L. Harrison 
115 
Philopterus bisignatus. Fig. 17 passes just in front of a stigmatic 
orifice. It will be seen that the pustulose area round the stigma is 
composed of a thin laminate cuticle ((73) differing in structure from, 
and many times less thick than, the chitin immediately surrounding 
it. The latter is homogeneous, the outer part (C 1) deeply coloured, 
the inner (C 2) translucent. Within the pustulose area the section has 
just sliced the edge of the vestibule (7). It will be noticed that the 
hypodermis {Hy) is pushed out into a cylindrical pocket under the 
stigma. The apex of the chitinous tab (T) has been cut off by the section, 
and running from it to be inserted into the body wall on the inner cir¬ 
cumference of the pustulose area is the extrinsic occhisor muscle {Occ. M 2). 
The next section (Fig. 18; the figure is slightly schematised, and shows 
more of the bulla than actually appears in the section) shows the 
continuation of the chitinous tab (T), w^hich rests upon the heavily 
chitinised walls of both vestibule and bulla. The heavy framewmrk 
showm on the upper side of the vestibule (7) is continued round the 
bulla (B) on the side opposite the tab. A short intrinsic muscle {Occ. 
M 1) runs from the tab to be inserted into this framework, which may 
be looked upon as the closing bow. 
The actual closing of the trachea is brought about at the junction 
between the bidla and the narrow initial part of the stigmatic trachea. 
By the simultaneous contraction of both extrinsic and intrinsic occlusors 
the tab is rotated inwards and forwards so that the knob-like process 
on its proximal surface (which apparently represents the closing piece 
of the typical apparatus) is brought directly over the narrow tracheal 
channel, and is depressed so that the thin elastic dorsal w'all is closely 
apposed to the more hea\'ily chitinous ventral wall. This seems to me 
the most reasonable explanation of the observed facts; but they are, of 
course, capable of other interpretation. I assume that the trachea 
reopens of its own elasticity when the muscles are relaxed. It may 
be that occlusion is completely brought about by the intrinsic muscle, 
and that the extrinsic muscle reopens by exerting a leverage on the 
tab against the fulcrum afforded by the opposite end of the closing bow. 
This type seems to be common to all Ischnoceea, and is, I believe, 
present in the same or a slightly modified form in many Amblycera. 
An intermediate type seems to be present in Heterodoxus (Fig. 19), the 
figure being drawn from a potash-cleared mount; but I have been 
unable either by dissection or by the examination of several series of 
sections to convince myself as to the relations of the occluding 
muscle or muscles. 
8—2 
