THE INTERNAL ANATOMY OF INSECTS 117 



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



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



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

 more transverse tracheae (Fig. 134). 



The structure of the tracheae. — The fact that 

 in their embryological development the tracheae 

 arise as invaginations of the body- wall, makes it 

 easy to understand the structure of the tracheae. 

 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 intima is the chitinoUs inner layer of the 

 tracheae. 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 

 tracheae is the fact 

 that it is furnished 

 with thickenings 

 which extend spirally. 

 These give the 

 of tracheae their charac- 

 transversely 



Fig. 134. — Larva 

 Cantharis vesicatoria, \^q^c\^\q 

 showing the distribu- 

 tion of tracheae (From striated appearance. 

 Henneguv after jf ^ -^^^ ^f ^^^ ^^ 

 Beauregard). ^ 



the larger tracheae 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 



-Section of a trachea 

 and the body-wall; c, cuti. 

 cula; h, hypodermis; hm, 

 basement membrane; sp, 

 spiral thickening of the in- 

 tima, the taenidium. 



