THE SIMPLE AND COMPOUND EYES OE INSECTS. 
583 
by the term rhabdion, as I think that there is evidence that it does not in any way 
correspond to the structure which I have named the stemon. I am rather inclined to 
regard it as the representative of the rod-like cells in the eyes hitherto described. 
If a facellus exist at all, it is placed beneath this structure — a fact that is clearly 
indicated by the position of the facellus in the eyes of the Lepidoptera, in which 
there can be no doubt of its presence. As will be seen, there is a structure in the 
nervous retina of the flies which resembles the facellus very closely, hut a true facellus 
is entirely wanting. 
Eristalis. (Figs. 9 to 13.) — -The eye in Eristalis does not differ in any way that I 
have been able to discover from that of Syrphus, but the parts of the latter are often 
more easily made out from their greater transparency. I shall describe the eye of 
the former insect, and refer to that of the latter when I have found the parts more 
distinct in it. 
The cornea is about Tsiroih of an inch in thickness, and the facets average -gijoth 
of an inch in diameter. In the centre their adjacent axes make an angle of about 
1° with each other. Those in the centre of the cornea are hexagonal and small : 
usually ufooth of an inch in diameter ; those at the edges are square, and as much 
as yyofh of an inch across. The facets in the immature imago and at the periphery 
of the cornea are surrounded by nuclei of a bright brown tint (the so-called nuclei of 
Semper) (fig. 9 «). These appear to be adherent to the substance of the cornea. In 
the mature imago and in the centre of the cornea the facets are surrounded by a frame- 
work of deep black pigment which conceals the nuclei, and is probably developed in 
or around them. Immediately beneath each corneal facet is a deep cup-like cavity 
(fig. 9) surrounded by flat cells filled with bright orange- coloured pigment ; at the 
bottom of this cup there are four nucleated cells (a) which rest upon the extremity 
of a quadrangular rod (figs. 10 a and 10 b). These parts attain a very high develop- 
ment in Acridium and in the Diurnal Lepidoptera. I shall call the four cells the 
tetrasome, and the quadrangular rod on which they rest the tetraphore. 
Between the tetrasome and the nervous retina is the rhabdion (a"). Tins consists of 
a protoplasmic sheath, containing a bundle of four fine highly-refractive threads, which 
are united together at the outer end of the rhabdion into an apparently single axial 
thread, which enlarges to form the tetraphore. I have been unable to make out the 
fourfold nature of this structure, but suspect that it consists of four elements. The 
outer extremity of the rhabdion is cylindrical, and is surrounded by a number of 
pigment cells (p), forming a structure which has been called the iris. I shall speak of 
these cells as the outer pigment cells of the rhabdion. In this region the rhabdion is 
seen to be grooved longitudinally, the grooves being filled by prolongations of the 
pigment cells (fig. 10 e). These details are best seen in sections of the eye of Syrphus. 
Beyond the region of the outer pigment cells the rhabdion is triquetrous, or more 
rarely quadrangular (figs. 10 f, and II); a double bundle of fine moniliform pigmented 
fibres lies at each angle. These pigmented fibres are partly derived from the outer 
MDCCCLXXVIIT. 4 F 
