98 THE CAUSES WHICH PIIOPAGATE 



visual org-ansj like those of hearings take rise in Arthropods^ in 

 low complex form. They are either compound [oculi) or simple 

 (stemmata) , and one or both kinds may exist in the same insect. 

 The stemmata or eye specks^ placed centrally in the head, in 

 Myriapoda sometimes number twenty _, in Arachnidae six or eight. 

 Among- insects, they are generally present in bees ; they exist also 

 in diurnal grasshoppers and crickets, in butterflies and moths, and 

 in terrestrial bugs, being here absent in nocturnal and aquatic 

 species ; they are also seen in many flies, and some beetles 

 {Omalium, A)tthopkagi) , as in many lepidopterous and coleop- 

 terous larvjB. They are usually three in number, hemispherical, 

 triangulated, adjusted for upward vision in Bees and Lepidoptera, 

 or front vision in Orthoptera, and Libellulidse ; but in Geocorisse 

 there are only two, as also in the Fulgoridse. In the larva of 

 Bytiscus, according to Dr. Grenachei*, the skin is slightly swollen 

 to form a lens ; and the vitreous humour and retina, with its 

 pigment, are differentiations of the hypodermis or cellular layer. 

 The larva of Acilius has a more convex lens, with the vitreous 

 humour more decidedly' differentiated. In Arachnidse, again, the 

 retina is more strongly differentiated, as in insects ; but in 

 the latter the vitreous humour is not much developed. 



The faceted eyes are generally two in number, variously 

 shaped and directed, and situated laterally in the head ; but in 

 some Long-horned Beetles they are more {Tetrops) or less 

 divided by a lap of the integument, as in some aquatic beetles 

 [Gi/rinus). These may be virtually said to possess four oculi. 

 The compound eyes structurally may be considered a contiguous 

 aggregation of the stemmata in the form of faceted six-sided 

 prisms — and, according to Professor Miiller, they act simul- 

 taneously in the formation of a single resultant image. In 

 section each element presents a cornea {a, p. U9), in the common 

 Stag Beetle of extraordinary thickness, and in Lihellula thickened 

 in the posterior facets. Behind the cornea in diurnal insects is 

 a ring- of black j)igment (r) (choroid), acting- as a diajjhragm in 

 diminishing the quantity of light which passes to the crystal- 

 like cone [e) , and consequently wanting in nocturnal cockroaches 

 and darkling beetles. The crystal cones are of great length in 

 the Libellulse, and at their extremity, where they become sud- 

 denly slender, they are continuous with the nervous filaments 



