SIGHT 



43 



Fig. 1056. Head of Male Honey- 



Bee (Apis </#&) and Beginning of 



of its home, and the eyes are under the shadow of the large 

 conical shell. Under such circumstances complex visual organs 

 are unnecessary. 



PICTURE- EYES. The development of refracting structures in 

 direction-eyes has led to the possibility of further specialization in 

 vision, and has resulted in what we may 

 call Picture- Eyes, capable of giving more 

 or less definite information about the form 

 and colour of external objects. Two kinds 

 of these may be distinguished, i.e. Com- 



j -T-* , f-^ -r-i 



pound Jiyes and Camera rLyes. 



Compound Eyes are characteristic of a 



great many Arthropods, SUCh aS Lobsters **d three smaU simple eyes near then- 

 IS J bases, but the most conspicuous struc- 



and Crabs, where they are placed at the turesare the enormous compound e yes , 



_ with their minute hexagonal facets. 



end of stalks, and Insects, where they are 



in the form of two large projections on the head (fig. 1056). 



Examination with a lens shows that such an eye is covered by 



a transparent patch of the hard covering of the body, which is 



divided into a multitude of minute square or polygonal areas, 



commonly known as facets. These may be exceedingly numerous, 



as will be seen from the following calculations made by Leeuwen- 



hoek more than a century ago: ^ __ m 



house-fly, 4000; gadfly, 7000; goat- 



moth, 11,000; death's-head moth, 



12,000; swallow-tail butterfly, 17,000; 



dragon-fly, 20,000; a small beetle 



(Mordella), 25,000. It was origi- 



nally believed that these elaborate 



structures were aggregates of simple 



50 



eyes, acting independently; and they 



., r 1 1 i i are ncae, eac conssng o exerna rerac- 



Wd'e therefore Called COmpOUnd ing structures (r., r.) and internal groups of retinal 



eyes, a rather misleading term. Sec- 

 tions through such eyes (fig. 1057) 



o / V o // / 



have demonstrated that each facet 

 is the outer end or base of a. very slender visual pyramid 

 (ommatidium), the external part of which consists of various 

 refracting structures, while internally is a group of sensitive 

 visual cells connected with nerve - fibres. Adjacent pyramids 

 are optically separated from one another by means of pigment. 

 Comparison of various compound eyes shows that there are 



Fi s- io s7 .Diagram of a compound E ye , in 



section, enlarged, to illustrate theory of "mosaic 



vision". Numerous radiating visual pyramids 



are indicated, each consisting of external refract- 



ro an exte al ob J ect is indicated for 



three pyramids; o., optic nerve. 



