred on the 

 setse of the body. Although it has not been experimentally 

 proved that the flies infect healthy plants with Claviceps, 

 there is no doubt that they carry the germs and that they 

 frequent rye-grass. 



Not a few insects are subject to fatal attacks of fungoid 

 parasites, and use is now being made of this to further the 

 destruction of injurious pests. By artificially favouring 

 the dissemination of the fungus it has been found possible 

 to cause a useful plague among the insects. Much good 

 has been done in this way in checking the scale insects 

 which attack the limes in Dominica and Montserrat and 

 similar islands. It has been recently suggested that an 

 artificial diffusion of a fungus, Empusa muscce, which is 



