232 
SYDNEY J. HICKSON. 
The basilar membrane is in all cases perforated by two sets of 
apertures, which are best seen in Agrion (PI. XVII, fig. 28, b.m .) ; 
the larger apertures are for the passage of the tracheal vesicles, 
the smaller for the nerve-fibrils passing to the retinulae. 
The thickness of the basilar membrane varies considerably. 
In Agrion, Aeschna, and the Libellulidse generally it is thick, 
but in the Diptera, Hymenoptera, and others very thin. 
§ 5. The Distribution of Tracheae to the Optic 
Tract and Ommateum. 
The tracheae of the insect may be divided under four heads : 
(1) large tracheal trunks lying in the various parts of the body ; 
(2) smaller tracheal vessels ramifying through the various 
viscera, with spiral markings on their walls; (3) very thin- 
walled vessels devoid of spiral markings, ramifying in the 
various tissues and organs of the body, and in some cases anas- 
tomosing with one another ; and finally, (4) tracheal vesicles, 
first recognised by Strauss-Durckheim (20), as closed sacs or 
dilatations of the tracheal vessels. Speaking of them he says, 
p. 319, “Dans les vesicules tracheenes qui ne se rencontrent 
que chez un certain nombre d’insectes, le fil spiral n'existe 
point, et elles sont reduites a la tunique exterieure ; car 
l’interieure ne s’y laisse aucunement apercevoir. ,! 
The extent to which spirally-marked tracheae are present in 
the nerve-ganglia seems to vary enormously, as may readily be 
seen by examining sections through the brain and optic tract 
of various insects, or consulting the figures which have been 
published by Leydig (12) and others. 
Taking the tracheae of what I have called the terminal anas- 
tomosis alone, we find that in Eristalis the spirally-marked 
tracheae in this region are very large and very numerous ; in 
fact they form a very dense network just behind the basilar 
membrane. In Musca they are also very numerous, but the 
network of them is not nearly so dense as it is in Eristalis. 
They are present in this region in Blatta and Dytiscus (Leydig), 
and in the Lepidoptera (Sphinx, Acherontia, Noctua), although 
