530 THE SENSES AND SENSOKY 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 frequent!} 7 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. I, i) 

 around the apex of the cone. The inner set (n 3 ) 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 Tracheae of the Dioptron. There are a number of fusiform 

 thick -walled trache;e between the great rods. These are 

 arranged with great regularity (PI. XXXVIII., Fig. 7, C. tr). 

 In some preparations they appear as dilated thin-walled sacs, 

 in others as thick-walled trachea; with a narrow lumen. The 

 distension of the tracheae accompanies the shrivelling of the 



