62 PARTIAL ORISMOLOGY. 



treat of in its proper place. The same observation refers likewise to 

 the lice (Pediculf). 



THE EYES. 



71- 



Having now concluded this detailed description of the oral apparatus, 

 we can pass on to the consideration of the other organs, and the eyes 

 occur as the most immediate objects to proceed with. 



The EYES plainly (oculi, PL III. f. 11, 12, 13, a. a., PI. V. f. 15, A., 

 PI. VI, 3 and 8, A. A.) also called COMPOUND EYES (oculi composite), 

 are placed at the sides of the head, above the mouth, and generally 

 present themselves as large hemispheres, the superficies of which, at 

 least upon close investigation, appear to consist of numerous regular 

 hexagonal surfaces. They are generally circular in circumference, but 

 many other figures (as OVAL or KIDNEY-SHAPED) are observable in 

 them. Each of the above hexagons is itself an eye (as we shall more 

 explicitly illustrate below in the Anatomy of the Eye), their surfaces 

 consequently are so many slightly convex horny cases, whence the quick 

 sight of these creatures is readily explained. Their margins of sepa- 

 ration are often thickly set with hair (oculi pilosi), in other instances 

 they are naked '(oculi nudi). The number of these lenses or facets 

 has been calculated by several authors, and their almost incredible 

 multitude has very justly excited astonishment. Hooke counted 7,000 

 in the eye of a house fly ; Leuwenhoek more than 12,000 in the eye of 

 a dragon fly ; 4,000 in the eye of a domestic fly ; and Geoffroy cites a 

 calculation, according to which there are 34,650 of such facets in the 

 eye of a butterfly. They must also necessarily be very numerous in 

 the eye of the Lamellicornia, in which, even under a tolerably strong 

 lens, the divisions are not perceptible, whence Fabricius * called them 

 simple eyes. 



The general rule is for the eyes to be separated by the brow (ocuh 

 distantes), but they frequently join closely together in male insects 

 (oculi approximati, for ex., in the dragon flies, the male Syrphi, the 

 Drones). There are, in general, but two of these compound eyes, but 

 a few exceptions are found to the universality of its application in the 

 whirlwigs (Gyrinus), and some Ephemera, which have absolutely four 



* Philosoph. Ent. p. six, 4. 



