62 BARKER : 



ulation we must not forget the discovery claimed by Exner, 

 and regarded by him as of great importance, that the pig- 

 ment masses in different regions of the compound eye are not 

 stationary, but shift their positions considerably under varia- 

 tions in the intensity of illumination, or at least when the 

 change is from darkness to light or vice versa. 



He states that his preparations show muscle fibres attached 

 to the iris pigment bundles, by whose contraction the bundles 

 are drawn inward when the eye is exposed to light. At the 

 same time the retinal pigment moves outward, so that the 

 inner ends of the crystalline cones and the zone of the gan- 

 glionic cells are both left free from pigment. A periodic dis- 

 placement of the pigment in the eyes of nocturnal lepidoptera 

 which were kept permanently^ in the dark, has since been 

 reported by A. Kiesel. This phenomenon he believed to be an 

 accompaniment of the insect's sleep.* 



Assuming that the reader to whom this paper is addressed 

 has little acquaintance with the subject under consideration, 

 it may also be assumed that he has at this stage recognized 

 the inconclusiveness of what has preceded and will inquire if 

 any definite judgment has been reached by those who can 

 speak with authority. 



In the text-book of Parker and Haswell f vve find a con- 

 cise description of the compound eye, as follows : — 



"The surface of the compound eye is marked out as in the case of 

 the crayfish with a great number of hexagonal facets, each of which 

 represents one of the elements (ommatidia) . When the eye is exam- 

 ined in section each ommatidium is found to consist of a cornea lens 

 (the outer surface of which forms the facet) , a crystalline cone and a 

 rhabdome. The crystalline cone is not always developed, its place 

 being taken in the eyes of some insects by four crystal cells. The 

 rhabdome is an elongated rod. Beneath the rhabdome is a fenestrate 

 membrane, beneath which again is a dense plexus of nerve fibres. 



*SB. K. Akad. Wiss. Wien, ciii (1894). 



fA Text-Book of Zoology by T. Jeffery Parker. D. vSc, F. R. S., 

 and William A. Haswell, M. A., D. Sc, F. R. S. London, 1897. 



