488 PHYSIOLOGY. 



consequently is most clearly seen closely, and more indistinctly at a 

 distance. If now each facet of the eye can survey but one small portion 

 of the field of vision, yet will the entire eye be able to survey a field 

 the larger in proportion to the size of the segment of the circle it forms 

 and to the convexity of its arch. Therefore, the larger the eye is, and 

 the more convex, the wider will be its horizon. The various structure 

 of the eyes of insects agrees also with this. Males, which are appointed 

 by nature to seek the female, have larger eyes than the latter. Insects 

 which live in and upon their pabulum itself, have small and flat eyes, 

 like all parasites ; others, again, which have greater difficulties to 

 contend with in procuring their food, as those insects which live upon 

 prey, like the Carabodea, Di/tici, Libdlulai, &c., have either large or 

 greatly convex hemispherical eyes. The position of the eyes also 

 corresponds with their size and convexity ; flat eyes, which are able to 

 survey but a short space only, are always more approximate and placed 

 more anteriorly than laterally, and are frequently contiguous, as in the 

 male Syrphi. Spherical and very convex eyes are placed laterally, and 

 their axis is frequently directly opposite, but they yet harmonise by 

 their greater convexity with their field of vision, as is distinctly 

 observable in the large Carabi. But the fields of vision of the two 

 eyes do not affect each other, there still remains a free space between 

 the eyes, which the insect can only survey by turning its head. 

 Sometimes, to compass a still wider field of vision, a complete divarica- 

 tion from the usual form becomes necessary, as in Gyrinns, a division 

 of each eye, the one half of Avhich lies upon the vertex, and the other 

 half is placed at the lower surface of the head ; the latter is for the 

 discovery of food which the insect finds in the water, and the former 

 to secure it from its enemies which approach out of the water ; or, as 

 in some male Ephemera;, in which two large flat eyes lie upon the 

 vertex, and two smaller but more convex ones are found at the margin 

 of the head. 



Joh. Muller appends to his beautiful and apparently perfectly 

 successful explanation of the sight of insects, the result of which we 

 have condensed above, some other more general observations, which we 

 cannot forbear briefly introducing here. Thus the relative proportions 

 of the distinctness of the image in various eyes increases, according to 

 him, in proportion to the size of the sphere of which the surface of the 

 eye forms a segment, with the number and smallness of the facets, and 

 with the length of the transparent lens. Whereas the power of sight 



