THE EYE AND OPTIC TRACT OF INSECTS. 
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in this order the walls are very thin and the spiral markings 
indistinct. Iu Apis, Aeschna, Nepa, Agrion, &c., spirally- 
marked tracheae are absent in this region. Again, in that 
region of the optic tract situated just behind the peri-opticon 
we find very few tracheae in Musca, and in some individual 
examples I have found none at all, but in Eristalis they are very 
numerous (PI. XVI, fig. 23), and even decussate with the optic 
nerve-fibrils. But although the spirally-marked tracheae may 
be absent in many parts of the optic tract there is very good 
evidence for believing that in all cases it is copiously supplied 
with thin-walled tracheal vessels. Thus, in Aeschna and 
Agrion the region marked 2 in the terminal anastomosis is 
undoubtedly occupied by thin-walled tracheal vessels, and in 
the former genus the spaces (PI. XV, fig. 7,s.) found in the 
peri-opticon are probably occupied by such vessels. 
The difficulty of differentiating these vessels in hardened and 
stained sections is remarkable. Thus, the tracheal vessels 
found in the ommateum of Musca (fig. 2, t. v.; fig. 3, t.v.) are 
quite invisible in many of my sections, although when the 
fresh eye is examined they are most conspicuous. It is true 
that in one series I have, that was stained in a remarkably 
good haematoxylon fluid [vide infra), I was able to see them 
fairly well, and it was in this series that I was able to trace 
their connection with the tracheae behind the basilar membrane 
[vide fig. 3, T. tv .) ; but, as a rule, they cannot be seen in 
hardened sections. I have turned my attention, however, more 
particularly to the distribution of the tracheae in this region in 
Eristalis and Musca. 
In both these forms two large tracheal trunks (Tt v Tt 2 ) 
are to be found at the sides of the optic tract lying in the 
groove between the epi-opticon and peri-opticon. The larger 
of these is external, the smaller internal. The larger one sends 
off special tracheae to the terminal anastomosis, and does not 
send any branches into the peri opticon or behind it. The 
smaller of them sends tracheae into the optic tract behind the 
peri-opticon, never in front of it. 
The tracheae of the terminal anastomosis send off thin-walled 
