370 Till-. TRACHEAL SYSTEM OE THE IMAGO. 



The exact relative share taken by the hypodermis and the 

 bulbous enlargement of the trachea in the formation of the 

 second spiracle is not known, but analogy renders it probable 

 that the vestibule originates from the hypodermis, as well as 

 the stigmatic plate. 



Anterior Spiracle. The development of the anterior spiracle 

 in the larva is not easily studied ; but if it is developed in the 

 sanu' way as that of the nymph, which is most probable, the 

 process must be very nearly as follows. 



The spiracular tracheal trunk is probably developed in the 

 usual way, by the proliferation of the peritoneal coat of a 

 lateral cutaneous branch of the main trunk, which first appears 

 as a solid bud ; and the hypodermis on the outer surface of 

 this bud is then invaginated. The finger-like processes are 

 developed within the invagination. Subsequently the invagi- 

 nation is turned inside out and forms the larval stigmatic 

 cornu. Hence there is no spiracular sac, and the outer surface 

 of the cornu apparently represents it. 



The question as to whether the stigmatic cornua of the 

 dipterous larvae and nymphs should be regarded as gills or 

 as spiracles, has been the subject of some difference of opinion. 



Weismann considered the anterior spiracular apparatus of 

 Corethra to be intermediate between the digitate spiracular 

 cornua of the larva in Musca and the tracheal gills of some 

 Tipulidae, and Palmen goes even further [153] and says : ' The 

 prothoracic horns of Corethra are identical with those of 

 Culex; these are not stigmata but true tracheal gills, which are 

 shed with the exuviae, and take no part in the formation of the 

 prothoracic spiracles of the imago.' \Yhether these respiratory 

 appendages are closed as Weismann and Palmen believed, and 

 as those of the Blow-fly apparently are in the adult larva, or 

 whether they are open and functionally spiracles, as Hurst has 

 recently shown them to be in Culex [155]; it is certain that they 

 are ^Ix-'l and are not converted into the spiracles of the imago. 



This fact is of interest in relation to Palmen's discovery 

 that the spiracles of the imago in the Ephemeridae are de- 

 v loped independently of the tracheal gills, and are not, as was 



