332 INSECTA. 



vibratory movements communicated to the antennae by the sonorous 

 undulations of the atmosphere. 



(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 them- 

 selves 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 com- 

 pound eyes ; but in general both kinds exist together. In the Sireae 

 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 

 Serrest, Joh. MiillerJ, Straus-Durckheim , and Duges|| have done 

 much to dispel the mistaken notions entertained by preceding anato- 

 mists. 



(870.) The simple eyes consist of a minute, smooth, convex, trans- 

 parent 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 con- 

 structed 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 hemi- 

 spherical. 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 corner (for 



* Annalen der Wetterau. Gesel. f. d. ges. Naturk. rol. i. 1809. 

 f Mem. sur les Yeux composes et les Yeux lisses des Insectes. Montpellier, 

 8vo, 1813. 



} Zur vergleichenden Physiologic des Gesichtssinnes. 8vo, 1826. 

 Ann. des Sci. Nat, torn, xviii. || Ibid. torn. xx. 



