Visiial Organs of Insects and Crustacea, 2S1 



iiisect; for, otherwise, the light would be intercepted in its 

 passage to the optic nerve. This second layer of pigment 

 terminates abruptly at the extremities of the nervous fila- 

 ments; the interstices of these latter being filled with a 

 third layer of a deep blue violet colour (c). At 6? a bundle 

 of filaments with the adherent pigment is seen partly detached 

 and bent aside ; and at e the under surface of the second 

 layer is represented, marked with dark violet spots, the traces 

 of its union with the third and more deeply coloured layer. 

 Of course, as the interstices of the optic filaments become 

 narrower in their approach to the bulb of the nerve, the 

 colour of the pigment becomes also, in appearance, of a fainter 

 hue. 



In the Gryllus hieroglyphicus, three distinct layers of 

 pigment are found ; and in the common oyster, the variation 

 of colour is still greater. 



The surface of the superficial layer also occasionally ex- 

 hibits diversities of colour. In the Gryllus lineola, the 

 pigment beneath the cornea, or, more correctly, between the 

 bases of the cones, is radiated with greenish brown tints, 

 giving to the eye, when examined externally, a striated aspect. 

 This is observed also in the Gryllus vittatus Fahr.^ whose 

 very large, slightly convex, and elliptical eye is radiated with 

 yellowish brown, in a direction from above and in front, 

 downwards and backwards. It is remarkable, however, that 

 this diversity of the superficial colours is commonly met 

 with only in the orthopterous insects, whose metamorphosis 

 is incomplete, and which, during the growth of their eyes, 

 several times renew the cornea, or its outer layer. In addi- 

 tion to the Orthoptera, this organisation is only met with in 

 some of the Diptera, in the species of Chrysops whose eyes 

 are marked with red purple points, and in the species of 

 Tabanus which have green eyes marked with red purple 

 stripes. In the orthopterous insects, it will be observed, also, 

 that the pigment nearest the cornea corresponds very closely 

 in hue, &c., with the colour of the common integuments of 

 the body ; the eye, in general, displaying the most brilliant 

 tints when the body is most vividly coloured. 



Compound Eyes "without Facets. — Schaffer (in his Naturge- 

 schichte des Krebsartigen Kiefenfusses, Regensburg, 1 756, sect. 

 68.) gave the first accurate description of the compound eye of 

 the Monoculus apus. The cornea, which is continuous with 

 the common integument of the body, and with it may be 

 removed, is smooth and without facets. After the removal 

 of the cornea, there is seen a dark-coloured mass, presenting 

 on its convex surface a dense aggregate of very small semi-* 



