THE STRUCTURE AND FUNCTION OF THE PROBOSCIS 65 



the surface so that the outline of the disc is clearly visible. It may 

 also be seen that the longitudinal prestomal sulcus was almost 

 completely closed. The lines of the pseudo-tracheae are marked 

 by double parallel rows of regularly placed clear oral areas 

 separated by thin black lines. Each of these areas from which 

 the pigment has been removed by suction represents the space 

 covered by an interbifid groove. PI. XIV, fig. 4, is a photograph of 

 a portion of a similar pattern more highly magnified. It will be 

 noticed that no traces of the zigzag fissures running longitudinally 

 along the pseudo-tracheae can be seen. These fissures are entirely 

 obliterated by the pressure of the proboscis on the surface causing 

 the free ends of the rings to meet, as can be readily understood 

 by reference to PI. XIII, fig. 4. 



The longitudinal axis of each pseudo-trachea is marked by 

 a zigzag black line. On each side of this line are clear areas 

 caused by the removal of pigment through the interbifid grooves. 

 The way in which this pattern is produced is best understood 

 by reference to PI. XIII, fig. i (left-hand side). 



The broader black lines represent the inter-pseudo-tracheal 

 plain areas of the disc. 



If fed on a drop of moderate depth the proboscis does not 

 seem to be so closely applied to the surface on which the drop 

 is placed, though the disc is in an erected condition. 



If a fly is allowed to feed on a large drop containing particles 

 of various sizes it often sucks up all the fluid and leaves the 

 larger particles in an irregular mass at one edge of the drop. 

 When the area originally covered by the drop is examined with 

 the aid of a lens it is found to be covered with numerous clear 

 oval proboscis marks, indicated by fine lines of pigment at their 

 peripheries. It is evident therefore that the fly at each applica- 

 tion of its proboscis has sucked up and swallowed the fluid and 

 smaller particles which are capable of passing through the inter- 

 bifid spaces, and that the suction has caused the larger particles 

 to adhere to the disc. After all the fluid has been swallowed 

 the larger particles adhering to the proboscis are deposited 

 either through the cessation of the suction, or by a small quantity 

 of fluid being- forced in a reverse direction to wash off the 

 deposit. 



G.-s. 5 



