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SYDNEY J. HICKSON. 
vessels devoid of spiral markings, which perforate the basilar 
membrane to end in the inter-ommatidial tracheal vesicles 
(fig. 3, t.v.). These vesicles are at the base broad, but taper to 
a point externally, and end in the region of the external 
pigment-cells. 
In addition to these the tracheae of the terminal anastomosis 
send off other thin- walled vessels, which perforate the peri- 
opticon and communicate with similar offshoots from the 
tracheae situated behind it. 
These cannot be seen very distinctly in Musca, but in 
Eristalis they are gathered together in bundles as they perforate 
the peri-opticon, and are easily distinguished by their com- 
paratively thick walls (PI. XVI, fig. 23). 
§ 6. Historical and Critical. 
The history of the research into the anatomy and develop- 
ment of the Arthropod eye practically dates from the publica- 
tion of Johannes Muller’s work ‘ Zur vergleichenden Physio- 
logic des Gesichtssinnes ’ in 1826 (17). From that time to 
the publication of Grenacher’s (7) great work ‘ Unter- 
suchungen fiber das Sehorgan der Arthropoden ’ in 1879, the 
study was carried on by numerous investigators both in England 
and abroad, and much light was thrown upon the numerous 
branches of the subject. 
It is not my purpose to enter here into the various points 
that have formed subjects for animated discussion in times 
gone by, nor is it my purpose to give any detailed account of 
researches previous to 1879, for all such information has been 
skilfully brought together and digested in the learned ‘ His- 
torischkritische Uebersicht ’ of Dr. Grenacher’s work. Since 
then, however, certain papers have been published describing 
certain structures in the eye and optic tract of insects, which 
cannot be allowed to pass by without some criticism. 
The eye of the blow-fly was described by Grenadier (7, p. 90) 
in considerable detail, and the description I have given above 
differs from his only in a few minor points. He says, for 
instance, that the corneal facets are plano-convex, but they 
