no 



ENTOMOLOGY 



In dipterous larvae the closure is effected by the contraction of a circular 

 muscle, but Coleoptera and Lepidoptera, among other insects, have a 

 somewhat complex apparatus for closing the trachea immediately behind 

 the spiracle. Thus, in the stag-beetle, a crescentic 

 bow (Fig. 174, b) extends half around the trachea, and 

 the rest of the circumference is spanned by a lever (I) 

 and a band (bd) ; these three chitinous parts, articulated 

 together, form a ring around the trachea. Further- 

 more, a muscle (m) connects the lever and the band. 

 As the 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 Tracheae. The tracheae originate 

 in the embryo as simple in-pocketings of the outer 

 germ layer, or e'ctoderm, and from these the countless 

 tracheal branches are derived by the same process of 

 invagination. The lining membrane of a trachea is, 

 then, continuous 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. 

 175). fitting closely together as a pavement epithelium. The chitinous 



FIG. 173. Larva 

 of Bittacomorpha 

 davipes showing res- 

 piratory tube. I^at- 

 ural size. After 

 HART. 



FIG. 174. Apparatus for closing the spiracular trachea in a beetle, Lucanus. A, trachea 

 opened; B, closed; b, bow; bd, band; c, external cuticula; /, lever; m, muscle; s, spiracle; 

 t, trachea. After JUDEICH and NITSCHE. 



lining, or intima, is thickened 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 



