150 THE COCKROACH: 



Tracked Thread. 



In the finest tracheal tubes ('0001 in. and under) the intima 

 is to all appearance homogeneous. In wider tubes it is 

 strengthened by a spiral thread, which is denser, more refrac- 

 tive, and more flexible than the intervening membrane. The 

 thread projects slightly into the lumen of the tube, and is often 

 branched. It is interrupted frequently, each length making 

 but a few turns round the tube, and ending in a point. The 

 thread of a branch is never continued into a main trunk. Both 

 the thread and the intervening membrane become invisible or 

 faint when the tissue is soaked with a transparent fluid, so as 



Fig. 84. Intirna (chitinous lining) of a large tracheal tube. The spiral thread 

 divides here and there. Copied from MacLeod, loc. cit., fig. 9. 



to expel the air. Both, but especially the thread, absorb 

 colouring matter with difficulty. The thread, from its greater 

 thickness, offers a longer resistance to solvents, such as caustic 

 alkalies, and also to mechanical force ; it can therefore be 

 readily unrolled, and often projects as a loose spiral from the 

 end of a torn tube, while the membrane breaks up or crumbles 

 away.* 



The large tracheal tubes close to the spiracles are without 

 spiral thread, and the intima is here subdivided into polygonal 



* Investigators are not yet agreed as to the minute structure of the tracheal 

 thread. Chun (Abh. d. Senkenberg. Naturf. Gesells., Bd. X., 1876) considers it an 

 independent chitinous formation, not a mere thickening of the intima. He describes 

 the thread as solid. The intima itself is, he believes, divisible in the larger tubes 

 iuto an inner and an outer layer, into both of which the thread is sunk. Macloskie 

 (Amer. Nat., June, 1884) describes the spiral as a fine tubule, opening by a fissure 

 along its length. He regards it as a hollow crenulation of the intima, and con- 

 tinuous therewith. Packard (Amer. Nat. Mag., May, 1886) endeavours to show that 

 the thread is not spiral, but consists of parallel thickenings of the intima. He is 

 unable to find proof of the tubular structure, or of the external fissure. We 

 have specially examined the trachea of the Cockroach, and find that the thread can 

 readily be unwound for several turns. It is truly spiral. 



