ORGANS OF THE SENSES. 255 



corneee, and their pointed apex is turned inwards to the pe- 

 ripheral or retinal ends of the separate minute optic nerves 

 (101. A. c.) These small conical horny lenses are easily 

 scraped off from the inner surface of the cornea when their 

 regular form and smooth surface (101. B. b, b,) are best ex- 

 amined, and they acquire a whitish opaque colour by the ac- 

 tion of alcohol, like the fibrous lenses of the higher classes 

 of animals. They are surrounded by the choroid pigment, 

 and in some insects, as the libellula, this coloured choroid 

 forms a sort of iris or uvea between the flat base of the 

 lenses and inner surface of the cornese, and appear to leave 

 space for a small quantity of aqueous humour. 



The lenses are very long in the eyes of the libellulse, 

 where they form lengthened slender nearly parallel cylinders 

 tapering very slightly to the retinal ends of the optic nerves. 

 They are generally shorter in proportion to their breadth in 

 other insects, like those of the melolontha (101. B. b, b.) The 

 separate slender optic nerves (101. A. c 9 ) proceed backwards 

 from the apices of the lenses, though the semifluid pigment 

 or vitreous humour, consisting of variously coloured globules 

 to the great optic ganglion within the globe of the eye, which 

 is almost a prolongation of the cerebral or optic lobes them- 

 selves (Fig. 83. D. 1.) The undulations or rays of light 

 have thus an uninterrupted passage in the compound eyes of 

 insects through the outer transparent homogenous epidermic 

 covering, then through the transparent central axes of the 

 prismatic hexagonal cornese, and lastly through the pupilar 

 openings of the choroid and the axes of the conical hard 

 lenses behind them, where they reach the terminal or retinal 

 papillae of the several optic nerves, by which the impressions 

 are felt or transmitted to the brain. 



Besides these compound eyes, insects in their mature state 

 often present at the back part of the head several simple de- 

 tached sessile eyes or ocelli, like those common in the infe- 

 rior articulated classes, and these are alone developed in the 

 larva-state of insects with perfect metamorphosis, as those 

 of orthoptera. In these ocelli of the larvae, as shown by 

 Lyonet in that of the cossus ligniperda, nearly the same 

 structure exists as in each of the component organs in the 

 compound eye of the perfect insect. There are six ocelli in 

 the cossus 9 disposed in a circular order on the parietal lamina, 



