254 ORGANS OF THE SENSES. 



large eyes like those of an insect, where however the 

 several cornese are more round and larger than in 

 the compound organs of the true hexapodous insects. 

 The compound eyes of insects consist merely of a closer ag- 

 gregate of many of these more detached organs of the myria- 

 pods, forming hemispherical sessile masses of sometimes ten 

 or twenty thousand eyes on each side of the head, and these 

 are often accompanied by a few small separate simple eyes 

 placed more posteriorly. Lewenhoek calculated 1200 facetts 

 in the eye of a libellula. They are compound even in the 

 apterous lepisma, where they are accompanied by three simple 

 eyes, as in many of the higher winged insects, but the coleop- 

 terous insects, which approach nearest to the higher crus- 

 tacea in the concentrated forms of many of their organs, 

 possess only the two compound eyes. The larvae of the co- 

 leoptera and hymenoptera are often destitute of eyes. In the 

 compound eyes of insects the epidermis appears to continue, 

 as we see in serpents, over the exposed surface of the 

 globe, but colourless, and transparent. Beneath this ex- 

 terior covering are placed the numerous transparent prismatic 

 hexagonal facettes, or corneae of the several minute compo- 

 nent eyes, as seen in the annexed section of a part of the eye 

 of melolontha vulgaris, (Fig. 101. A. .) Within this thick 

 continuous exterior layer of aggregated transparent hexagonal 

 corneas are placed the small conical transparent lenses, (101. 

 A. b, B. b 9 ) of a regular tapering form and smooth rounded 

 surface, of a firm horny texture and colourless transparency. 

 The hexagonal corneas of the sphinx atropos are only the 

 sixtieth of a line in diameter. The flat bases of the lenses 

 directed outwards are applied to the inner surface of the 



