THE INTERNAL ORGANS OF RESPIRATION. 171 



that they consist of three distinct layers, which, taking them from the 

 exterior, appear in the following form : 



The outermost membrane (PL XXII. f. 11.) is transparent, very 

 smooth, without being perceptibly librous, but hard, and generally 

 colourless. Coloured tracheae, which we now and then observe, for 

 example, brown in Locusta viridissima, red in Phasma gjgfis, or 

 black, as in the larvae of Dyticus and Hydrophilus, derive their colour 

 from this exterior skin, whereas both the others, especially the second, 

 are constantly of a silvery white, and shining. A dark colour facilitates 

 very much the detection and unravelment of the extremely delicate 

 tracheae, particularly when they run upon the clear ground of other 

 organs. But in those cases where the tracheae are not coloured their 

 investigation is not very difficult when freshly killed individuals are 

 selected for the purpose, for in them the tracheae are still filled with 

 air : they then display themselves as silvery white, glittering threads, 

 which here and there appeardull and transparent, from moisture having 

 at those parts already penetrated them. In general, the last and most 

 delicate ends are still filled with air, which, however, is forced out 

 when the creature has been long immersed in spirits of wine, and it 

 then becomes difficult to obtain a satisfactory view of their distribution. 

 The exterior membrane of the tracheae consequently is structureless, 

 nor is it in very close connexion with the second, but loosely surrounds 

 it, leaving everywhere a free space between them, which is quickly 

 perceived upon a microscopic investigation, and thereby readily con- 

 vinces us of the presence of at least two layers. 



The second layer consists of a single, tense, elastic, and very delicate 

 filament, which twines spirally around the innermost membrane, so 

 that its windings are everywhere, or at least very generally contiguous. 

 This thread appears to be simple and round, but which is occasionally 

 difficult to ascertain from its delicacy, but the microscope displays how 

 it distributes itself about the circumference of the vessel, and that it 

 scarcely leaves the smallest space between its successive windings, and 

 which is filled only by membrane. In some instances, for example, in 

 Locusta viridissima, and indeed in all insects provided with large 

 tracheal stems, the filament becomes broader, resembling a band, and 

 can be distinctly distinguished as such. Sprengel * detected in such 

 larger tracheae ramose filaments, or perfectly closed rings, which were 



* Commcutar. tie Par!., &c PL II. f. 14. 



