412 
PROF. B. T. LOWHE ON THE 
In most insects the outer, highly refractive ends of the retinal 
end-organs are imbedded in abundant pigment. The flies are 
tbe only exception, and in these the cells surrounding the bacilli 
are free from pigment. 
The Tracheae (Plate XXVII. fig. 3) form a dense network around 
the inner segments of the retinal end-organs in insects, and 
branches extend to and perforate the basilar membrane. These 
fine tracheae are without any spiral markings, and are easily 
mistaken for fine nerve-twigs. The figure given (fig. 3) shows 
these trachese in a moth, and it can be readily seen that they 
lie between the nerve-end organs, and that they branch 
dichotomously between the great rods. The aniline stains at 
once colour the tracheae, whilst they have no effect upon the 
nerves. These stains, however, attack the nerve- sheaths, but 
not the outer ends of the retinal end-organs. By the use of 
aniline stains, especially Spiller’s purple, I have been able to 
trace the finer tracheal vessels, which have been constantly mis- 
taken for nerves, to the larger tracheal trunks and in one of my 
photographs this relation is sufficiently evident. 
The illustrations on Plate XXVII. show the large size of 
the bundles of optic nerve-fibres with their terminations in the 
retinal end-organs ; they also show that nothing bearing any 
proportion to the magnitude of these nerve-cords passes through 
or even up to the basilar membrane. The basilar membrane is 
chitinous and has a cellular layer on both its inner and outer 
surface ; that on its inner surface consists of branching or stellate 
cells, which are continuous with the sustentacular framework of 
my retina ; the outer layer consists of pigment-cells, continuous 
with the pigment-sheaths of the great rods. The j>erforations in 
the chitinous layer of the basilar membrane are between and not 
opposite to the extremities of the great rods, and transmit the 
tracheal vessels. 
The structure of the great rods has with some been the diffi- 
culty in accepting my views. The appearance of these structures 
in many sections is certainly perplexing. The reason is that 
which I have already insisted upon. In life they are hollow tubes 
filled and distended w r ith fluid. In bad preparations they appear 
stellate in transverse sections and present no central cavity ; in 
radial sections they are separated from each other by wide spaces, 
often filled by distended tracheal vessels. 
