276 INSECTA. 



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 antennse, 

 spread themselves out ; but this apparatus has not been detected 

 in other insects. 



(321.) The eyes of insects are of two kinds, simple and com- 

 pound ; 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. 



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 Sir ex gigas (Jig. 128), 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. 



The structure of the eyes has been most minutely investigated 

 by several distinguished entomotomists, and the labours of Marcel 

 de Serres,j- Joh. M tiller, J Strauss Durckheim, and Duges,|| have 

 done much to dispel the mistaken notions entertained by preceding 

 anatomists. 



The simple eyes consist of a minute, smooth, convex, transpa- 

 rent 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 dis- 

 tinct-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 com- 

 pound eyes of insects, for these are constructed upon principles so 

 elaborate and complex, that we feel little surprise at the amaze- 

 ment expressed by early writers who examined them, although 

 their ideas concerning their real structure came far short of the 

 truth. 



* G. R. Treviranus, Annalen der Wetterau. Qesel. f. d. Ges. Naturk. vol. i. 1809. 

 t Mem. sur les Yeux composes, et les Yeux lisses des Insectes. Montpel. 8vo. 

 1813. 



t Zur Vergleichenden Physiologie des Gesichtssinnes, 8vo. 1826. 



Annales des Sciences Nat. torn, xviii. || Ibid. torn. xx. 



