THE SO-CALLED SPIRAL THREADS 



445 



most usually four turns. Fig. 408, part of a trachea of Dyticus marginatus, 

 shows that at a slight bend in a trachea the tsenidium is interrupted, and short, 

 incomplete, wedge-shaped tsenidia (e) are interpolated ; at A, d is seen a split 

 in one of the tsenidia (compare also 

 MacLeod, PI. 1, Fig. 9). The 

 threads are quite irregular in width. 

 In the axils of the branches there 

 is, as seen in Fig. 409, a basket- 

 work of independent, short, often 

 spindle-shaped tsenidia ; these are 

 succeeded by longer ones, until 

 we have threads passing entirely 

 around near the base of each new 

 branch ; these being succeeded by 

 others which make from two to 

 five spiral turns. 



The shape of the teenidia 

 appears to vary to a great 

 extent. In lepidopterous in- 

 sects we have observed them 

 to be in their general shape 



ra.ther flat and slightly concavo-convex, the hollow looking towards 

 the centre of the trachea. Minot's section (Fig. 393) shows that 

 in Hydrophilus they are cylindrical and solid, and Chun states 

 that those of Stratiomys are round, while in Eristalis they are 

 round, with a ridge projecting into the cavity of the trachea; in 



V 



FIG. 409. Tfenidia of Dyticus in an axil of two branches : c, e, ends of ta?nidia. 



JSschna the thread is quadrangular. MacLeod states that some- 

 times it is cylindrical, in other cases flat, likewise prismatic ; Mac- 

 loskie believes that the spiral threads of the centipede are "fine 

 tubules, externally opening by a fissure along their course." 



Stokes confirms Macloskie's statements, stating that in the hemip- 

 terous Zaitha fluminea "the tsenidia are fissured tubules formed 

 within and from chitinized folds of the intima, the convexity of the 



