This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
* Vermischte Schriften, vol. ii. 1 Op. cit. p. 296.
(866). In some moths, Treviranus* has discovered structures which would seem to be indubitably real auditory organs. He found in front of the base of each antenna a thin membranous drum, behind which, large nerves, derived from those supplied to the antennae, spread themselves out; but this apparatus has not been detected in other insects.
(867). The eyes of insects are of two kinds, simple and compound, - the former being insulated visual specks; while the latter consist of agglomerations of numerous distinct eyes, united so as to form most elaborate and complex instruments of sight.
(868). Some insects, as the Dictyotoptera and Thysanoura, only possess simple eyes; others, as for example the Coleoptera, have only compound eyes; but in general both kinds exist together. In the Sirex gigas (fig. 171), for instance, besides the large hemispherical organs of sight, situated at the sides of the head, three simple spots are seen upon the vertex, which are likewise appropriated to vision.
(869). The structure of the eyes has been most minutely investigated by several distinguished entomotomists; and the labours of Marcel de Serres1, Joh. Muller2, Straus-Durckheim §, and Duges|| have done much to dispel the mistaken notions entertained by preceding anatomists.
(870). The simple eyes consist of a minute, smooth, convex, transparent cornea, in close contact with which is a small globular lens; behind this lens is placed the representative of the vitreous humour, upon which a nervous filament spreads out, so as to form a retina: the whole is enclosed in a layer of brown, red, or black pigment, which, bending round the anterior surface of the eye, forms a distinct coloured iris and pupillary aperture. Such an arrangement evidently resembles what is met with in higher animals, and is remarkable for its simplicity: but it is far otherwise with the compound eyes of insects; for these are constructed upon principles so elaborate and complex, that we feel little surprise at the amazement expressed by early writers who examined them, although their ideas concerning their real structure came far short of the truth.
(871). The compound, eyes of insects are two in number, situated on the lateral aspects of the head, the form of each being more or less hemispherical. When examined with a microscope, their surface is seen to be divided into a multitude of hexagonal facets, between which minute hairs are generally conspicuous. The number of facets or corneae (for such, in fact, they are) varies in different genera: thus, in the Ant (Formica) there are 50; in the common House-fly (Musca domestica), 4000; in some Dragon-flies (Libellula), upwards of 12,000. In Butterflies (Papilio) 17,355 have been counted; and some Coleoptera (Mor-della) possess the astonishing number of 25,088 distinct corneae.
* Annalen der Wetterau. Gesel. f. d. ges. Naturk. vol. i. 1809. 1 Mem. sur les Yeux composes et les Yeux lisses des Insectes. Montpellier, 8vo, 1813.
2 Zur vergleichenden Physiologie des Gesichtssinnes. 8vo, 1826. § Ann. des Sci. Nat. torn, xviii. || Ibid. torn. xx.
(872). But in order to appreciate the wonderful organization of these remarkable organs of sight, it is necessary to examine their internal structure: every cornea is then found to belong to a distinct eye, provided with a perfect nervous apparatus, and exhibiting its peculiar lens, iris, and pupil; thus being completely entitled to be considered a distinct instrument of vision.
(873). By attentively examining the annexed figure, representing a section of the eye of the Cockchafer (Melolontha), as displayed by Straus-Durckheim, the whole structure of the organ will be readily understood. The optic nerve (fig. 166, a), derived immediately from the supra -oesophageal mass of nervous matter, swells soon after its origin into a rounded ganglion nearly half as large as the brain itself. From the periphery of the ganglion so formed arise a considerable number of secondary nerves (b), which are very short, and soon come in contact with a layer of pigment (d), that in the Cockchafer is of a brilliant red colour, and is placed concentrically with the convex outer surface of the eye. Behind this membrane (called by Straus - Durckheim the common choroid), the secondary optic nerves (b) unite to form a membranous expansion of nervous matter (c), which may be denominated the general retina. From the nervous expansion so formed arise the proper optic nerves (e), appropriated to the individual eyes, or ocelli, as we shall term them. These nervous filaments are as numerous as the facets of the cornea, and traverse the common choroid to radiate towards the individual eyes whereunto they are respectively destined, and the structure of which we must now proceed to examine.
In fig. 166, b, a portion of the circumference of the compound eye is represented upon a very large scale, in order to show the construction of the hexagonal ocelli that enter into its composition. Each cornea (i) is a double convex lens, adapted by its shape to bring to a focus the rays passing through it. Behind every lens so constituted is placed a hexahedral transparent prism (h), which from its office may be compared to the vitreous humour of the human eye; and it is upon the posterior extremity of these prisms that the proper optic nerves (fig. 166, a, e) spread themselves out, so as to form so many distinct retinae. When we reflect upon the extreme minuteness of the parts above alluded to, we may well expect slight discrepancies to occur between the accounts given of them by different anatomists. Straus-Durckheim represents every optic nerve as terminating in a minute pyriform bulb (fig. 166, b, f), and points out a dark layer of pigment (g), which forms a choroid tunic proper to each ocellus; while, according to Muller and Duges, the vitreous humours (h) are conical, and terminate posteriorly in a sharp point, upon which the terminal expansion of the optic nerve spreads out, without any pyriform enlargement; they likewise deny the existence of the proper choroid (g) in the situation indicated by Straus-Durckheim, but find a black pigment situated immediately behind the cornea, that at first sight would appear to be continuous over the whole surface of the eye. Even Cuvier seems at one time to have adopted this opinion. Muller, however, found that, upon carefully removing the internal structures of the organ, leaving the pigment untouched, the dark varnish in question, although very thick at the lines of union of the different facets, where it is continuous with a choroid that separates the individual ocelli, yet towards the centre of each facet becomes exceedingly thin, and at the very centre is quite wanting, so that a minute perforation or pupil is thus left, through which the rays of light enter.