510 7V//-: SENSES A XD SKNSORY ORGANS. 



3. THE MEDIAN OCELLI, STEMMATA, OR SIMPLE EYES. 



a. The Stemmata of the Blow-fly. 



(PI. 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 /* to 120 /z, 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 w p ith 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 hypoderrnal 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 expansion 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- 



