^34 INSECT MISCELLANIES. 



at least twice as broad as those of the other part, 

 which is anterior and below, and of a greyish tint. 

 When a section of the eye is made, we see behind 

 the cornea (a) a layer of black pigment (c) ; then a 

 broad zone (e), orange-coloured posteriorly, and 

 black in front ; and a second zone (g-), situated 

 within the first, and appearing to be nearly wholly 

 of a somewhat deep black hue. This latter imme- 

 diately surrounds the white swelling or ganglion of 

 the optic nerve (J). Each of these parts shall now 

 be described more in detail. 



" The cornea («) is thickest at the posterior part of 

 the eye ; the facets there being about four times as 

 thick as they are broad ; in that part also it may be 

 readily seen that each facet is separated from the 

 adjoining ones by an opaque line, a kind of suture, 

 which gives to the whole of this transparent layer a 

 bluish tinge, and thus softens, when the eye is exa- 

 mined externally, the intense colour of the pigment 

 beneath. 



*'The black pigment (c) forms a layer of a very 

 dark colour ; but its thickness is not so great as that 

 of the cornea. Miiller very justly regards it as iden- 

 tical with the pigment situated more deeply in the 

 eye. At a first and cursory examination it might 

 very readily be supposed that this layer is perfectly 

 continuous beneath the cornea, so as to intercept 

 completely the passage of light to the parts within 

 it ; but a careful removal of the internal structures 

 of the eye, leaving this pigment untouched, will 

 show that, although very thick at the sutures of the 

 facets, where it is continuous with the pigment of the 

 more internal textures, it becomes, towards the cen- 

 tre of each facet, exceedingly thin, and at the very 

 centre no pigment can be seen ; a minute perfora- 

 tion, as it were, in the layer being there observed. 

 If a cornea, with its layer of pigment still attached 



