44 THE BOOK OF THE FLY 



compared with those of other kinds of insects ; but 

 their structure viewed under the microscope is intricate, 

 and they may be efficient organs of sense perception, 

 probably in part auditory. The really unique feature 

 is the retractile and suctorial proboscis, which is often 

 incorrectly regarded as the tongue ; it is normally held 

 doubled up and withdrawn towards a hollow under the 

 head, whence it is from time to time extruded. The 

 structure of this member is characteristic of the entire 

 tribe to which the house-fly belongs ; it is a fusion or 

 combination of mouth parts, which in other insects are 

 used more or less separately for the various functions 

 of inspecting, biting, masticating, drinking and 

 swallowing. In the house-fly the proboscis is 

 absolutely suctorial, and is not provided with the 

 lancets used by the blood-sucking flies for piercing the 

 skin. Two maxillary palpi are attached to the upper or 

 basal part of the proboscis, which is called the rostrum 

 (a snout) ; the lower part is called the haustellum (a 

 pump), and it has at the end a pair of soft cushion-like 

 lobes or lips, which, when spread apart, form a heart- 

 shaped pad with an opening in the centre. The max- 

 illary palpi are used for feeling and probably smelling. 

 Each mouth-lobe has amain collecting central channel 

 and thirty subsidiary cross channels of a wonderfully 

 complex character. Imbibed fluids pass from the 

 mouth-lobes to the gullet along a passage in the 

 haustellum and the rostrum. 



As with many other flies and other insects, there are 

 on the top of the head very small simple and rather 

 inconspicuous eyes called ocelli, three in number, 

 between the large and prominent compound eyes, 



