436 THE SENSORIAL FUNCTIONS. 



magnified views of smaller portions given in the four 

 succeeding figures, (4'25 to 428) in all of which the 

 same letters of reference are used to indicate the 

 same objects.* The whole outer layer (c c) of the 

 compound eye may be considered as corresponding 

 to the cornea ; each separate division of which has 

 been termed a Corneule, being composed of a horny 

 and perfectly transparent material. Each corneule 

 (c) has the form of a truncated pyramid, the length 

 of which (l, Fig. 42-),) is between two and three 

 times the diameter of the base (b). The outer sur- 

 face (b) is very convex ; but the internal or trun- 

 cated end (d) is concave ; and the concavity of the 

 latter being smaller than the convexity of the 

 former, its optical efiect is that of a meniscus, or 

 concavo-convex lens, with power of converging to 

 a distant focus the rays of light which traverse it. 

 Within these corneules there is extended a layer of 

 an opaque black pigment (x), probably connected 



the Cordylura, and in all the Muscidce. In the Syrphus, the largest 

 facets are above : and Mr. Ashton has observed the same circum- 

 stance in those of the Libellnla. 



In a few coleopterous insects each composite eye is divided into 

 two, thus giving the appearance of four eyes : this occurs in the 

 genus Tetrops ; and the North American Longicora tetraopes ; and 

 also in the Gyrinus, among the water beetles ; and in some of the 

 Petalocera. 



In many insects, especially of the orders Hymenoptera and Dip- 

 tera, the compound eyes are thickly beset with short erect hair. 

 The compound eyes are in some instances placed at the extremity of 

 a long pedicle, as in the Diopsis and Achias, among the Diptera ; 

 and a shorter pedicle of the same kind Occurs in the Strepsiptera ; 

 from which circumstance Mr. Kirby gave to the parasite of the Bee 

 the name oi Sty lops. 



* These figures, as well as the account of the anatomy of the eye 

 of the Libellula, are taken from the memoir of Dug^s, in the Annales 

 des Sciences Naturelles, xx. 341. 



