THE TR AC HEAL SYSTEM OF THE BLOW- FLY. 



369 



the reticular connective tissue, which are also formed from the 

 proliferating cells of the peritoneal coat of the new tracheal 

 vessels. The process is intrinsically identical with that by 

 which the principal tracheal trunks are formed in the embryo ; 

 except that the germs of the new vessels are developed as 

 buds, instead of being directly differentiated from the parablast. 

 The Posterior Spiracles of the newly-hatched larva are 

 circular orifices in the transverse stigmatic furrow (Fig. 41), 

 but at the first moult these simple spiracles are replaced by 

 the stigmatic plates. Each of these is at first perforated by 

 two slit-like openings, but at the next ecdysis a third slit-like 

 opening is developed between them, and the stigmatic plate 

 assumes the characters of that of the adult iarva already 

 described (see p. 49). 



FIG. 49. The Posterior Spiracles of the Embryo and Larva : /, the posterior 

 spiracles of the fully-developed embryo, showing the bulbiform enlargements of 

 the peritoneal coat of the main tracheae from which the second set of tracheae are 

 developed ; 2, the posterior spiracles of the larva after the first ecdysis ; j, the 

 posterior spiracles of the larva after the second ecdysis ; sf, the first ; sf-, the 

 second, aad sf*, the third spiracles. 



The Stigmatic Plate is developed from the hypodermis, and 

 exhibits a scar on its inner edge produced by the closure of 

 the circular spiracle of the mature embryo. The second spiracle 

 is not formed by a modification of the first, but is a new 

 formation. In the newly-hatched larva the great tracheal 

 trunk exhibits a bulb-like projection of its peritoneal coat. 

 This is at first solid, but ultimately becomes hollow ; its cavity 

 is continuous with a space formed by the separation of the 

 peritoneal coat of the tracheae from its intima. It is in direct 

 relation with the hypodermis. 



252 



