THE ORGANS OF THE SENSES. 293 



the case in all insects with an imperfect metamorphosis. Nevertheless, 

 each lens is flatter in them than in other insects, and we must here 

 consequently regard every individual lens as cut at its margin, so that 

 merely the central most elevated portion remains. Were this not the 

 case in thick and flattish lenses, objects would necessarily appear 

 indistinct. In Gryllus hieroglyphicus, Joh. Mailer * detected the 

 proportion of the breadth to the thickness to be 1 : 7- The space at 

 the circumference of the facets is covered by the pigment collected 

 between the filaments of the optic nerve, so that each individual facet 

 is surrounded with a ring of pigment or kind of iris ; the disk, how- 

 ever, remains free and transparent. Upon the superior surface we 

 occasionally observe, particularly in the bees and flies, fine hairs pro- 

 jecting, which may be considered as analogous to the eye-lashes, as 

 they doubtlessly prevent the approach of external bodies, but at the 

 same time limit the visual circle of each facet to the space itself 

 occupies. 



Upon the inner surface of each individual lens we find a transparent 

 crystalline cone, the convex surface of which touches merely the centre 

 of each facet, but leaves a small space around the circumference free 

 for the ring of pigment. The circumference of each of these cones is 

 for a certain space not inclined but perpendicular, thus giving the 

 crystalline body a more cylindrical form, which, however, gradually 

 diminishes, and they internally run to a point, to which a delicate 

 filament of the radiating optic nerve passes. The pigment or peculiar 

 colouring matter, which occupies the whole inner space of the eye, 

 passes between these cones, enveloping the filaments of the optic nerve 

 as far as the facets forming the iris around the circumference of the 

 base of the cone. In this manner each individual facet with its crys- 

 talline body is separated from the other, and may therefore be considered 

 as a distinct eye. The length of these cones varies not only in different 

 insects, but often in the same, from its position being either marginal 

 or central. We may consider, in general, that, in such eyes which 

 form no segment of a circle, those cones which are found at the flattest 

 part of the eye are the longest, and the others situated at the more 

 convex parts, the shortest, but the basal surface of the cone does not 

 vary, but is always regulated by the form of the facet. Their length 

 cannot be precisely determined, but, in such eyes which form the 



* Where cited above, p. 241. 



