EXTERNAL ANATOMY OF INSECTS. 495 
fifty, and are distinctly visible to the naked eye*. These 
lenses vary in magnitude, not only in different, but some- 
times in the same eyes. This is the case in those of 
male horse-flies and flies, those of the upper part of the 
eye being much larger than those of the lower®, The 
partitions that separate the lenses, or rather bezels, in 
which they are set, are very visible in the eyes just men- 
tioned, and those of Xenos; but in many insects they 
are only discernible at the intersecting lines of separa- 
tion between the lenses. In hairy eyes, such as those of 
the hive-bee, the hairs emerge from these septa. Every 
single lens of a compound eye may be considered as a 
cornea, or acrystalline humour, it being convex without 
and concave within, but thicker in the middle than at 
the margin: it is the only transparent part to be found 
in these most remarkable eyes. Immediately under the 
cornea is an opaque varnish, varying according to the 
species, which produces sometimes in one and the same 
eye spots or bands of different colours. These spots and 
bands form a distinguishing ornament of many of the 
Tabani and other flies. And to this varnish the lace- 
winged flies (Hemerobius, &c.) are indebted for the beau- 
tiful metallic hues that often adorn them. When insects 
are dead, this varnish frequently loses its colour, and the 
eye turns white: hence many species are described as 
having white eyes which when alive had black ones. 
The consistence of this covering is the same with that of 
the varnish of the chorord in the eyes of vertebrate ani- 
mals; but it entirely covers the underside of the lens, 
without leaving any passage for the light. Below this 
* Plate XXVI. Fic. 38. » Hooke Microgr. schem. xxiv. 
