THE EYE AND OPTIC TRACT OF INSECTS. 
227 
I have called the elements of the peri-opticon ; in bees, wasps, 
and many Lepidoptera the elements of the peri-opticon are 
long, slender, and close-set ; in Aeschna they have partially 
fused with one another ; and in Bombyx, Eristalis, and the 
Crustacea they have completely fused to form a complete and 
continuous ganglion, similar in every way to the opticon and 
epi-opticon. 
§ 3. Further Remarks upon the Histology of the 
Optic Tract. 
In the optic tract of insects we can distinguish the following 
nervous structures : nerve-fibrils and fibrillse, neurospougium, 
nerve-cells, and occasionally ganglion-cells. The nerve-fibrils 
are found in the regions between the opticon and epi-opticon, 
between the epi-opticon and the peri-opticon, and in the 
terminal anastomosis. They must be considered to be naked 
axis cylinders, are devoid of any medullary sheath, and fre- 
quently break up to anastomose freely with their neighbours. 
In the vicinity of the main ganglia they break up into a number 
of minute fibrillse, which, anastomosing together, form a dense 
plexus or meshwork, which forms the main part of the ganglia, 
and which I have called a neurospongium. This connection be- 
tween nerve-fibrils, fibrillse, and neurospongium maybe well seen 
in the epi-opticon of the young bee (PI. XVII, fig. 25, e. op.). 
Each nerve-fibril seems to be composed of a number of very fine 
fibrillse closely cemented together, and to be capable of splitting 
up into its component parts, where an intimate anastomosis is 
requisite or necessary. The different stages of the complexity 
of this anastomosis have been seen in the peri-opticon of the 
various insects I have described above. Thus, in Blatta the 
fibrils split up into finer fibrils in this region, which anastomose 
with one another, but the most ultimate division of each fibril 
seems never to occur. In Nepa the anastomosis is still more 
complicated, and in some cases the nerve-fibrils seem to split 
up into their ultimate fibrillse. In Musca finally the sub- 
division of the fibrils is complete, and a true neurospongium is 
formed by the anastomosing fibrillse. 
