ORGANS OF SPECIAL SENSE. 



dening, be adopted.) Beneath the crystalline cones 

 we have the great rods (rhabdia), consisting of several 

 straight chitinous threads, partly fused together 

 {gr, D), which pass inwards towards, and actually 

 perforate, the basilar membrane, which is represented 

 by a line running across the lower part of the fan- 

 like form, in the section C. These rods are surrounded, 

 throughout their length, by eight retinulae, about 

 which are placed pigment cells, preventing the wan- 

 dering of light from one optic element to another. 

 This will be best understood by the cross section 

 of the rods (F). The crystalline cones, for a similar 

 purpose, are protected by pigment cells, which have 

 also greater density at the upper, lower, and middle 

 portions of the rhabdia (marked ppp, C). Between 

 these microscopic telescopes, pointing in every direc- 

 tion, run long and perfectly straight tracheal tubes, 

 which find their entrance by passing through per- 

 forations in the basilar membrane. Immediately 

 behind the basilar membrane lies a complex nerve 

 structure, which Dr. Sydney J. Hickson,* in his 

 admirable paper, denominates the periopticon ; 

 thence, running backwards, a bundle of optic nerve 

 fibrils {on, C), decussate (or cross), and then enter 

 a ganglionic swelling — the epiopticon, if we follow 

 the nomenclature of Dr. Hickson. Yet another 

 bundle of decussating fibrils brings us to the opticon 

 {<r in our figure), beyond which lies the cerebrum, 

 described and illustrated at page 53. The structures 

 united by the decussating fibrils are complex, and 



#"The Eye and Optic Tract of Insects." The Quarterly Journal of 



Microscopical Science, April, 1885. 



L 



