CHAPTER IX 



THE RESPIRATORY SYSTEM 



All insects breathe by means of tracheae, i.e. tubes designed 

 to carry the oxygen supply direct to all the organs of the body. 

 These tracheae are of ectodermal origin, being developed as paired 

 segmental invaginations in the embryo. The mouth of the in- 

 vagination becomes the spiracle or stigma. The tubes unite in 

 the body of the insect to form a complete tracheal system consisting 

 of paired longitudinal trunks, from which 

 numerous branches arise. These branches 

 divide again and again, until they end in 

 exceedingly fine tracheae known as capil- 

 laries. 



The structure of a trachea (fig. 72) is 

 easily understood, if we bear in mind its 

 ectodermal origin. In transverse section 

 it is seen to be formed of two layers, 

 an outer ectotrachea (ec), and an inner endo- 

 trachea (en). The ectotrachea is a cellular 

 layer corresponding with the hypodermis. 

 Its cells are flattened, hexagonal, with 

 distinct broadly oval nuclei, which are 

 often seen bulging out from the contour of 

 the smaller tracheae. The endotrachea is 

 a chitinous non-cellular layer secreted by 

 the cells of the ectotrachea, and corre- 

 sponding with the cuticle. On its inner 



side it is strengthened by a peculiar thickening known as the spiral 

 thread (sp). This is not a single complete spiral, but consists of 

 a number of short coils, each the product of a single cell. These 



Fig. 72. A. Portion of a 

 small trachea from lar- 

 va of Syrilestes weyersi 

 Selys. B. T.S. through 

 same ( x 800). ec ec- 

 totrachea ; en endotra- 

 chea ; sp spiral thread. 

 Original. 



