114 BEES AND BEE-KEEPING. 



difficult of examination. The periopticon is made up 

 of a number of cylindrical elements, seen just beneath 

 the basilar membrane (C), and consisting mainly of 

 divisions and sub-divisions of the decussating fibrils, 

 traversing a granular matrix, and for which structure 

 Dr. Hickson proposes the name of neurospongium. 



All physiological students know perfectly that, in 

 our own eye, by example, vision depends upon the 

 presence of nerve end cells, which lie behind the 

 expansion of the fibres of the optic nerve. The 

 retinulae, previously mentioned, have been very care- 

 fully investigated by Grenacher,* and many other 

 naturalists, who have, with great unanimity, regarded 

 these structures as the nerve end cells of insects ; but 

 Mr. B. T. Lowne has recently published a treatise,t in 

 which he endeavours to show that the true nerve 

 end cells are situated behind the basilar membrane, 

 in the periopticon of Hickson. The controversy is 

 beyond our limits, and those desiring to follow it may 

 consult the works mentioned in the footnotes. 



It is clear at once that the multitude of simple 

 eyes (directed to almost all points of the horizon), 

 which, by partial fusion, constitute the compound eye 

 of insects, permits a far wider range of vision than 

 would have been possible with a simple fixed eye ; 

 but difficulties have been felt in explaining how these 

 parts produced a single true impression of surround- 

 ing objects. Miiller suggested that each ocellus saw 

 only the point just before it, and so a picture was 



* " Untersuchungen iiber das Sehorgan der Arthropoden." 

 f " Compound Vision and the Morphology of the Eyes of Insects." 

 Linnsean Society's Transactions, Second Series, vol. ii. } Part n. 



