40 The Form of Insects 



The large conspicuous eyes of all adult insects 

 are compound eyes, such as have been described, 

 and no other kind of eye is present in the Cockroach. 

 But very many insects are provided with simple eyes 

 in addition to the compound eyes. Examining the 

 top of a Bee's head, for instance, we notice three 

 simple eyes {ocelli) (fig. :^l). The skin over such 

 a simple eye is transparent and swollen, forming a 

 doubly convex lens ; beneath this a cup-shaped mass 

 of elongate pigmented cells form a retina, each cell 



Fig. 30. — Section of part of optic tract of Blowfly, r. rhabdoms \/. fine branching 

 nerve-fibrils ; /. <?/. periopticon ; Nf. decussating nerve-fibres ; e op. epiop- 

 ticon with outer cellular layer and inner, radially arrayed nerve-fibres. 

 Highly magnified. After Hickson, Quart. Journ. Mic. Sci. vol. 25 (n.s.). 



ending above in a transparent rod directed toward 

 the lens and being connected below with a nerve- 

 fibril (fig. 32). The nerves in connection with these 

 simple eyes come ofF from the front upper region 

 of the fore-brain, independently of the nerve tracts 

 which supply the compound eyes. 



Few questions in natural history have given rise to 

 more discussion than the nature of insect vision. It 

 is generally believed that the lens of a simple eye 

 forms an inverted image of an outside object, which 

 is perceived, as in our own eyes, by means of the 



