584 
MR. B. T. LOWNE ON THE MODIFICATIONS OF 
pigment cells of the rhabdion, and partly from four pigmented nuclei which are situated 
at the inner extremity of this structure (fig. 12). These pigmented fibres become very 
tortuous when they are acted on by water, and apparently produce the contorted 
conditions of the rhabdion which have been attributed to the elasticity of its axial 
structure. The interspaces between the prismatic portions of the rliabdia are occupied 
by large sac-like tracheal tubes. These are, so far as I can tell, confined to the Diptera, 
and are quite characteristic in this group. 
The inner extremities of the rhabdia rest on a strong chitinous membrane (fig, 12, m), 
which is perforated for the rhabdia to communicate with the nervous retina beneath, and 
for the tracheal tubes. The rhabdia appear to be continuous with the thick outer pro- 
cesses of the large stellate cells of the nervous retina (fig. 12). In a few cases I believe 
I have seen two rhabdia connected with one cell. I have been unable to trace any 
continuation of the axial structure of the rhabdion into the nerve cells of this region, 
but in some specimens I have seen four fine processes continued from the rhabdion into 
the region of the nervous retina, but in these the cells had disappeared in the pre- 
paration, so that I cannot state whether these were mere connective elements, or 
whether they belong to the proper structure of the nervous apparatus. 
Fig. 12 shows the inner extremity of the rhabdia and their relation to the nerve 
structures beneath them. The oval cells are probably embryonic, as I have not found 
them in the adult imago. The drawing is from the eye of a small species of Syrphus, 
from which I succeeded in getting a very beautiful series of preparations. 
The rhabdia of the two peripheral rows of facets are united into bundles at their 
inner extremities, four or more forming a compound structure, which is surrounded by 
elongated pigment cells (fig. 10 /). These compound rhabdia have six or more 
pigmented nuclei at their inner extremities. The form of the transverse section of 
the rhabdia is very variable (fig. 10, d to i). 
The Nervous Retina of Eristalis and Syrphus. — As my most successful investiga- 
tions of the nervous retina have been made in these insects, and as the modifications 
of the other parts are best understood when the nervous retina is included in the 
description, I shall describe the nervous retina in these insects, and afterwards state 
the points in which the same part appears to differ from it in other insects. 
Fig. 1 3 represents this structure. From without inwards there are (<f ) two layers 
of ganglion cells, (n) a layer of small round cells, (/') a very remarkable layer of 
bundles of fusiform cells, so like the cells of the facellus in the eyes of the insects 
already described that it can hardly be regarded as anything but its physiological 
representative ; and (</') a third layer of stellate ganglion cells. These structures 
form the outer ganglionic retina, and are connected by a decussating optic nerve (n) 
with a still deeper layer of staff-shaped cells, or, rather, with several layers of fusi- 
form cells (c) superimposed one on the other. This inner ganglion is connected with 
the supraoesophageal ganglion by a distinct peduncle. 
The intercommunication of the elements of the external ganglion or ganglionic 
