Visual Organs of Insects and Crustacea, 221 



of hot climates, the eyes are fixed upon filaments, yet even 

 here they invariably maintain the same position in relation to 

 each other. In crabs, and some others of the Crustacea, the 

 eyes are movable, being placed upon footstalks. 



The compound eyes are two in number, but the males of 

 some JSphemeridae possess four. The two eyes are rarely 

 united together, so as to form one mass : this, however, is the 

 case in the males of som.e hymenopterous and dipterous in- 

 sects ; as in the genera Cyclops, Polyphemus, &c. More 

 frequently they are placed very near to each other ; as in some 

 dipterous insects, especially in the males of the genera Culex 

 and Jabanus, 



As the structure of the compound eyes of the articulated 

 animals has hitherto been either disregarded or misunder- 

 stood, it will, I think, be best to select for description a few 

 of the insects and Crustacea whose eyes appear to have been 

 most carefully investigated by Muller and others. This will, 

 it is presumed, render much more intelligible the succeeding 

 remarks on the diversities of form, &c., observed in the several 

 parts which enter into the construction of these compound 

 organs of vision. The compound eyes of the grey dragon- 

 fly have been very recently examined, apparently with great 

 care, by M. A. Duges; and my own observations, as far as 

 I have been able to carry them, confirm the accuracy of his 

 description r I shall begin, therefore, with the eyes of this 

 insect. 



Compound Eyes with Facets, — The compound eye of the 

 common, or grey, dragon-fly (Libellula vulgaris), when ex- 

 amined externally, may be divided into two parts ; one superior 

 and posterior, of an obscure red colour, and provided with 

 facets at least twice as broad as those of the other part, which 



is anterior and below, and 



^^ .<<^iiiitii2l7>\ ^^ ^ greyish tint. When a 



^<^^\m\ 11 l/y7^AK?\ section of the eye is made, 



f^^^^^^XMultLt //^^^\, ^® ^^^ behind the cornea 



/^^^^^wiiilw^^^^^^^VA (^* ^^' ^) ^ layer of black 



y^^^^^r^^^<^^<^^^^ pigment (c) ; then a broad 



^^^^^'^ \\ I / ^^^^3i ^^^^ (^^' orange-coloured 



C^^^ \\ \ I // £::;:— itrl posteriorly, and black in 



«^^^^^H //^^^^Wa ^^^^^» ^^^ ^ second zone 



i i I Y \ Y I inS^'^ " te)> situated within the 



6^ I A ^\aet ^^^^> ^^^ appearing to be 



./ nearly wholly of a somer 



what deep black liue. 



This latter immediately surrounds the white swelling ox 



ganglion of the optic nerve (j). Each of these parts shall 



now be described more in detail. 



