960 



INSECTA. 



Organs of vision. The eyes of Insects are of 

 Iwo kinds, simple and compound, and have 

 been attentively examined by Professor Miiller, 

 Straus Durcklieim, Duges, and others, from 

 whose admirable investigations we shall chiefly 

 derive the following description. The large 

 convex cornea; which cover the external surface 

 of the head are divided, as we have seen, into 

 an immense number of facets, generally of an 

 hexagonal shape, or in very rare instances 

 somewhat quadrangular. Each facet, (Jig. 

 419, B,) or as it has sometimes been called, 

 corneule, is the proper cornea of a distinct eye, 

 and is perfectly transparent. It is somewhat 

 broader at its base or external surface (n, c) 



Fig. 419. 



A, section of the eye of Melolontha (Straus). 

 B, section of eye of Libellula (Muller). C, sec- 

 tion of do. (Duges). b, the external convex 

 surfaces of the facet or corneule c, c, c; d, base 

 of the corneules ; a, the anterior chamber be- 

 tween the corneule and iris j e, pupillary aper- 

 ture in the iris, formed by the reflexion inwards of 

 the choroid ; /, the cones filled with the vitreous 

 humour j g, the nerve ; h, the choroid surround- 

 ing the fibres of the optic nerve. 



than at its internal, (rf,) and each one is separated 

 from those by which it is surrounded by the inter- 

 position of a layer of dark-coloured pigment, so 

 that the rays of light can pass through it only in a 

 direction converging to its centre. In some in- 

 stances, as in Lepidoplera, it is convex both on 

 its external and internal surface, or lens-like, 

 but in general is nearly plane. In the Lilicllu- 

 lina, according to Duges, (c,) it is convex exter- 

 nally, and slightly concave internally, and it va- 

 ries considerably in thickness in different in- 

 sects, and in different facets in the same 

 compound cornea. Immediately behind each 



corneule is a layer of dark-coloured pigment, 

 (//,) which is believed to be continuous with the 

 delicate pigment that is interposed between the 

 cornese. It covers the whole of the inner surface 

 of the cornea, excepting only in the centre, 

 where it is perforated by a minute hole or pupil- 

 lary aperture, (e,) to admit the rays of light that 

 have passed through the cornea. Between this 

 pigment, which is the curtain or iris of the eye, 

 and the end of the cornea, Duges found a 

 space, (,) filled with an aqueous humour. Be- 

 hind the iris of each cornea is a little cone- 

 shaped transparent body, ^/",) with its apex 

 directed backwards in the axis of the eye. It is 

 filled with a perfectly transparent tenaceous 

 fluid, the vitreous humour of the eye,into which 

 the rays of light received through the cornea 

 and iris are admitted, to fall upon the retina, 

 or termination of the nerve, (g,) at the apex of 

 the cone. The length of the cone differs greatly 

 in different insects. It is shortest in the Dip- 

 tera, and scarcely exceeds its breadth. In the 

 Coleoptera and Lepidoptera it is five or six 

 times longer than it is broad, and perhaps even 

 exceeds this in some of the Libellulina. The 

 apex of each cone is received upon the extremity 

 of one of the many thousand of fibres (g) 

 which we described as radiating from the bulb 

 of the 'nerve, immediately after it has passed 

 through the optic foramen. The choroid of 

 dark pigment that forms the iris (A) is conti- 

 nued backwards over the surface of the cone 

 and optic fibre to the bulb of the optic nerve, 

 thus completely insulating every individual 

 cone and fibre rljm those by which they are 

 surrounded. It is in the spaces thus occupied 

 by the choroid that the tracheal vessels and cir- 

 culatory passages ramify, so that the choroid in 

 the eyes of insects, as in those of the verte- 

 brata, is the proper vascular structure of the 

 organ. It is subject to much variety of colour 

 in different insects, being in some nearly black, 

 in others dark blue, violet, green, purple, 

 brown, or yellow. In some there are two or 

 three layers of pigment of different colours. 

 The usual arrangement of these is first a dark 

 coloured portion near the bulb of the optic 

 nerve, then a lighter colour, and lastly, again, a 

 darker near the cornese. According to Duges, 

 the base of the cone is rounded where it is co- 

 vered by the iris, but Miiller states that this is 

 the case only when the cornea is devoid of 

 facets or corneules, and is perfectly smooth. 

 According to the same authority, the pupillary 

 aperture is most distinct when the cones are 

 short, as in Diptera. This aperture was disco- 

 vered by Miiller, and also the nature of the 

 cones, which had been thought by Straus- 

 Durckheim to be expanded terminations of the 

 optic fibres. We have seen the iris and pu- 

 pillary aperture very distinctly in the eye of 

 Pontia brassicie, the white cabbage butterfly, 

 and also in Sphinx ligustri and Nepa grundis. 

 In the two latter instances it is of a dark brown, 

 or nearly black, and is particularly large in 

 Nepa. In Pontia it is yellow, in the centre 

 of which the pupil exhibits a glassy brightness. 

 The manner in which the extremity of the ner- 

 vous fibre is connected with the apex of the 

 cone has recently been investigated by Professor 



