(162 



COMPOUND EYES OF INSECTS. 



Section of the Eye of Melolontha vulgaris 

 (Cockchafer) : — a, facets of the Cornea ; 

 b, transparent pyramids surrounded with 

 Pigment ; c, fibres of the Optic Nerve ; 

 d, trunk of the Optic Nerve. 



the number of Ocelli thus grouped-together is usually small. In 

 the higher Crustacea, however, the Ocelli are very numerous ; 



their compound Eyes being 

 Fig. 339. constructed upon the same 



general plan as those of 

 Insects, although their 

 shape and position are 

 often very peculiar (Fig. 

 406). The individual 

 Ocelli are at once recog- 

 nized, when the Composite 

 Eyes are examined under 

 even a low magnifying 

 power, by the ' facetted ' 

 appearance of the surface 

 (Fig. 338), which is 

 marked-out by very re- 

 gular divisions either into 

 hexagons or into squares : 

 each facet is the 'Corneule' 

 of a separate Ocellus, and 

 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. Behind 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 j and this is perforated by a central aperture or ' Pupil,' 

 through which the rays of light that have traversed the 

 corneule gain access to the interior of the eye. The further 

 structure of these bodies is best examined by vertical sections 

 (Fig. 339) ; and these show that the shape of each Ocellus (b) 

 is conical, or rather pyramidal, the Corneule forming its base (a), 

 whilst its apex abuts upon the extremity of a fibre (c) pro- 

 ceeding from the termination of the Optic Nerve (d). The details 

 of the structure of each Ocellus are shown in Fig. 340; in which 

 it is shown that each Corneule is a double-convex Lens, made up by 

 the junction of two plano-convex lenses, a a and a' a', which have 

 been found by Dr. Hicks to possess a different refractive power ; by 

 this arrangement (it seems probable) the Aberrations are dimi- 

 nished, as they are by the combination of ' humors ' in the Human 

 eye. That each ' Corneule ' acts as a distinct Lens 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 



