300 
Psyche 
[September-December 
what in comparison with the hypothetical plan of figure la, and 
are at an angle of about 45° from the vertical. This is in contrast 
with the mesothoracic coxae of Gerris, which have rotated so 
that their longitudinal axes are parallel with that of the meso- 
thorax (Fig. 7). In addition the pleurocoxal attachments of Gelas- 
tocoris and Belostoma are medial whereas those of Gerris are 
anterior. The orientation of the coxae and the position of the 
pleurocoxal attachments in Gerris result in legs which project 
almost completely horizontally. In Gelastocoris and Belostoma 
the projection of the legs has both vertical and horizontal com- 
ponents. In all three insects the bend in the trochanter causes 
the femur to project anterolaterally. 
The major change in the musculature has been the enormous 
development of muscles 46 and 47. In Belostoma these two muscles 
are rather small and have discrete origins, M. 46 on the lateral 
wall of the tergum and M. 47 on the pleural apophysis. Both 
insert on a single trochanteral apodeme. In Gelastocoris these 
muscles are also rather small. Muscle 46 has an origin and inser- 
tion similar to that of Belostoma; M. 47, however, is in two parts, 
one originating on the sternum, the other on the pleural apophysis. 
In Gerris these two muscles are so large as to be contiguous and 
it is difficult to distinguish between them; Guthrie (1961) in fact, 
considers them a single muscle. They originate over a wide area 
of the tergum, first phragma, pleuron and sternum and insert 
Figure 7: Coxae of the left mesothoracic legs of a) Belostroma b) Gerris to 
illustrate the relative positions of the coxal axes. 
D 
C D 
C 
F 
T 
coxa 
femur 
trochanter 
