THE COMPOUND EYE. 5^5 



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 riot split into longitudinal 

 segments. 



The so-called axial threads, or longitudinal striae, are not 

 axial at all, but are mere ridges in an elastic sheath (PI. 

 XXXVI., Fig. 2, b and c) and are so placed that they could 



A. 



FIG. 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 ; d d, so called axial threads ; C, the rhabdome still surrounded by its 

 protoplasmic sheath; b l>, vacuoles ; c, granules ; d, so-called axial threads; 

 <?, 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. 



