114 
The Respirntory System of Mallophaga 
wider vestibule, the inner narrow end of which is directly continuous 
with the extremely narrow initial portion of the stigmatic trachea. 
This inner end serves as a fulcrum for the closing lever, which in the 
open position stands at right angles to the trachea, closely apposed to 
the bulla. From the distal end of this tab a muscle runs to be inserted 
into the integument alongside the vestibular rim. On contraction of 
Fig. 17. Part of first of two (.-onsecutive section.s through an abdominal stigma of 
Philopterus bisignatus. 
Fig. 18. Part of second of two consecutive sections through an abdominal stigma of 
Philopterus bisignatus. 
this occluding muscle the tab is revolved through an angle of about 
45°, and kinks the elastic trachea against the rigid inner end of the bulla, 
thus effectually cutting off the air. This form is best observed in the 
abdominal stigmata of Trichodectes and Gyropus, the prothoracic stigmata 
being more complex. 
The second type is illustrated in Figs. 17 and 18, which represent 
a part of two successive transverse sections through the abdomen of 
