562 INSECTS AND ARACHNIDA. 



has a convexity of its own ; hence by counting the facets, 

 we can ascertain the number of ocelli in each composite eye. 

 In the two eyes of the common Fly, there are as many as 4000 ; 

 in those of the Cabbage Butterfly, there are about 17,000 ; in 

 the Dragon fly, 24,000 ; and in the Mordella beetle, 25,000. Be- 

 hind each " corneule" is a layer of dark pigment, which takes 

 the place, and serves the purpose, of the "iris" in the eyes 

 of Vertebrate animals ; and this is perforated by a central aper- 

 ture or "pupil," through which the rays of light that have 

 traversed the cornea gain access to the interior of the eye. The 

 further structure of these bodies is best examined by vertical 

 sections ; and these show that the shape of each ocellus is coni- 

 cal, or rather pyramidal (Fig. 284), the cornea forming its base 

 (a), whilst its apex abuts upon a bulbous expansion of the optic 



Fia. 284. 



A, Section of the eye of Melolontha vuJgaris (Cockchafer): B, a portion more highly magnified : 

 a, facets of the cornea; 6, transparent pyramids surrounded with pigment; c, fibres of the optic 

 nerve; d, trunk of the optic nerve. 



nerve. Each " corneule" acts as a distinct lens ; as may be 

 shown by detaching the entire assemblage by maceration, and 

 then drying it (flattened out) upon a slip of glass ; for when this 

 is placed under the microscope, if the point of a knife, scissors, 

 or any similar object, be interposed between the mirror and the 

 stage, the image of. this point will be seen, by a proper adjust- 

 ment of the focus of the microscope, in every one of the lenses. 

 The focus of each " corneule" has been ascertained by experi- 

 ment to be equivalent to the length of the pyramid behind it ; 

 so that the image which it produces will fall upon the extremity 

 of the filament of the optic nerve which passes to its point. 

 This pyramid consists of a transparent substance (B, b), which 

 may be considered as representing the " vitreous humor;" and 

 the pyramids are separated from each other by a layer of dark 

 pigment, which completely encloses them, save at the pupillary 

 apertures which admit the rays that have passed through the 

 " corneules," and at their smaller ends, where the pigment is 

 perforated by a set of apertures that give passage to the fibres of 



