THE HESI'UI.VTOKY SYSTEM. 



Section VIII. The Itespiratofy tii/stem. 



The cliief modification of structure dependent upon the 

 peculiarity of the circulation is in the respiratory organs, the 

 air being carried to all the tissues in a system of tubes, called 

 trachea?, from their cavity being kept open by a spiral fibre, 

 or more often by a series of rings which run into each other so 

 as to appear spiral. These tubes open externally by pores called 

 spiracles, and are merely an involution, so to speak, of the proto- 

 derrn or external layer of the skin ; they ramify in the interior 

 of the insect, dividing in an arborescent manner, until at last 

 they attain the minute diameter of something like 100,000th of 

 an inch, when they anastomoze freely with each other and form 

 a network of tubules over the surface, and in the substance, of 

 every organ. 



The trachea! tubes consist of two coats. The internal, in the 

 substance of which the spiral fibre is developed, corresponds 

 to the protoderm, and like it, is formed from cells, which in the 

 case of the perfect fly originate in the pupa case, in the earliest 

 stages of the formation of the imago, Similar rings to those 

 in the tracheae are developed, either from cells or nuclei of the 

 protoderm, in the false tracheae of the proboscis, an important 

 fact, as it is confirmatory of the view I have taken of their 

 homologies.* The external layer is easily detected before 

 the fly attains its full development ; it is thick and struc- 

 tureless at first, containing nuclei, and thickest in their vicinity; 

 it seems to be a mere collection of protoplasm. As the 



* The origin of the rings of the false trachcaj is even better seen in the 

 tongue of the Cricket, where the nuclear form is retained throughout life, 

 near the edges of the organ ; this was first pointed out to me by my 

 friend, Mr. C. Stewart. 



