OF SUCTION IN LYGUS PABULINUS. 713 
These muscles fall into two groups, according to the mode of 
their attachment and arrangement :— 
(a) The fan- or feather-shaped muscles. (Text-figs. 14-21).— 
These are arranged like barbson a feather vein or rachis, alternating 
with one another on either side. This arrangement begins in the 
clypeal region and gradually disappears posteriorly. They are 
attached to the opercular ligament, which is the continuation of 
the soft chitin lining the operculum. In the clypeo-labral region, 
1.€.,1n the region where the gustatory organs are situated, the 
ligament passes between those organs to give attachment to these 
muscles. The gustatory organs consist of four lobes arranged in 
pairs on either side of the dorsal wall of the pharynx. (Text- 
figs. 27, 28.) 
The fan-shaped arrangement is found in the region where there 
isa great amount of work done; the work being to produce a 
vacuum sutticiently powerful to suck in the sap which is being 
accumulated in the pharyngeal duct. This vacuum is produced 
by contractions of these muscles, the operculum being pulled ont 
of the ventral wall of the pharynx. The arrangement described 
gives a large surface for the attachment of the muscles. 
(b) The strap-shaped muscles.—These are found in the epi- 
cranial region, where they are directly attached to the upper wall 
of the pharynx, which has here lost its character of operculum. 
Here its ventral wall also loses its chitin, and the whole structure 
is gradually transformed into the soft and thin cesophagus which 
is no longer concerned with suction. These muscles are attached 
to the dorsal wall of the head in the median line. 
The divaricators occupy a large area in the head, beginning in 
the clypeo-labral region and ending in the anterior part of the 
epicranium. 
(2) The Pump muscles, or Aspirators. (Text-figs. 2, 4, 5, 7, 
17, etc. P.M.) 
These are next to the divaricators in order of importance, 
though they do not occupy such a large space. They are very 
compact and powerful, as may be seen from the mode of their 
arrangement and attachment. They lie below the pharynx in the 
median line, but they diverge on either side towards the posterior 
arms of the tentorium, to the ventral surface of which they are 
fused. At the other end they are attached to the handle of the 
piston of the pump. 
When they contract they pull out the piston of the pump, the 
posterior arms of the tentorium being very strong and rigid at the 
sides. This pulling out cannot be an easy thing, small as the 
pump is, because the posterior wall, of which the piston forms a 
part, is made of thick, unyielding, though elastic, chitin, the 
resistance of which must be enormous. Hence their dispropor- 
tionate size and the mode of their attachment can easily be 
understood. 
