578 
MR. B. T. LOWNE ON THE MODIFICATIONS OF 
to state that the eyes described are typical examples of the structure in the Order in 
question. 
I shall conclude with some remarks on the function of the compound eye. 
All the preparations, except where the contrary is stated, were prepared from 
insects hardened in a 2 per cent, solution of chromic acid. I have not found the 
peroxide of osmium so good in their preparation, and have only used it in a few 
instances. 
I. On the Structure of the Stemmcita of Eristalis tenax. (Plate 52, fig. 1.) 
I have but little to add to what is already known of the structure of the simple eye. 
I have at present only examined it in a few of the Diptera, but have found such com- 
plete accordance between the descriptions of the authors already named and my obser- 
vations, that I shall only briefly describe the structure of the ocellus of this insect, as 
it affords the best starting point for the correct interpretation of the structure of the 
compound eye. 
Fig. 1 represents the ocellus. It consists of a very convex lens, rather more convex 
on its inner than its outer surface. Immediately behind the lens are the recipient 
structures, rods (fig. 1, a), consisting of an outer and an inner segment. The outer 
segment (a), which is next the lens, is a cylindrical, highly-re tractive rod ; the inner 
( h ) is a fusiform nucleated cell. The inner segments are surrounded and separated 
from each other by an orange-coloured granular pigment. 
The outer segment of each rod is from togo^ 1 t° Tooth of an inch in length, and 
oufooth of an inch in diameter ; it is finely striated in the longitudinal direction. 
These rods are not closely packed together, but seem to lie in a fluid ; this may, how- 
ever, be a post-mortem change. Those at the periphery of the eye appear to be twice 
as long as those at the centre. I have not found them to be doubly refractive, nor 
have I ever observed any transverse division into disks. 
The inner extremity of each rod-cell is connected with a fusiform cell (fig. 1, c), or 
with several fusiform cells arranged one beyond the other, and these are connected 
with the central nervous ganglion by fine nerve fibres. The fibres are surrounded by a 
few minute granules of a highly refractive substance. The nerves of the three ocelli 
unite into a single trunk. 
The principal fact to which I would draw special attention is the apposition of the 
recipient elements of the retina with the lens, and the entire absence of anything like 
a vitreous body. In the young eye, the percipient structures are separated from 
the lens by a layer of cells. I have not observed this condition in the present species, 
but it is seen in the ocellus of the larva of D ficus, Acilius, &c. 
The great convexity of the lens in the ocellus of Eristalis must give it a very short 
focus, and it is manifestly but ill adapted for the formation of a picture. The com- 
paratively small number of rods must further render the production of anything like 
a perfect picture, even of very near objects, useless for purposes of vision. I strongly 
