THE INTERNAL ANATOMY OF INSECTS 117 



ments by one or more longitudinal tracheas, and is also connected 



Fig. 133. — The trachcce of Machilis (From Oudemans). 



with the group on the opposite side of the same segment by one or 

 more transverse trachese (Fig. 134). 



The structure of the tracheae. — The fact that 

 in their embryological development the tracheas 

 arise as invaginations of the body-wall, makes it 

 easy to understand the structure of the tracheas. 

 The three layers of the body-wall are directly 

 continuous with corresponding layers in the wall 

 of a trachea (Fig. 135). These layers of -a 

 trachea are designated as the intima, the epithe- 

 lium, and the basement membrane. 



The mtima is the chitinous inner layer of the 

 trachese. It is directly continuous with the 

 cuticula of the body- 

 wall, and like the 

 cuticula is molted at 

 each ecdysis. 



A peculiar feature 

 of the intima of 

 trachea is the fact 

 that it is furnished 

 with thickenings 

 which extend spirally. 

 These give the 

 1 34-— Larva of tracheae their charac- Fig. 135.— Section of a ixachea 



teristic transversely ^",<^ ^^^f body-v/all; c, 



Cantharis vesicatorta, 

 showing the distribu- 

 tion of tracheae (From striated appearance. 



the larger trachea be 

 pulled apart the intima will tear between the folds of the spiral 

 thickening, and the latter will uncoil from within the trachea like a 



basement membrane; sp, 

 spiral thickening of the in- 

 tima, the tasnidium. 



