STRUCTURE OF THE COMPOUND EYE. 149 



The size of the facets seems to bear some relation to 

 the size of the insect, but even in the smallest species 

 none have been observed less than 20*^^ of an inch in 

 diameter. Butterflies, which fly in the day, have the 

 facets smaller than those of moths, which are generally- 

 evening insects. 



The facets are in most cases similar, six-sided, and 



S^Sht.. 



Fig. 101.— Section through the eye of a fly (after Hickson). b.m, Basilar membrane ; 

 c, cuticle; cop, epioptic ganglion; n.c, nuclei; n.c.s., nerve-cell sheath; N.f, 

 decussating nerve-fibres ; op, optic ganglion ; pc, pseudocone ; pg, pigment cells ; 

 p.op, perioptic ganglion ; r, retinula ; ,Eh., rhabdom ; T, trachea ; t.a., terminal 

 anastomosis ; Tt, trachea ; ti, tracheal vesicle. 



very regular. In locusts, however, they vary a good 

 deal both in form and size. In some flies (Diptera) 

 and dragon-flies (Libellulidse) those in the upper part 

 of the eye are larger than the lower ones, and the 

 junction of the two often forms a well-marked, curved 

 line. 



