582 
MR. B. T. LOWNE ON THE MODIFICATIONS OF 
provided with a chamber like the eye of a true dipterous insect, surrounded with 
palisade-like rods of pigment. 
The stemon (st) is much shorter than that of the Wasp. Each has four elongated 
cells attached to its surface ; these, as well as the stemon itself, are coloured with violet 
pigment. This pigment is in fine granules, and, like that of the rods in the eye of the 
Lobster, according to Kuhne,* and that of the eye in all the insects which I have 
examined, is unaffected by light. The transverse section of the stemonata (fig. 8a) 
shows that they are cylindrical and not prismatic ; they exhibit four or more bright spots 
on their periphery, and are surrounded with granules of purple pigment. I am at a 
loss to understand the bright spots, but am inclined to view them as the result of 
molecular change ; they may, however, be the indications of highly refractive threads. 
I have not, however, been able to detect any such threads in the vertical sections. 
The stemonata rest on a limiting membrane of chitin ( m ). 
I have been more fortunate in the examination of the ganglionic retina of the Ant 
than in that of the Wasp. The stemon is connected with the nuclear layer by a 
single thick nerve fibre (n) ; but from what I have seen in the Lepidoptera I have no 
doubt that by appropriate preparation this would be found to consist of a large 
number of component fibril he. My preparations of the eye of this insect were made 
from specimens which were killed some two or three years ago by immersion in spirit, 
and which had been put away and forgotten. The ganglionic retina (g) consists exclu- 
sively of small nuclei, or perhaps of very small round cells : these are connected with 
the deeper ganglia by bundles of nerve fibres. 
I have not detected any stellate cells, nor have I found the fusiform cells so 
universally present ; but I have not obtained sections of the deeper ganglia ; neither 
have I as yet, in any of the insects which have a semi-compound eye like the Ant, 
detected the presence of any decussation of the fibres of the optic nerve. I have not, 
however, obtained a thoroughly satisfactory section of all the parts of the nervous 
structures connected with the optic tract, owing to the difficulty of getting a section 
in the plane which includes them all, if such a plane exists, as it certainly does in 
many of the Diptera and Lepidoptera. I am inclined to believe, however, from what 
I have seen, that no such decussation occurs in these insects. 
IV. On the Structure of the Compound Eyes of Eristalis tenax, Syrphus, Musca 
vomitoria, Stomoxys, and Tabanus. (Figs. 9 to 20.) 
The eyes of all the Brachycerous Diptera which I have examined are formed on one 
type, which differs entirely from that on which the eyes hitherto described are 
formed. They all have a cavity beneath each facet of the cornea containing a slightly 
coagulable fluid. At the inner extremity of this cavity, which is conical, there is a 
body consisting of four nuclei or small cells, and beyond this a rod-like structure 
which apparently differs but little from the stemon. I shall, however, distinguish it 
* Loc. tit. 
