1914] Tower—Mechanism of the Mouth Parts of the Squash Bug 103 
with the action of the longitudinal muscles of the sheath, play 
some part in passing the food down the cesophagus. 
The pharynx is double U-shape in cross section. The lateral 
margins of the dorsal plate (dp) are attached to the dorsal edges 
of the rigid chitinous ventral plate. To the dorsal surface of the 
dorsal plate are attached the powerful pharyngeal muscles (phm), 
see Pl. 1 f. 6, which retracting draw up the dorsal plate creating 
a vacuum thereby drawing the plant juices up through the suction 
tube of the setze and into the pharynx. ; 
The pharyngeal muscles are attached dorsally to the inside of 
the head capsule. There are three distinct series of pharyngeal 
muscles attached to the dorsal plate of the pharynx. The first or 
anterior series is short and composed of small muscles. They are 
attached to the dorsal plate just posterior to the epipharynx. The 
second is the longest series and is attached to the widened middle 
portion of the pharynx. The third or posterior series is attached 
to the more flattened upcurved posterior portion of the pharynx. 
As the dorsal plate lies on the ventral plate, when the pharyngeal 
muscles are relaxed it seems reasonable to suppose that this pump 
acts in the following manner. The middle portion of the pharynx 
is filled when muscle series one contracts and is followed by the 
contraction of series two, the third series remaining relaxed while 
the middle portion of the pharynx is filling. When the middle 
portion is full the first series of muscles relaxes, allowing the dorsal 
plate to drop. The third series now contracts, opening the way 
into the cesophagus as the second series relaxes forcing the con- 
tents of the pharynx down into the esophagus. Series three now 
relaxes to complete the emptying of the pharynx. At the 
completion of this series of contractions and relaxations at the 
posterior end of the pharynx, no doubt a new series of similar 
contractions and relaxations begin again, or possibly they start just 
before the completion of the first series. As the above descrip- 
tion describes a wave-like motion and as no distinct valves have 
been found as are seen in the salivary pump, the above described 
process is no doubt the correct one. 
Anteriorly the hypopharynx is marked off from the remainder 
of the ventral plate of the pharynx by raised irregular thickenings 
situated at the base of the hypopharynx on its lateral walls. The 
epipharynx is similarly marked off from the remainder of the dorsal 
