510 THE SENSES AND SENSORY ORGANS. 
8. THE MEDIAN OCELLI, STEMMATA, OR SIMPLE EYES, 
a. The Stemmata of the Blow-fly. 
(Pl. XXXV., Fig. 2.) 
Median ocelli, or simple eyes, occur in the imago state in 
most Diptera, Hymenoptera, and Homoptera, and in many 
of the Orthoptera. They are usually three in number, rarely 
reduced to two, as in some Homoptera, or to one, as in some 
Ants. In the latter insects they are usually obsolete. 
In the Blow-fly the ocelli are three in number, and placed in 
a triangle on the vertex; two are posterior, and one in front. 
They all have the same structure, a corneal lens, a vitreous, 
and a retina enclosed in a pigmented prolongation of the 
nerve-sheath, which consists of flattened cells. 
The diameter of the cornea varies from 70 p to 120 p, and is 
less in the males than in the females; whilst the great com- 
pound eyes are larger in the males than in the females. 
The Corneal Lens is very convex both on its external and 
internal surface, and exhibits a thin external layer which is 
undoubtedly a continuation of the external layer of the 
epidermis. Beneath, and intimately united with this external 
layer, is a biconvex lens, with a distinctly laminated structure 
which assumes a pink tinge under the action of Millon’s 
reagent. This lens is developed at the expense of a layer 
of hypodermal cells continuous with the general hypoderm. 
The inner extremities of the lens-cells form a vitreous, and 
in the mature insect only exist as a very thin layer, which may 
in some cases be entirely overlooked in sections. The gradual 
thinning of the vitreous layer is of importance, as the ocelli 
have been described as of two kinds in many Insects and 
Arachnids by some observers, and much difference of opinion 
exists as to whether the vitreous is absent or present in one of 
these forms. 
The Retina is a cup-shaped expansiqn of the optic nerve, 
formed of a single layer of typical rod cells (Fig. 67). Each 
rod consists of two segments—an anterior extremely trans- 
