The Tracheal l^ystciii of Simulium. 711 



cuticular gill of the pupa of Simulium as a pitted 

 stigma. 



The development of the pupal gill begins in the young 

 larva about the same time as that of the imaginal rudi- 

 ments. It is noteworthy that this early origin of the gills 

 in Simulium is not paralleled in Chironomus, where Miall 

 and Hammond (No. 4, p. 124) find that "the dorsal pro- 

 thoracic rudiments, from which the pupal tracheal gills of 

 Chironomus proceed, are the last to be developed," and 

 that it " is not till the larva is almost full-grown, and long 

 after the other thoracic appendages are visible, that they 

 appear." In the case of Simulium, the pupal gill and the 

 imaginal rudiments make their appearance together. Of 

 these, the dorsal prothoracic rudiment arises as an in- 

 vagination of the epidermis, which takes place at the 

 hinder end of the prothorax, immediately in front of the 

 mesothoracic spiracle. The invagination gives rise to a 

 shallow depression, lying at the side of the prothorax, 

 whose wall becomes the sheath of the developing gill. The 

 sheath, as seen from the outside, is semicircular in outline, 

 with a straight and a curved margin. The straight margin 

 runs obliquely upwards and forwards from the mesothoracic 

 spiracle, and the rounded margin lies towards the anterior 

 side. The wall bends sharply inwards along the straight 

 edge, and is formed of a thickened epidermis. At its dorsal 

 end an outgrowth of the thickened epidermis develops, 

 and projects forwards and downwards into the cavity of the 

 sheath as the rudiment of the gill. It soon divides more 

 than once, and the branches become coiled up within the 

 sheath. The gill-rudiment is hollow, and the cavity is 

 continuous with the cavity of the body. 



Dorsal to the gill-base and immediately behind the 

 sheath a second invagination of the epidermis gives rise to 

 a groove leading from the gill-base to the mesothoracic 

 spiracle. The groove closes to form a tube (the tracheal 

 extension), one end of which becomes continuous with the 

 epithelium of the spiracle-chamber ; the other ends blindly 

 immediately below the gill-base. The epithelium of the 

 tube secretes an intima on its free surface, which shows a 

 retiform thickening. A thickening of the epithelium of 

 the gill-base, which almost obliterates the cavity, grows 

 inwards to meet the tracheal extension. The chitinous 

 wall of the gill then becomes differentiated. The superficial 

 layer with the fibrilla3 forms first, and the homogeneous 



TRANS. ENT. SOC. LOND. ir>02. — PART IV. (dEC.) 47 



