296 ANIMAL PHYSIOLOGY. [LESSON 92. 



1337. The crystalline lenses fit into the concave surfaces of the 

 cornea, but how accurately is not known, nor whether a space is 

 reserved for an aqueous humor these, at present, are matters of 

 conjecture. 



1338. From the fact that the pigment, which limits the aperture 

 of the lens by coating its sides, from the circumference towards the 

 centre, and thus forms an iris, there is great probability of the 

 presence of an aqueous humor. 



1339. If a carpenter had to fill a given space with boxes of uni- 

 form size, he must adopt one of three mathematical figures : they 

 must be either square, triangular, or hexagonal the first and last 

 are used in the formation of the corneas of insects. 



1340. The eyes in the centre of the facetted organ are always 

 the largest, and most perfect hexagons in form. As they approach 

 the margins, they begin to assume a square form, which, at the ex- 

 treme edge, is perfected ; so that both these figures exist in the same 



FIG. 420. FIG. 421. FIG. 422. 



Centre of the transparent Transparent cornea, towards Transparent cornea, at 



cornea, M. carnaria. the edge, M. carnaria. the edge, M. carnaria, 



eye, chiefly ^ it would appear, for the economy of space. This is well 

 shown in the transparent cornea of Musca carnaria (Fig. 420) ; first 

 we see the perfect hexagons in the centre of the cornea ; secondly 

 (Fig. 421), the gradual change to a square form; the process of 

 change is still further continued, till at last a series of perfectly 

 square cells are formed (Fig. 422). 



1341. In the cornea of a Beetle a similar arrangement occurs, 

 but the square cells are not so sharply formed ; so, too, in the Drag- 

 on-fly, although in this insect the hexagons glide into parallelograms 

 rather than squares, the like arrangement generally prevails ; but 

 there are exceptions. 



1342. Such may be the structure of the eye in some insects, but 

 some important differences, not yet recorded, occur in the eyes of 

 other insects. A large Beetle, Prionus longimanus (its specific 

 name signifying " long arms," and applied to the great length of the 

 first pair of legs), commonly known as the " Harlequin-beetle," from 

 the many colors it possesses, and the peculiarity of their arrange- 



