ANATOMY AND PHYSIOLOGY vif 
Thus, in the stag-beetle, a crescentic bow (Fig. 173, b) extends 
half around the trachea, and the rest of the circumference is 
spanned by a lever (1) and a band (bd) ; these three chitinous 
parts, articulated together, form a ring around the trachea. 
Furthermore, a muscle (mm) connects the lever and the band. 
ms, tne muscle: shortens, the lever 
turning upon the end of the band as 
a fulcrum, pulls the bow toward the 
lever and band until the enclosed 
trachea is pinched together. When 
the muscle relaxes, the trachea opens 
by its own elasticity. 
Structure of Trachez. — The 
tracheze originate in the embryo as 
simple in-pocketings of the outer 
germ layer, or ectoderm, and from 
these the countless tracheal branches 
are derived by the same _ proccess 
of invagination. The lining mem- Structure of a trachea. h, 
Drneomsa ‘trachea: is) then, Con=, “Ce 
2 2 t, tenidium. 
tinuous with the external cuticula, 
and the cellular wall of a trachea is continuous with the 
rest of the hypodermis. This wall consists of a layer of 
polygonal cells (Fig. 174) fitting closely together as a pave- 
ment epithelium. The chitinous lining, or imtima, is thick- 
ened at regular intervals to form thread-like ridges, which 
course around the inner circumference in essentially a spiral 
manner, though the continuity of the so-called spiral thread is 
frequently interrupted. These elastic threads, or temidia, 
serve to keep the trachea open without affecting its flexibility. 
The ultimate tracheal branches (Fig. 175) are extremely 
delicate tubes, which do not end blindly, but anastomose with 
one another, forming a capillary network of confluent tubes. 
Some authors have held that the finest tracheal filaments pene- 
trate epithelial or other cells. 
Respiration.—The external signs of respiration are the 
