590 



ZOOLOGY 



SECT. 



s 



large compound eyes. The surface of the compound eye 

 marked out, as in the case of the Crayfish, into a great number of 

 minute hexagonal facets, each of which represents one of the 

 elements ( i,,i mutvUn) of the eye. Of these there may be as 

 many as 28,000 (dragon-fly). 'When the eye is examined in 

 section, each ommatidium is found to consist of a cornea-lens, the 

 outer surface of which forms the facet, a crystalline cone, and a 

 i'liiiljfl<>i,-n\ The crystalline cone is not always developed, its place 

 being taken in the eyes of some Insects by four crystal cells. The 

 rhabdome is an elongated rod. Beneath the rhabdomes is a 

 fenestratc membrane, beneath which again is a dense plexus of 

 nerve-fibres. Nerve-fibres pass through the fenestrate membrane 

 and terminate in a delicate sheath which incloses each rhabdome, 

 the sheath, together with the nerves that end in it, constituting 

 the retinula. Pigment surrounds the crystalline cones and retinulae. 

 The ocelli, or simple eyes (Fig. 486), consist of a bi-convex 

 transparent thickening of the cuticle the lens and beneath it 



of a group of specially 

 modified epidermal cells. 

 Of these, some, situated 

 beneath the lens, form 

 a transparent mass, the 

 vitreous body ; another 

 set of elongated cells 

 being arranged to form 

 the retina. 



The antennae and 

 palpi are the organs 

 of touch, and these 

 appendages seem to be 

 also the seat of the 

 olfactory sense. A num- 

 ber of minute processes 



sometimes sunk in pits, and each having a special nerve-plate 

 connected with it, are regarded as being specially concerned 

 with this sense, and similar processes or pits on the maxilla? 

 and the epipharynx, are perhaps connected with the sense of 

 taste. 



Peculiar nerve-endings, supposed to be auditory, have been 

 found in the most various parts of the body. Each consists of a 

 ganglion-cell (Fig. 487, gz.) giving off a process which is inclosed 

 in an elongated tube, and which ends externally in a slender 

 rod (.ST.). Groups of these are associated together to form the 

 auditory organ. 



In certain Insects the Fireflies and Glowworms belonging to 

 the order Coleoptera, there occur organs the luminous organs _ 

 l'"r tin- production of light. 



.nn 



Fir.. 4^t'.. Section through the ocellus of a -young 

 Dytiscus 1 u-va ; </. curick- ; <//,-. cells of the vitreous 

 linily; l,n, epidermis; /, cuticular lens; no, ojitk- 

 nerve ; re, retinal cells ; st, rods. (From Lang, after 

 Grenacher.) 



