THE INTERNAL ANATOMY OF. INSECTS 



139 



Ocelli of Ephemerida. It has been found that the ocelli of certain 

 adult Ephemerida differ remarkably from the more common type of 

 ocelli described above. These peculiar ocelli have been described and 

 figured by Hesse ('01) and Seiler ('05). In them the cuticula over the 

 ocellus, the cornea, is arched but not thickened and the corneal hypo- 

 dermis is a thin layer of cells immediately beneath it. Under the 

 hypodermis there is a lens-shaped mass of large polygonal cells ; and 

 between this lens and the retina there is a layer of closely crowded 

 columnar cells. 



The development of these ocelli has not been studied; hence the 

 origin of the lens-shaped mass of cells and of the layer of cells between 



it and the retina is not known. 



C. THE COMPOUND EYES 



A compound eye consists of many 

 quite distinct elements, the ommatidia, 

 each represented externally by one of 

 the many facets of which the cuticular 

 layer of the eye is composed. As the 

 ommatidia of a given eye are similar, 

 a description of the structure of one 

 will serve to illustrate the structure of 

 the eye as a whole. 



The structure of an ommatidium. 

 The compound eyes of different insects 

 vary in the details of their structure; 

 but these variations are merely modi- 

 fications of a common plan ; this plan is 

 well -illustrated by the compound eyes 

 of Machilis, the structure of which was 

 worked out by Seaton ('03). Figure 

 155 represents a longitudinal section 

 and a series of transverse sections of an 

 ommatidium in an eye of this insect, 

 which consists of the following parts. 



The cornea. The cornea is a hexa- 

 gonal portion of the cuticular layer of 

 the eye and is biconvex in form (Fig. 

 155, c). 



The corneal hypodermis. Beneath 

 each facet of the cuticular layer of the eye are two hypodermal cells 



Fig. 155. An ommatidium of 

 Machilis. The lettering is ex- 

 plained in the text. 



