THE TRACHEAL SYSTEM OF THE BLOW-FLY. 371 
formerly supposed, openings left by the shedding of the 
gills. 
The researches of Dewitz [159], which show that the tracheal 
gills are developed from spiracles, render it probable that the 
stigmatic cornua of dipterous larvae are homologous with 
tracheal gills, as Palmén supposed them to be. The question 
whether they are pervious or not is a secondary one, and it 
is possible that they may be pervious only for a short time. 
They are certainly closed when they are first formed, so that 
it is probable that Hurst and Palmén examined insects in 
different stages of development. 
Development of the Stigmatic Cornua of the Pronymph.—The 
great cornu of the prothorax of the pronymph is undoubtedly 
developed, as has been already stated, from the upper pro- 
thoracic disc. It first appears as an invaginated sac surround- 
ing the tracheal trunk, which connects the anterior stigma of 
the larva with the great lateral trachea. This sac afterwards 
encloses the cuticular cornu, which comes to the surface as the 
prothoracic disc is everted ; it is indubitably the homologue of 
the great stigmatic cornu of the nymph of Culex, which it also 
closely resembles. It is shed at a very early period in the 
development of the nymph, and no new spiracle is developed in 
relation with it. ; 
The Development of the Stigmatic Intersegmental Cornua of the 
Nymph.—The manner in which the intersegmental cornu of 
the nymph is developed may be easily studied. A new tracheal 
trunk originates by the enlargement of the distal extremity of 
a cutaneous tracheal vessel, in the vicinity of the old spiracle. 
The wall of this vessel is separated from the old intima by 
fluid. The hypodermis is then invaginated in relation with 
the enlarged trachea, and the finger-like extremities of the 
cornu are developed within the invagination; this is afterwards 
everted, and forms a new digitate stigmatic cornu. 
Development of the Anterior Spiracles of the Imago.— The 
anterior spiracles of the imago are developed in immediate 
relation with the stigmatic cornua of the nymph, by the further 
invagination of the integument and the ecdysis of the external 
