98 



EYES 



the structures of the Insect, and in the higher and more active 

 forms, such as the Dragon-flies and hover- 

 ing Diptera, attain a complexity and deli- 

 cacy of organisation that elicit the highest 

 admiration from every one who studies 

 them. They are totally different in 

 structure and very distinct in function 

 from the eyes of Vertebrata, and are 

 seated on very large special lobes of the 

 brain (see Fig. 65), which indeed are 

 so large and so complex in structure 

 that Insects may be described as possess- 

 ing special ocular brains brought into 

 relation with the lights, shades, and 

 movements of the external world by a 

 remarkably complex optical apparatus. 

 This instrumental part of the eye is 

 FiG.53.— Twoomraatidiafrom called the dioptric part in contradistinc- 

 c':, 7ieo. "iSSr) tion from the percipient portion, and con- 

 a, Cornea ; 6, crystalline gigts of ail outer Corneal lens (a, 1 ig. 5 3 ), 

 f:n":stVat:' :!::^J.^ whose exposed surface forms one of the 

 nerve structures below it ; facets of the eye ; Under the lens is placed 

 pignt'?:'""*'^' "'""" the crystalline cone (6), this latter Ijeing 

 borne on a rod-like object (c), called the 

 rhabdom. There are two layers of pigment, the outer (e), 

 called the iris -pigment, the inner (/), the retinal- pigment ; 

 underneath, or rather we should say more central than, the 

 rhabdoms is the fenestrate membrane (d), beyond which there 

 is an extremely complex mass of nerve - fibres ; nerves also 

 penetrate the fenestrate membrane, and their distal extremi- 

 ties are connected with the delicate sheaths by one of which 

 each rhabdom is surrounded, the combination of sheath and 

 nerves forming a retinula. Each set of the parts above the fene- 

 strate membrane constitutes an omniatidinm, and there may be 

 many of these ommatidia in an eye ; indeed, it is said that the 

 eye of a small beetle, if or delta, contains as many as 25,000 

 ommatidia. As a rule the larvae of Insects with a complete 

 metamorphosis bear only simple eyes. In the young of Dragon- 

 flies, as well as of some other Insects having a less perfect meta- 

 morphosis, the compound eyes exist in the early stages, but they 



