The Tracheal System of Simulium. 



707 



Separating tlie superficial from the deep stratum is a space 

 about as wide as the deep stratum. This space is filled 

 with air. It is to the presence of air in the chitinous 



Fig. 4. 



Tracheal extension and base of pupal ^\\\. The gill-base is shown in vertical sect'on 

 tlie tracheal extension in surface view from the inner side, x 75. 



X, conimunication between gill and tracheal extension ; //, chitinous flbrillse ; ds, 

 deep stratum of wall of gill ; ss, superficial do.; ex, tracheal extension; -nis, meso- 

 tlioracio spiracle, with initial thread leadint; from the spiracle to the siiiracle-chaniber. 

 Tlie line a h indicates the plane of the transvei-se section (fig. 6). 



wall that the silvery appearance of the gill when viewed 

 by reflected light is due. Traversing this air-containing 

 space and connecting the superficial with the deep stratum, 

 are numerous delicate fibrilla3 (figs. 4 and 5,/). These 

 fibrillse are simple at their deep ends, but tend to branch 

 as they pass towards the superficial layer. They resist the 

 action of hot caustic potash, and are probably chitinous 

 like the rest of the wall. When the surface of a cleared 

 filament is viewed by transmitted light, the points 

 where the fibrilloe meet the superficial layer appear as 

 bright dots which 

 simulate pores. No 

 apertures, however, 

 occur in the wall of 

 the gill. 



The gill - stem 

 differs from the rest 

 of the gill ill the 

 arrangement of its 

 chitinous layers. 

 Four external sur- 

 faces may be distin- 

 guished : an upper, 

 a lower, one turned 

 towards the axis 

 of the body, and 

 another turned 



2>W 



Fig. 5. 



Transverse section of pupal gill, while still within the 

 larval skin, x 300. 



vto, compact ventral wall ; ss, superficial stratum ; (Zs, 

 deep stratum ; ep, epithelium ; /, branched chitinous 

 HbrilUie. 



