530 THE SENSES AND SENSORY ORGANS. 
The conclusion at which I have arrived, from the appearance 
of the great rods before they contract and expel their contents, 
is that they consist of an elastic sac filled with a highly refrac- 
tive fluid. This sac is apparently divided by radial septa into 
six chambers. It frequently happens that parts of the tubes 
are more completely emptied than the rest, so that they exhibit 
the appearances represented in Fig. 68. I regard the rhabdome 
as a lens of great thickness and very short focus, which forms 
a magnified image of a part of the sub-corneal image, upon 
a retina situated beneath the basement-membrane of the 
dioptron. 
The Pigment Cells.—These cells are situated at the inner and 
outer ends of the great rods. 
The outer set form a kind of iris (Pl. XXXVI., Fig. 1, 2) 
around the apex of the cone. The inner set (m*) rest on the 
basilar membrane. Each set gives off a close series of parallel 
pigmented fringes, which interlock with those of the other set. 
The pigment is granular, and in the recent state has a brilliant 
crimson colour. In sections it usually assumes an orange- 
brown colour, but, in specimens fixed in absolute alcohol and 
stained with Erhlich’s logwood, the natural colour is very nearly 
restored. This is possibly due to the staining of the cells, and 
not to a restoration of the normal condition of the pigment. 
It indicates, perhaps, that the normal pigment is complex, and 
that it loses its blue element in spirit, chromic salts, etc., 
and that the blue of the logwood takes its place. Except in 
stained sections and recent eyes I have never seen the pigment 
crimson, it is always yellow or orange-brown. Light has no 
action upon this pigment, so far as its colour is concerned, but 
its position is undoubtedly changed by the intensity of the light 
to which the eye is exposed in the living insect (see p. 532). 
The Trachew of the Dioptron.—There are a number of fusiform 
thick-walled tracheze between the great rods. These are 
arranged with great regularity (Pl. XXXVIIL, Fig. 7, C. t). 
In some preparations they appear as dilated thin-walled sacs, 
in others as thick-walled trachez with a narrow lumen. The 
distension of the tracheze accompanies the shrivelling of the 
