INVERTEBRATE PHOTORECEPTORS 647 



the lateral mass becomes organized into a battery of ommatidia were 

 followed carefully by Watase (1890). 



The normal compound eye has been termed an "omma" and dissected 

 tangentially into a dioptric system and a layer of soft parts (the "omma- 

 teum" of Lankester and Bourne, 1883) or radially into ommatidia 

 (Carriere, 1884). The dioptric system consists of a corneal component 

 that is molted and regenerated. In some eyes each ommatidium is dis- 

 tinct on the outside, since the corneal lens bulges as a separate facet, 

 either square or hexagonal in outline. In other eyes the outer surface 

 is perfectly smooth. In a few the corneal lenses are slightly concave 

 externally. 



Below the corneal lens and the hypodermal cells that secrete it, the 

 ommatidium has some additional dioptric parts. None of these is molted. 

 The ommatidium is described as of "exocone" type if there is an inward 

 extension of the corneal-lens material — as in crustaceans, trilobites, 

 xiphosurans, and beetles of the famihes Dermestidae, Elateridae, and 

 Lampyridae. The ommatidium is of "eucone" type if special cone cells 

 ("Semper's cells") secrete a separate solid body within themselves. 

 Ordinarily this process leaves the cone-cell nuclei distal to the cone, and 

 sometimes there is also an anuclear portion of the cone cells basal to the 

 cone. Eucone ommatidia are characteristic of the insect orders Thysa- 

 nura, Orthoptera, Homoptera, Neuroptera, Trichoptera, Lepidoptera, 

 and Hymenoptera and of some members of Odonata, most beetles, and 

 some nematocerous Diptera. The brachycerous dipterans are unique in 

 possessing "pseudocone" ommatidia, in which the cone cells secrete distal 

 to themselves a fluid or paste extending to the corneal lens and supposedly 

 aiding in the refraction of light. "Acone" ommatidia, in which the cone 

 cells become translucent and refract light and occupy all the space 

 between the receptor cells and the corneal lens, are characteristic of the 

 insect orders Dermaptera, Heteroptera, some Odonata, some Coleoptera 

 (chiefly the families Silphidae, Histeridae, Coccinellidae, and Curculioni- 

 dae), and some nematocerous Diptera. 



At the basal end of the dioptric parts of each ommatidium is a ring of 

 receptor cells, or two rings, one distal to the other. Sometimes there is 

 an eccentric receptor cell outside the ring but extending a terminal seg- 

 ment to reach toward the end of the crystalline cone in a position central 

 to the ring of receptors — like a core to the group. More commonly there 

 is no eccentric cell, and the ring of receptors secretes a translucent rod in 

 the core position as a rhabdom bringing light energy to the basal parts 

 of the receptors along the optical axis of the ommatidium. Two rings of 

 receptor cells may be more primitive than a single ring. 



Investing the ommatidium like a sheath is a pigment-cell mantle. 

 Often the pigment closes in around the base of the crystalline cone, 

 limiting the passage of light to a very small aperture (van der Horst, 



