120 Mr. O. H. Latter on the Prothoracie Gland of 
discharge of acid and cause evagination of the vestibule, 
which then appears as a green band externally; at the 
same time they bring the orifice of the true gland (vide 
infra) up to the external surface. The retractor muscles 
are arranged in four groups of three each; a pair are 
inserted a little to the right and left respectively of the 
middle line on to the extreme posterior wall of the 
vestibule (Fig. 1, 7mv). In the resting position of the 
larva these muscle bands have a curved course to their 
origin from the ventral body-wall, but in the attitude 
assumed under alarm the course would be almost, if not 
entirely straight. The other retractors of the vestibule 
are inserted on to the extreme lateral corners of the 
vestibule (Fig. 1, /7 mv) and originate from the ventro- 
lateral body-wall. 
From the middle of the posterior wall of the vestibule 
there leads backward a short cylindrical or funnel-shaped 
duct (Fig. 1, d), which issues from the large flask-shaped 
glandular sac (Fig. 1, gs). The duct is strengthened 
and doubtless kept open by a number of stout chitinous 
hoops, which project some distance in the lumen of the 
duct (vide Fig. Ni 
The gland sac in a full-grown larva may be 8 mm. 
long, 5 mm. wide, and about 3 mm. deep. Except for 
tracheal attachments it lies almost free in the body cavity ; 
its anterior end by which it is connected to the duct lies 
just behind the first thoracic ganglion and (generally) to 
the right of the nerve cord which is thrust over slightly 
to the left in consequence (vide Fig. 5); the bulk of the 
gland is dorsal to the nerve cord between it and the 
alimentary canal. 
The minute structure of this gland has been described 
by Klemensiewicz (8), and I have not much to add to his 
description. ‘The epithelial cells that secrete the chitin 
lining the vestibule gradually pass into those of the gland 
without any sharp line of demarcation, all stages of 
transition being found along the course of the discharging 
duct. The cells of this duct get larger in passing towards 
the gland, and there is a corresponding increase of 
nucleus. ‘The cells are arranged in more or less circular 
(spiral ?) rows round the duct, and each row produces a 
well marked ridge of chitin projecting into the lumen of 
the duct; the anterior ridges are very small, and they 
gradually become more pronounced in passing backwards 
