No. I.] EYES OF ARTHROPODS. 221 



eye in Vespa, which I regard as the homologue of the acces- 

 sory eye in Bibio and Cloe, is visible at a much earlier period. 



In the larvae of the Libcllulidcs, we have an indication, simi- 

 lar to that in Vespa, of the dual nature of the compound eye 

 in the fact that the ventral part of the eye is separated by 

 a furrow from the dorsal part, which is flatter, and contains om- 

 matidia much less developed than those in the ventral part. 

 Still other evidence of this nature is to be found in the stalked 

 eyes of Euphausia, which I have found to be divided into two 

 distinct parts by a thick layer of black pigment. In one part 

 the corneal facets and ommatidia are much larger than those in 

 the remaining part, while there are several striking differences 

 in the structure of the ommatidia. 



A consideration of these facts has led me to the supposition 

 that the dorsal and ventral eyes of Plironima and Gyrinnus, and 

 those of the males of Bibio a?id Cloe as well as the dorsal and 

 ventral parts of the eyes in LibelliilidcB and Enphansia are homol- 

 ogous with the dorsal and ventral halves of the larval compound 

 eyes of Vespa. The parts of the compound eyes of Vespa, and in 

 all probability of most other Insects, are in turn homologous with 

 the posterior upper ocelli of Acilius and their dorsal extensions. 

 In such cases as those seen in the larvce of Corethra and Phry- 

 ganids, the ocellus has already become a compound eye, while its 

 dorsal extension does not attain that perfection until the imago 

 stage has been reached. 



In Coleopterous larvae with six pairs of ocelli I have never 

 seen more than a single pair with dorsal extensions. Although 

 the position of these double ocelli appears to vary somewhat in 

 different genera, I see no reason to doubt that they are, in all 

 these cases, modifications of the same pair of ocelli. 



In my paper on the " Eyes of Molluscs and Arthropods," 

 I stated that the arrangement of the retinal cells in the ocelli 

 of Spiders closely resembles the arrangement of cells in the 

 ommateum of the compound eye. Since then I have ex- 

 amined the ocelli of Phalangium, and find the resemblance 

 even closer than I anticipated. 



The ocelli of Phalangium consist of three layers of cells, a 

 thick, sublenticular one, the corneagen, beneath which is a delicate, 

 cuticular-like membrane, overlying a very thin, middle layer of 



