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305 
large collection of these into which goes a tracheal trunk from each of 
the dorsal vessels. The function of these cells surrounding the heart 
in insects does not appear to be known, although in the bug at least 
they would appear to be in the nature of fat bodies since the}'' are very 
inconspicuous in a famished specimen, whereas in a well-fed one they 
are clearly seen. 
Respiratory System. 
The respiratory system of the bug is more or less of the ordinary 
insect type. As already mentioned, there are nine pairs of spiracles, 
seven on the abdomen and one each between pro- and meso-thorax, 
and meso- and meta-thorax. 
Round each spiracle the chitin is much thicker than elsewhere and 
the opening is narrow in the thickness of the wall, so that the hole is 
of hour-glass shape. 
The trachea does not rise directly from the inside of the spiracle, 
but from a diverticulum like a glove finger. Possibly the presence of this 
diverticulum, which renders difficult the passage of foreign bodies into 
the trachea, explains why there is no apparatus for closing the spiracle, 
as is so common in the insect world (Fig. 27). 
The main trunk rising from this vestibule is at first slightly swollen, 
this being possibly a remnant of a system of air sacs, such as are found 
in flying insects, since the bed bug is evidently a degenerate flier. 
Very near the spiracle the trachea divides, one branch going directly 
across to, and anastomosing with, the corresponding branch from the 
opposite spiracle, and on the way giving off branches on either side. 
The other branch, with a good deal of “slack,” joins the main longi¬ 
tudinal trunk of its own side. These lateral trunks run from the seventh 
abdominal spiracle right up to the head, where, after passing through 
the circum-oesophageal commissure, in compjany with the oesophagus, 
they divide into capillaries in the substance of the head supplying all 
the organs, even the tips of the antennae and the rostrum getting their 
share. 
Since the bug is about 0-5 mm. thick when hungry, and when full- 
fed increases to 3 mm., it can readily be seen that there is great necessity 
for the above-mentioned “ slack ” since the longitudinal trunks are 
slightly “ tacked ” to the dorsal surface of the body wall, and follow 
its movements. 
In structure the tracheae are internally strengthened with the typical 
chitin rings, although the diverticulum connecting the trachea with the 
