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496 EXTERNAL ANATOMY OF INSECTS. 
varnish there are numbers of short white hexagonal 
prisms*, 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), perpendicular to the cornea, is 
a membrane which serves them all for a base, and which 
consequently is nearly parallel 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 trachea, 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 nervous 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 number of almost imperceptible 
holes’. From this description it appears that the eyes 
of insects have nothing corresponding with the wvea or 
humours of those of vertebrate animals, but are of a type 
peculiar to themselves. 
Having explained to you the wonderful and complex 
structure with which it has pleased the Creator to di- 
stinguish the organs of vision of these minute beings, 
proving, what I have so often asserted, that when ani- 
* Pirate XXIII. Fic. 3. 
> Cuvier Anat. Compar. ii. 442 —. Compare Swammerdam Bid/. 
Nat. i, 211. t. xx. f. 45. 
