AND THE SEXUAL DIFFERENCES OF THE EYES IN DIPTERA. 381 



chitin ; and insects are met with having a similar development. 

 I have several undetermined parasites in my cabinet which have 

 such eyes fitted with very globular lenses. From such a con- 

 dition it is only a short stage to the eyes of some Mycetophilidae, 

 such as the Sciara figured (Fig. 4). 



The next stage would be that these chitinous divisions would 

 take less room, or interrupt less light ; and we see from the 

 third experiment that a depth of the side might be of service in 

 concentrating rays of light. Doubtless many modifications of the 

 original pierced opaque plate with circular apertures arose, till we 

 find in the highly specialised Muscidae that all the non-transparent 

 material of the eye-surface has disappeared. If my hypothesis as to 

 Haematopota being close to an ancestral form of the Muscidae is 

 correct — and the condition of the eye gives increased probability 

 to this belief — the fact that in the eye of II. pluvialis <$ we find a 

 transition state is of great significance, and would show that the 

 structure of the Muscid eye, like the pubescence and the com- 

 plete covering of the head with eyes, is a character inherited 

 from the male, and adopted by both sexes, and that it originally 

 arose from the advantage of increased powers of vision in the 

 male sex. 



I have prepared tables giving a rough idea of the varying 

 conditions of the eyes in most of the families of Diptera, and these 

 will, I trust, be of assistance in the comparison of the different 

 types. 



Methods of Work. 



These structures are best seen in preparations cleared in 

 caustic potash and mounted without pressure ; but they can be 

 recognised in those mounted with pressure, although in many 

 cases high powers will be necessary to show the pubescence and 

 the shape of the facets. 



The structures on Haematopota pluvialis S are best seen if the 

 head is cleared in caustic potash and the eyes dissected out, and 

 flattened (and consequently fractured) on the slip, but here again 

 a whole head mounted with pressure will show them. 



