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 
