THE COMPOUND EYE. 519 



then all the discussions which form a great part of the litera- 

 ture of the subject will become a mere matter of history, and 

 the structure of the organ of vision becomes intelligible on 

 the principles of mechanical optics, just as vision is intelligible 

 on these principles in the Vertebrata. 



Now, I shall first endeavour to prove that the view generally 

 received, which is mainly due to Grenacher, is by no means 

 established either by facts or hypothesis. 



Grenacher [216 and 222] commences by showing that the 

 crystalline cone (' Krystallkegel'), although commonly found in 

 the compound eye, is entirely absent during the whole of life 

 in many forms of compound eye, and that when it is present 

 in the ultimate stage of development, it is not present during 

 the earlier stages. He says : 



' We always find in early stages of development a certain 

 number of cells behind each corneal facet, the function of 

 which, in the first place, is the secretion of the facet itself. 

 They have commonly, taken together, the form of a cone, with 

 its apex directed inwards. In by far the greater number of 

 cases there are four of these cells, seldom two or five. 



; This is the starting-point, the embryonic stage. The eyes 

 of the definitive form are, however, sharply distinguished by 

 the metamorphosis which these cells undergo. 



' In numerous insects these cells persist in an unaltered form ; 

 as in Coleoptera, at least all except the Pentamera, Hemi- 

 ptera, Heteroptera, Tipularidae, and Orthoptera, at least in the 

 Forficulina.' This form of eye Grenacher terms aconic. 



' In other insects, and in the Crustacea, these cells exist as 

 such only for a short time. At an early period a secretion, 

 which is more or less solid, begins in them.' With the increase 

 of this, he states that the cells disappear in great part. 

 ' Generally only the nucleus and cell-wall remain. The cell- 

 secretions run together and form the so-called crystalline cone, 

 the segments of which are equal in number to the original 

 cells.' Such eyes Grenacher terms euconic. 



The eyes of the Diptera brachycera are pseudo-conic, accord- 

 ing to Grenacher, and he says these differ from the aconic and 



