EYES OF INSECTS. 563 



the optic nerve (<;), of which one proceeds to each ocellus. Thus 

 the rays which have passed through the several " corneules" are 

 prevented from mixing with each other; and no rays save those 

 which pass in the axis of the pyramids, can reach the fibres of 

 the optic nei've. Hence it is evident that, as no two ocelli on 

 the same side have exactly the same axis, no two can receive 

 their rays from the same point of an object; and thus, as each 

 composite eye is immovably fixed upon the head, the combined 

 action of the entire aggregate will probably only afford but a 

 single image, resembling that which we obtain by means of our 

 single eyes. Although the foregoing may be considered as the 

 typical structure of the eyes of Insects, yet there are various 

 departures from it (most of them slight) in the different members 

 of the class. Thus in some cases the posterior surface of each 

 "corneule" is concave; and a space is left between it and the 

 iris-like diaphragm, which seems to be occupied by a watery 

 fluid or "aqueous humor;" in other instances, again, this space 

 is occupied by a double convex body, which seems to represent 

 the "crystalline lens;" and this body is sometimes found behind 

 the iris, the number of ocelli being reduced, and each one being 

 larger, so that the cluster presents more resemblance to that of 

 Spiders, &c. Besides their composite eyes. Insects usually pos- 

 sess a small number of rudimentary single eyes, resembling 

 those of the Arachnida ; these are seated upon the top of the 

 head, and are termed stemmata (Fig. 283, a, a, a). It is remark- 

 able that the larvce of Insects which undergo a complete meta- 

 morphosis, only possess single eyes ; the composite eyes being 

 developed, at the same time with the wings and other parts 

 which are characteristic of the Imago state, during the latter 

 part of Pupal life. 



384. Various modes of preparing and mounting the Eyes of 

 Insects may be adopted, according to the manner wherein they 

 are to be viewed. For the observation of their external facetted 

 surface by reflected light, it is better to lay down the entire 

 head, so as to present a front-face or a side-face, according to 

 the position of the eyes ; the former giving a view of both eyes, 

 when they approach each other so as nearly or quite to meet (as 

 in Fig. 283) ; whilst the latter will best display one, when the 

 eyes are situated more at the sides of the head. For the 

 minuter examination of the " corneules," however, these must 

 be separated from the hemispheroidal mass whose exterior they 

 form, by prolonged maceration ; and the pigment must be care- 

 fully washed away by means of a fine camel-hair brush, from their 

 inner or posterior surface. In flattening them out upon the glass 

 slide, one of two things must necessarily happen ; either the 

 margin must tear when the central portion is pressed down to a 

 level ; or, the margin remaining entire, the central portion must 

 be thrown into plaits, so that its corneules overlap one another. 

 As the latter condition interferes with the examination of the 



