STRUCTURE OF THE OPTIO LOBES. 153 



on each side of the head five or six eye-spots, contain- 

 ing each a crystalline body, but, as we shall presently 

 see, they can probably do little more than distinguish 

 between light and darkness. 



I do not propose to attempt to give here any detailed 

 account of the structure of the insect brain, but I must 

 say a few words on the subject. Between the brain 

 proper and the eye itself there are, in, for instance, the 

 blow-fly {Musca vomitoria), three distinct ganglionic 

 swellings, which Hickson, a copy of whose beautiful 

 figure I have given (Fig. 101), terms the"opticon" 

 (op), epiopticon (cop), and periopticon (p.op). It will 

 be seen that the nerve-fibrils do not pass in a direct 

 course, but actually decussate, or cross from one side to 

 the other, three times, once between each two ganglionic 

 swellings. The optic lobes of the two sides are also con- 

 nected by a fibrous bundle. The structure of the three 

 nervous swellings is also very complex. It consists of a 

 fine granular matrix, traversed by a mesh work of very 

 minute fibrillae, and, at least in the periopticon, is col- 

 lected into a series of cylindrical masses. It is entirely 

 beyond our present range of knowledge to explain the 

 origin or purpose of these complex arrangements, 

 though we cannot doubt that they do serve important 

 functions. It is remarkable that these arrangements, 

 though apparently very constant in individual species 

 and genera, differ greatly in different groups of insects ; 

 for instance, Hickson asserts that in the water-scorpion 

 (Nepa), there is no decussation, and Carriere makes the 

 same statement as regards Libellula; but it seems 

 very extraordinary that this arrangement should be 

 present in some insect eyes, and absent in others 

 formed apparently on so nearly the same plan. 



