254 
Mr. R. J. Ashton on the Cornea 
respond exactly with each other, and cause the hemispherical, or 
at least convex, figure of the superior surface of the eye but 
this general description requires some qualification, as will pre- 
sently be seen. On an examination of the cornea of the eye of 
certain insects, it will be found that it is divided into facets of 
different sizes, varying both in relative proportions and mode of 
arrangement in different species. Thus in some of the Libellulce, 
as in Libellula vulgata, we find the whole of the upper part of the 
cornea, comprising nearly one-half of the eye, composed of large 
equal facets, and the remainder of the cornea divided into small equal 
facets, not above one-fifth of the size of the former in superficial 
measurement, the transition from the former into the latter being 
very abrupt, and the line of demarcation between them of a some- 
what serpentine form (PI. XXI. fig. 2), and in some other insects 
the appearance presented is even more remarkable. A peculiarity 
of this nature in the cornea, as far as my observation goes, only 
obtains in those insects which have large and prominent eyes, or 
such as are particularly exposed to the light ; in others the facets 
being perfectly uniform. Thus the only orders in which I have 
noticed it are the Neuroptera and Diptera, to which orders the in- 
sects most conspicuous for their highly developed eyes belong. 
It appears to me that the diversity of size and arrangement of the 
facets depends upon the form of the eye, and that the large facets 
are always placed where the light strikes most strongly upon the 
cornea. This in the before-mentioned insect, Libellula vulgata, is, 
I think, sufficiently apparent (see fig. 2, a, b). Thus also in the 
dipterous insect, Scceva selenitica (?) , the large facets occupy the 
top of the cornea, and run down the centre of the eye in the form 
of a tongue ; the louder part of the eye, and a narrow slip on each 
side (especially posteriorly), to the top, being composed of uniform 
facets considerably smaller (fig. 3), presenting a very remarkable 
distribution ; but on examining the outlines of the cornea (fig. 3, 
a, b), it will appear that the part thus occupied by the large facets 
is precisely that most exposed to the light. Again, in Asilus era- 
broniformis there is only a narrow' portion of the anterior part of 
the cornea consisting of large facets, the rest of the cornea (in- 
cluding the upper part) being composed of small ones (fig. 4 a); 
but on observing the position in which this insect holds its head, 
we arrive at the same result, viz. that the large facets receive the 
strongest light. 
In the before-adduced instances of Libellula vulgata and Scceva 
Shuckard’s Translation, p. 292. 
