VISUAL ORGANS . 
15 
tlie usual polygonal lenseSj tiiese last approacli tlie 
forms of liemispheres. 
The colour of the eyes varies from brilliant red to 
brown or black. The pigment behind the choroid is, 
in some few species, developed into dots, which gives to 
the eye a uniformly spotted appearance. When the 
imago first issues from its pupa the eyes often appear 
of a bright green colour, or else they take a paler, hue 
than that afterwards seen in the adult insect. 
Latreille and Burmeister first noticed the remarkable 
supplementary eye or tubercle which is superposed on 
the cornea of each compound eye. Bach of these 
small tubercles is furnished with from’fiv^to ten hemi- 
spherical lenses, which do not seem to differ in struc- 
ture from those of the compound eyes. 
It is not easy to conceive what modification of 
vision these smaller eyes subserve. We find them 
present in the winged forms, which also possess the 
ocelli. These last organs generally are supposed to be 
adapted to distant vision. 
The winged insects, therefore, are provided with no 
less than three different kinds of eyes. 
The Larvse of some species are quite blind ; others 
show the merest rudiments of eyes. Thus the larva 
of Vacuna dryopMla possesses only an eye composed of 
a small tubercle, garnished with four or five lenses. 
Again, some subterranean Aphides are wholly destitute 
of sight. 
The presence of this supplementary tubercle is of no 
value for the discrimination of genera. 
The males and winged females of all Aphides show 
three, or very rarely six ocelli or stemwiMta, 
They are absent in the larv®, but sometimes may 
be traced as red spots, on the vertex, through ■ the 
integument of the pup®. In the imago an ocellus is 
placed at the inner margins of each compound eye, 
close to the base of the antenna. The third ocellus 
is on the vertex, between the antenn®, and it often 
gives a pointed outline to the head. 
