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Psyche 
[September-December 
Muscle 41 (M. notocoxalis) is also a coxal rotator and is antag- 
onistic to muscle 40. It originates on the lateral wall of the 
mesotergum and inserts on an apodeme on the lateral coxal 
rim. 
Muscle 44 (M. furca-coxalis) is a third coxal rotator. It is a very 
small muscle originating on the furca — a sternal process - 
and inserting on the anterior coxal wall. 
Muscle 46 (M. nototrochanteralis). This is an extremely well 
developed muscle which originates on the anterior two thirds 
of the mesotergum and the dorsal region of the first phragma 
and inserts on several well developed apodemes from the medial 
edge of the trochanter. It is a major retractor of the leg. 
Muscle 47 (M. pleura-trochanteralis) is also a well developed 
muscle. It originates on the ventral surface of the first phragma 
and the anterior half of the pleuron and sternum, and inserts 
on the same apodemes as muscle 46. It acts with muscle 46 to 
retract the leg. 
Muscle 49 (M. coxa trochanteralis medialis). This muscle origi- 
nates on the dorsal wall of the coxa and inserts on the same 
apodemes as muscles 46 and 47. It is a depressor of the tro- 
chanter. 
Muscle 50 (M. coxa trochanteralis lateralis). This muscle orig- 
inates on the ventral wall of the coxa and inserts on a flat 
apodeme which extends from the lateral rim of the trochanter. 
It is antagonistic to muscles 46 and 47 and protracts the leg. 
Muscles within the leg distal to the coxa were not considered. 
Nervous System 
The nervous system in Gerris is remarkable in being highly 
fused (Guthrie, 1961). All the thoracic and abdominal ganglia 
form a single ganglionic mass, which lies mostly in the prothorax. 
The three pairs of main nerves to the muscles of the mesothoracic 
leg arise from the more posterior region of this ganglionic mass — 
the region which Guthrie has identified as containing the meso- 
thoracic ganglia. Nerve 1 sends branches to muscles 46, 40, and 
41. Nerve 2 has a branch to muscle 41, but the main portion goes 
into the distal part of the leg and probably contains a large num- 
ber of sensory fibres from the sensory receptors of the leg. Nerve 
3 has branches to muscles 47, 44 and the distal part of the leg. 
