THE COMPOUND EYE. 525 
stabchen of Grenacher. In the living insects these form the 
segments of an ovoid lens divided by longitudinal septa. 
In the Blow-fly the uninjured great rods before they undergo 
any post-mortem change appear as very long, highly-refractive, 
cylindrical bodies, the anterior extremities of which project into 
the pseudo-cones, and are nearly hemispherical. These rods 
when dissociated become twisted or curled, and exhibit pheno- 
mena similar to those which I observed in the Plume Moth 
(Pterophorus) except that they do not split into longitudinal 
segments. 
The so-called axial threads, or longitudinal striz, are not 
axial at all, but are mere ridges in an elastic sheath (Pl. 
XXXVI., Fig. 2, b and c) and are so placed that they could 
F'1G. 68.—Isolated portions of the rhabdomes of a Blow-fly in various stages of dis- 
integration. A and B, isolated portions of the internal membrane ; a, filled with 
fluid; dd, so called axial threads; C, the rhabdome still surrounded by its 
protoplasmic sheath; 4 é, vacuoles; ¢, granules ; d, so-called axial threads ; 
e, sheathing substance. 
not be affected by an axial pencil of light. I shall, however, 
return to this subject when I discuss the optics of the com- 
pound eye. 
I would especially draw attention to Grenacher’s figures 
[222, Figs. 62, 63, 80-82, 85, 88, and 126] and his optical 
longitudinal sections [222, Figs. 44, 47, 49, and 65], as these 
are quite consistent with the views here adopted and the phe- 
nomena observed by me, and they ill accord with the views 
he has put forward as to the nature of the great rods in his 
TEXt. 
