406 A. A. MERLIN ON THE TRACHEAL TUBES OF INSECTS, ETC. 



attention to structural details which, although not described in 

 the text -books, are probably familiar to at least many of you. 



The points to which I especially wish to call attention are the 

 following : — 



(a) The strengthening chitin fibres are coiled within the tube 

 in very short lengths, often not sufficing to form one complete 

 turn. 



(b) Extremely short and narrower threads are occasionally 

 observable between the larger fibres, extending through only a 

 small fractional part of the tube's circumference. These may 

 possibly indicate the manner in which the fibres aie evolved 

 during the growth of the pipe. 



(c) The fibres are not of uniform breadth, but vary consider- 

 ably, and exhibit an uneven w^avy aspect. 



(d) All the fibres towards their extremities taper off into very 

 finely pointed and long, undulating filaments. In one instance a 

 fibre has been seen split into two such terminal filaments. 



In most mounted slides of this object, short portions of the 

 thread are observable partially unwound and projecting from the 

 severed ends of the trachese. This fact has been referred to by 

 the old writers, but I have never seen any considerable continuous 

 length of thread so unwound, say sufficient for several coils, as has 

 been depicted in one work. In the centipede trachese, a longitudi- 

 nally widely ruptured tube has been found, showing many of the 

 filamentous terminations of the main fibres slightly disarranged 

 and projecting entirely clear of the torn envelope. 



Liastly, I may mention that the chitin fibres themselves have 

 yielded no indications of any internal structure under an oil- 

 immersion objective. 



Journ. Quekett Microscopical Club, Ser. 2, Fol. VH., No. 47, November 1PO0. 



