58 THE STRUCTURE AND FUNCTION OF THE PROBOSCIS 



The cuticle lining the oral surface of the labellum dips down 

 into the pseudo-trache;u through the longitudinal fissure and 

 also forms the lining of these tubes, as may be seen by reference 

 to PI. XIII, fig. 4. In passing downwards into a pseudo-trachea 

 the cuticle accurately follows its chitinous margins, being closely 

 adherent not only to the chitinous sides of the interbifid spaces 

 but also to the intervening elevations between them produced 

 by the projection of the expanded ends of the alternate rings, 

 and the application to them of the adjacent forks of the neigh- 

 bouring rings on either side. 



Owing to this arrangement a remarkable series of folds is 

 produced in the cuticle forming channels or grooves leading into 

 the interbifid spaces. If the cuticle is traced along the edge of 

 the longitudinal fissure from the bottom of one interbifid space 

 to the bottom of the next it can be observed to be very closely 

 attached to the chitin along the base of an interbifid space and 

 up the side of a fork to its pointed extremity. It then passes 

 over the expanded portion of the alternate ring and down the 

 adjacent fork of the next ring, binding the two forks mentioned 

 and the expanded portion of the intermediate ring into an 

 elevated mass which lies between the deep depressions of the 

 interbifid spaces. The arrangement described can be most easily 

 understood by reference to PI. XIII, fig. 5, a longitudinal section 

 through a pseudo-trachea just to one side of the central fissure. 

 The depressions caused by the cuticle adhering to the bases of 

 the interbifid spaces are continued outwards as folds or grooves 

 in the cuticle for a considerable distance which are gradually 

 lost on the surface of the labellum. Each 'interbifid groove' 

 thus forms a well-defined channel leading into the pseudo-trachea 

 through the interbifid space with its long axis at right angles to 

 the line of the pseudo-trachea. The deepest part of the groove 

 is at its entrance into the pseudo-trachea, and at this point it 

 loses its groove-like character and becomes a tunnel, though still 

 communicating with the surface by a very narrow slit. When 

 the proboscis is erected b)' slight pressure on the head and the 

 oral sucker viewed with a microscope these grooves can be easily 

 seen as regularly placed channels running at right angles to each 

 pseudo-trachea. 



