ORIGIN OF THE TRACHEAE, 



447 



arranged so that one has its fork on one side of the opening of the 

 channel, the next ring the fork on the opposite side of the channel, 

 and so on, in alternation. Their true struct- 

 ure is revealed when flattened out." 



The use of the elastic taenidia is to render 

 the tracheae elastic, and to keep them per- 

 manently open, as is the case with the 

 parallel rings of the trachea of the higher 

 vertebrates. The tracheae are thus rendered 

 firm and solid, at the least expense of chiti- 

 nous material. The spiral thread, as Mac- 

 Leod remarks, " is the realization in nature 

 of what engineers call a form of the greatest 

 resistance." 



The tsenidia are wanting in the fine endings of 

 the tracheae (tracheal capillaries) ; also in the cock- 

 roach, according to Miall and Denny, they are not 

 developed in the large tracheae close to the spiracles, 

 and the intima or wall of the tube has a tessellated 

 instead of a spiral marking (Fig. 411). The same 

 structure is seen in the Perlidae (Nemoura, Ger- 

 staecker, Zeit. f. wissen. Zool. xxiv, Taf. xxiii, Figs. 

 5 and 7) ; also in ^Eschna (Hagen, Zool. Anz. 1880, 



Fro. 411. Abdominal spira- 

 cle (left side) of cockroach (P. 

 aniericana}, side view, show- 

 ing the bow: p, lateral pouch of 

 spiracle (in centre) seen from 

 within. The tessellated struct- 

 ure of spiracle and trachea shown 

 at A, and the margin <>f the ex- 

 ternal aperture at B. After 

 Miall and Denny. 



p. 159). In certain fine tracheae of the eyes of the 



fly no spiral threads are developed. (Hicksou.) The air-sacs or dilated tracjiese 



are also without tsenidia. 



While in the living insect the main and smaller tracheae are filled 

 with air, it is stated by Von Wistinghausen that the fine capillary, 

 ends contain a fluid. 



e. Origin of the tracheae and of the "spiral thread " 



While we owe to Btitschli the discovery of the mode of origin 

 and morphology of the tracheae, which as he has shown a arise by 

 invaginations of the ectoblast ; there being originally a single layer 

 of epiblastic cells concerned in the formation of the tracheae; we 

 are indebted to Weismann 2 for the discovery of the mode of origin of 

 the "intima," from the epiblastic layer of cells forming the primi- 

 tive foundation of the tracheal structure. 



1 Zur Entwicklungsgeschichte der Biene, Zeitschr. wissens. Zoologie, xx, p. 519, 

 1870. 



2 Die Entwicklung der Dipteren im Ei, Zeitschr. wissens. Zoologie, xiii, 1863. 



