68 GNATS OR MOSQUITOES — CHAPTEK V 



and form has led to their beinj^ termed horns or trumpets. 



The wings are nearly flat, oblong plates, arising behind 

 the bases of the syphons, and extending downwards and 

 backwards. 



Immediately behind them are a pair of triangular plates, 

 enclosing the halteres of the future gnat. The legs are 

 mostly hidden by the wings, but the femur, tibia and first 

 tarsal joint of the first leg, and the tibia and first tarsal of 

 the second are visible. The respiratory syphons are nearly 

 cylindrical, narrowed at their bases, and curved forwards 

 to be attached by flexible membranes to slight prominences 

 on the sides of the pro thorax. Above, they are obliquely 

 truncate and open, and the margin is slightly notched on 

 the inner side. The outer surface is marked so as to 

 resemble imbricated scales, each with a minute spine at 

 its apex. The cavity of the syphon communicates directly 

 with the tracheal trunk at its base. Palmen (" Zur Mor- 

 phologie des Tracheen systems," Helsingfors, 1877) has 

 denied the communication of the syphons with the trachea?, 

 and imputed to them the function of " tracheal gills " ; but 

 apart from the fact that their dense chitinous structure 

 renders them entirely unsuitable for the performance of any 

 such function, the reality of their communication with the 

 trachea3 can easily be proved by watching the imbibition 

 of suitable fluids through the syphons into them. All these 

 appendages originate as protrusions of the epidermic layer, 

 enclosing mesoblastic tissue. Those of the dorsum are all 

 at first flat, wing-like plates, but those of the mesothorax 

 alone retain this form as the wings of the adult, while 

 the halteres become club-shaped, and the anterior appen- 

 dages become rolled up to form the syphons of the pupa, 

 only to disappear on attaining the adult form. The legs, 

 on the other hand, appear from the first as cylindrical 

 processes. They are at first unjointed, but by the end of 

 the pupal period have segmented themselves into the 

 various joints of the adult. 



The abdomen is flattened dorso-ventrally, and when at 

 rest is curved under the thorax. It is jointed and flexible, 

 and forms with the pair of large fins, borne by the eighth 



