986 INSECTS AND ARACHNIDA 



be now examined with a microscope (the C of Zeiss was employed), 

 the 'lenses' will be distinctly seen, but if the focus be readjusted 

 to the focal plane of the image in the eye this image will be seen 

 and magnified. This will be understood from I) (fig. 735), where 

 e, f represent the image, h the cornea with its * lenses ' g, e'-f being 

 the image of the object thrown upon the position from which the 

 retina has been removed, and which is now made the focal plane of 

 the objective employed. 



It was this image (e'-f) which was photographed in the ordinary 

 manner with a Zeiss photo-micrographic apparatus and the C object- 

 glass. The manner in which this was done is seen diagrammatically 

 at E (fig. 735), where i indicates the cornea of the eye exposed 

 to air, k the image thrown though the ' lenses ' as a unified 

 picture at the focal point of the microscope, and I is the sensitised 

 plate on which the image was photographed. This piece of admi- 

 rable research and its clear results have a value not only physio- 

 logical but philosophical. 



Although the structure already described may be considered as 

 typical 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 dia- 

 phragm, which seems to be occupied by a watery fluid or ' aqueous 

 humour.' 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 num- 

 ber of ocelli being reduced, and each one being larger, so that the 

 cluster presents more resemblance to that of spiders, &c. Besides 

 their ' compound ' eyes, insects usually possess a small number of 

 * simple ' eyes (termed ocelli or stemmata) seated upon the top of the 

 head (fig. 731, a, a, a). Each of these consists of a single very con- 

 vex corneule, to the back of which proceeds a bundle of rods that 

 are in connection with fibrils of the optic nerve. Such ocelli are 

 the only visual organs of the larvse of insects that undergo complete 

 metamorphosis, the ' compound ' eyes being only developed towards 

 the end of the pupa stage. 



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 faceted 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. 731), 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 pig- 

 ment must be carefully washed away by means of a fine camel-hair 

 brush from the 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 



