THE SIMPLE AND COMPOUND EYES OP INSECTS. 
587 
these cells which line the chamber. They are moniliform, with small granules of 
dark brown pigment. The walls of the chamber are chitinous. 
The rhabdia are hexagonal in transverse section in their outer extremity (fig. 23, a), 
and in the young imago at least are easily seen to be made up of six cells surrounding 
the central highly refractive threads. The inner portions of the rhabdia are round in 
transverse section (fig. 23, b and c) ; these organs are everywhere pigmented with fine 
black pigment. In many of my preparations they contain bright globules like oil ; 
1 suspect this is due to degenerative changes during the preparation of the specimens. 
I have observed the same in the stemonata of Formica. 
The rhabdia are surrounded by a network of stellate cells containing black pigment 
(fig. 22). 
The trachea of the rhabdia form a network in the spaces between them ; but there 
is nothing like the large blind tracheal sacs found in the same region of the eye in 
the Diptera. 
The external ganglionic retina (fig. 24) differs from that of the Diptera in the large 
quantity of black pigment developed in it : this is contained in the stellate cells of 
the neuroglia. The granules or round cells (n) are more numerous than in the 
Diptera, and form several layers, and the place of the facelloid layer of the Dipterous 
eye is occupied by a triple layer of large prismatic cells (/ and n). These also 
contain a large amount of pigment. I have not been able to make out the structures 
of this portion of the eye with the same clearness as in the Diptera, owing to the 
pigment in the cells of the neuroglia. 
YI. On the Structure of the Eye in Acridium (Stenobothrus). (Figs. 25, 26, and 27.) 
The cornea is not divided into facets in this insect, but both its surfaces are 
continuously curved. Beneath the cornea is a framework of chitinous chambers like 
the cells of a honeycomb ; these are xmroth of an inch in diameter. In each chamber 
there is an exceedingly complex tetrasome ; this consists of two parts, which I shall 
call the tetrasome (t) and the tetraphore (t') (figs. 25, 26, and 27). 
The tetrasome is placed immediately beneath the cornea. In the young Acridium, 
just before the development of the wings, it consists of four transparent nucleated 
cells (fig. 25) ; but in the adult insect these are developed into four spherical highly 
refractive bodies containing numerous minute vacuoles* (figs. 26 and 27). They are 
supported on the sides of a square rod-like body formed of four segments, which are 
enlarged below into the body of the tetraphore. 
The tetraphore in the adult insect consists, like the tetrasome, of a highly refractive 
substance, probably chitin ; but in the immature insect it consists of four cells, which 
first become chitinous where they are in contact with each other, or they develope a 
Similar vacuoles exist in the tetraphore of Vanessa; these have been described by Olaparede, luc. cit. 
