1878.] the Simple and Compound Eyes of Insects. 263 



structures in other insects which are perhaps very different, and as it 

 tends at least to give an erroneous idea of its nature, I have preferred 

 to call it the facellus. 



The facellus consists of a cylindrical or fusiform bundle of seven or 

 more — twelve at least in the ant — fusiform cells, each of which has a 

 fine axial thread of highly refractive material. This thread is prolonged 

 from the outer extremity of the facellar cell into the rod- cell layer 

 above described. These threads pass also into the deeper structure 

 beneath the facellus, which I have named the stemon. 



The stemon is the large rod-like organ which connects the facellus 

 with the ganglionic retina. The stemon of each central facet remains 

 distinct through its whole course, but those of the peripheral facets 

 unite in bundles of four or more near their inner extremities. In 

 Tipula each stemon, whether simple or compound, divides at its inner 

 extremity into several irregular fine branches which pass into stellate 

 highly pigmented cells (ganglion cells ?). The inner processes of these 

 cells connect them with the round cells of the ganglionic retina. In 

 the ant and in the wasp, the stemonata do not divide at their inner 

 extremities. In the latter I have not been at present successful in 

 tracing the connexion of the stemon and the nervous structures 

 beneath ; but, in the former, each stemon is connected with the gang- 

 lionic retina by what appears at first sight to be a thick nerve fibre. 

 From observations on the eyes of Lepidoptera, where similar thick 

 nerve fibres exist, I conclude that, in both cases, these are compound 

 fibres consisting of a large number of primitive fibrillse. The gang- 

 lionic retina in the ant consists exclusively of small round cells ; I 

 have been unable to detect either stellate corpuscles or fusiform cells, 

 which are so constant in this organ in insects. 



In the third form of eye, the true compound eye, the facets are each 

 provided with a single complex rod-like structure, which I have called 

 the rhabdion, in accordance with the nomenclature proposed by 

 Dr. Grenadier. 



The nature of this structure is best seen in the eyes of the crepus- 

 cularian Lepidoptera. In the eyes of the Sphingidae a single rhabdion 

 rests on a distinct facellus, quite comparable, indeed almost identical, 

 with that of the semi-compound eye. The rhabdion is separated from 

 the corneal facet by a cone, described under the term crystal-cone 

 (jkrystal-Tcegel) . The facelli are continuous with thick nerve fibres, 

 which are undoubted compound. These are gathered together into 

 nerve trunks, the fibres from thirty or forty facelli being united into a 

 single trunk, which branches at its inner extremity, and is connected 

 with numerous ganglion cells. 



The cone consists of eight cells — Four are superficial, that is, they 

 are in contact with the corneal facet; these remain soft; they have 

 been spoken of by previous writers as " Sempers' cells," and have been 



