THE E7E AND OPTIC TRACT OF INSECTS. 
221 
In Musca vomitoria only a few tracheae with spirally- 
marked walls are found behind the peri-opticon, and these 
arise from separate tracheal trunks situated at the sides of the 
optic tract. 
§ 2. The Minute Anatomy of the Optic Tract 
of various Insects compared. 
When the brain and optic tract of a very young Periplaneta 
is examined, it is found that the optic nerve separating the 
cerebral ganglion from the opticon is much longer in propor- 
tion than it is in the adult blow-flv. The opticon is a well- 
marked ganglion, and the epi-opticon is small, though distinct. 
From the epi-opticon, however, the optic fibres pass straight 
to the ommateum without passing through any intermediate 
neurospongium. In fact, the peri-opticon described above in 
Musca does not exist in the young Periplaneta (fig. 18). The 
optic fibres leaving the epi-opticon do not decussate, but as they 
approach the basilar membrane they break up into numerous 
branches which undergo a loose anastomosis. In this anasto- 
mosis a few large nerve-cells may be seen (fig. 18, n. c.). In 
the adult Periplaneta the opticon is very large and separated 
from the cerebral ganglion by a very considerable optic nerve. 
The fibres connecting the opticon and the epi-opticon decus- 
sate (fig. 19, d.f.). The epi-opticon is a smaller hemispherical 
well-marked ganglion, and the fibres which pass from it to the 
ommateum partially decussate (Nf.). My figure (19) is taken 
from a preparation of the head of an adult male cockroach with 
fully-developed wings, so that I am inclined to think that in 
this genus the optic fibres never completely decussate in this 
region. The number of fibrils passing from the epi-opticon 
to the ommateum is relatively much larger than in the young, 
and the nerve-cells in the meshes of the anastomosis are more 
numerous. The anastomosis is, perhaps, rather denser and 
more complicated than it is in the young, but I cannot find 
anything comparable to the loopings of Nepa to be described 
immediately, nor anything that could be described as a neuro- 
spongiura. 
