372 THE TRACHEAL SYSTEM OF THE IMAGO. 
portion of the intima and of the whole stigmatic cornu. The 
new tracheal trunk is at first constricted at its orifice, where 
the peritoneal coat remains in contact with the old intima, but 
internally to this point it becomes widely separated from the 
intima by fluid (see Pl. XXII.). 
The stigmatic sac is developed from the invaginated hypo- 
derm, and the outer valve-plates are also formed from the 
hypoderm. The internal valve is probably developed from 
the constricted orifice of the new trachea. 
The Order of Succession of the Anterior Spiracles in the larva 
nymph and imago is as follows: The stigmatic cornu of the 
prothorax of the larva is shed with the larval integument which 
forms the pupa-case, but is replaced by the newly-formed stig- 
matic cornu of the pronymph. This resembles the respiratory 
siphon of Culex very closely; it is developed from the dorsal 
prothoracic imaginal disc, and is shed with the pupa-sheath. 
Before the shedding of the pupa-sheath, however, a third 
spiracular apparatus appears behind and below the stigmatic 
cornu of the pronymph. This is the inter-segmental spiracle 
of the nymph. It is a digitate cornu similar to, but more 
simple than, the stigmatic cornu of the larva, inasmuch as 
it has fewer digitations. It is developed in relation with a 
new tracheal vessel given off behind the prothoracic spira- 
cular trunk of the larva. 
Lastly, a new spiracle replaces this, the anterior thoracic 
spiracle of the imago, formed by the invagination of the cellular 
integument of the nymph in immediate relation with it, and 
with the tracheal trunk which opens through it. 
The existence of two sets of spiracles which are shed in the 
pupa stage, the stigmatic cornua of the pronymph, and the 
digitate inter-segmental spiracles of the nymph, indicate the 
existence of two virtual ecdyses, resulting in the pupa-sheath, 
and not one as has hitherto been supposed—the separation of 
the stigmatic cornu from the prothorax certainly occurs at an 
earlier period than that of the inter-segmental spiracle, and 
the latter appears to be covered by the pupa-sheath in some 
preparations (see Pl. XX., Fig. 3). 
