THE TRACHEAL 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 larva already 
described (see p. 49). 
FIG. 49.-—The Posterior Spiracles of the Embryo and Larva: z, the posterior 
spiracles of the fully-developed embryo, showing the bulbiform enlargements of 
the peritoneal coat of the main trachez from which the second set of trachezx are 
developed ; 2, the posterior spiracles of the larva after the first ecdysis ; 3, the 
posterior spiracles of the larva after the second ecdysis ; s¢, the first; s¢2, the 
second, and s¢%, 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 trachez from its intima. It is in direct 
relation with the hypodermis. 
25—2 
