THE COMPOUND EYE OF THE BLOW-FLY. 529 



The pseudo-cone lies immediately beneath the corneal lens, 

 and the outer end of the axis of the rhabdome is imbedded in 

 its apex. 



The Great Rods (PL XXXVI. , Fig. i, rh) occupy the greater 

 part of the dioptron ; each rod extends from the apex of a 

 pseudo-cone (c, c) to the basilar membrane (m b), and consists 

 of an axial rhabdome (rh} and its sheath. Five pigment cells, 

 the iris cells (i), surround its outer extremity ; a similar number 

 (n 3 ) are seen at its inner extremity on the basilar membrane. 

 Both sets of cells give off pigmented fringes, which lie upon the 

 surface of the great rods, those of the iris cells meeting and 

 interlocking with those of the inner pigment cells. 



The great rods in transverse section (PI. XXXVI. , Fig. 2) 

 exhibit a distinct lumen, surrounded by a transparent wall. 

 The wall stains readily with carmine stains, and has the 

 appearance of granular protoplasm ; it is not stained by nuclear 

 stainvs. 



On the inner surface of the transparent wall there is a deli- 

 cate, apparently cuticular membrane, resembling the intima 

 of a tracheal tube ; but, instead of transverse thickenings, it 

 exhibits six or seven longitudinal threads. In the recent state, 

 and in well-preserved sections, the lumen of the great rod is 

 very distinct, but there is a strong tendency for the sheath to 

 contract ; in many transverse sections this contraction is so 

 marked that the axis of the great rod appears to be completely 

 occupied by the highly refractive dots, which are the transverse 

 sections of the fine longitudinal threads. Under these circum- 

 stances the great rods are seen to be separated from each other 

 by considerable spaces, but in good preparations they are only 

 separated by very narrow ones, owing to their larger diameter 

 and the considerable lumen they enclose. 



Grenadier's figures almost invariably represent the lumen 

 more or less contracted by the infolding of the inner cuticular 

 layer. 



Very generally in the fully formed imago each great rod 

 exhibits a single nucleus, midway between its ends, but in an 

 earlier stage there are usually several nuclei at its extremities. 



352 



