EXTERNAL ANATOMY OF INSECTS. 497 



of this covering is the same with that of the varnish of 

 the choroid in the eyes of vertebrate animals ; but it en- 

 tirely covers the underside of the lens, without leaving 

 any passage for the light. Below this varnish there are 

 numbers of short white hexagonal prisms a , every one of 

 which enters the concavity of one of the lenses of the 

 cornea, and is only separated from it by the varnish just 

 described : this may be considered as the retina of the 

 lens to which it is attached ; but at present it has not been 

 clearly explained how the light can act upon a retina of 

 this description through an opaque varnish. Below this 

 multitude of threads (for such the bodies appear), per- 

 pendicular to the cornea, is a membrane which serves 

 them all for a base, and which consequently is nearly pa- 

 rallel with that part. It is very thin, of a black colour, 

 not produced by a varnish ; and in it may be seen very 

 fine white tracheae, which send forth branches still finer, 

 that penetrate between the prisms of the cornea : this 

 membrane may be called the choroid. Behind this is a 

 thin expansion of the optic nerve, which is a true nerv- 

 ous membrane, precisely similar to the retina of red- 

 blooded animals. It appears that the white pyramidal 

 threads which form the retina of each lens are sent forth 

 by this general retina, and pierce the choroid by a num- 

 ber of almost imperceptible holes b . From this descrip- 

 tion it appears that the eyes of insects have nothing cor- 

 responding with the uvea or humours of those of verte- 

 brate animals, but are of a type peculiar to themselves. 

 Having explained to you the wonderful and complex 



a Plate XXIII. Fig. 3. 



b Cuvier Anat. Compar. ii. 442—. Compare Swummertlam Bibl. 

 Nat i. 211. t. xx. /. 45. 



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