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 Palmen 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 Palmen 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 maybe 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 



