28 THE HOUSE FLY DISEASE CARRIER 



of an upper and a lower portion, the upper portion 

 bearing two curved bristly lobes. The lower portion 

 or true haustellum expands at the tip into two lobes 

 which are called the oral lobes. On their under sur- 

 face they have transverse chitinous bars which are 

 called false tracheae (pseudotracheae). The presence 

 of these hard ridges under the oral lobes fit it to a 

 certain extent for rasping solid food. The orifice to 

 the haustellum occurs between the lobes. 



In feeding upon fluid or semi-fluid substances, the 

 oral lobes are simply applied to the surface and the 

 fluid is sucked up. When, however, they feed upon 

 soluble solids the process is somewhat different. Doc- 

 tor Graham-Smith has carefully watched them feeding 

 upon crystals of brown sugar, and has done this 

 through the Zeiss binocular microscope. He states that 

 the oral lobes of the proboscis are very widely opened 

 and closely applied to the sugar. Fluid (saliva) seems 

 to be first deposited on the sugar and then strong suck- 

 ing movements are made. Doctor Graham-Smith 

 watched a fly sucking an apparently quite dry layer 

 of sputum. It put out large quantities of saliva from 

 its proboscis and seemed to suck the fluid in and out 

 until a fairly large area of the dry layer of sputum 

 was quite moist; then as much as possible was sucked 

 up and the fly moved away to another spot. The same 

 observer noticed that flies which had the opportunity 

 of feeding either on fluid or partly dried milk often 

 chose the drier portions, and states that under natural 

 conditions they can often be seen sucking the dried 



