THE FLY'S SENSES 35 



crops such bacteria will thrive in the flies' food ; and 

 then, after many hours perhaps, the germs will be 

 regurgitated into a baby's mouth or into the invalid's 

 beef-tea. Or the germs can multiply in the digestive 

 processes of the fly and be passed on to the sugar, or on 

 to the spout of the milk- jug even in a London drawing- 

 room. Nature is so complete in her methods she 

 takes no risks lest her schemes shall fail ; if she desires 

 to perpetuate a disease she leaves nothing to chance, 

 but makes sure of every means to attain her ends ; but 

 why she does it is difficult to explain. 



The domestic fly must be possessed of some subtle 

 sense. It has a large compound eye with which, pro- 

 bably, it can see in all directions, for the eye is faceted 

 in every plane. But it must also have a peculiar power 

 of smell. Note how flies will swarm to a kitchen, a 

 larder, a privy, an ash-bin, a manure-heap. It is the 

 food therein which attracts them from all quarters. 

 But how this is done exactly is difficult to understand. 

 The way in which a mosquito also finds her food in 

 a house is very extraordinary. It can hardly be sight 

 only, for she works in the dark, and can recognise the 

 presence of living blood in a distant room; it can 

 hardly be the sense of smell, for where are there smell- 

 organs in gnats or flies large enough to attract these 

 insects to their food and to cause them to go long 

 distances for it ? These insects must be possessed of 

 some subtle sense which we human beings cannot 

 realise. Their nervous mechanism must be wonderful 

 for such senses to be contained in such small bodies. 

 Yet the nerve ganglia of flies are small affairs. Why 

 do some mosquitos bite only at night while others will 

 bite in the daytime too ? Some species do the one 

 and others do the other. A house-fly has wonderful 



