1976] Bowdan — Functional Anatomy of Gerris remigis 297 
tion: the ventral surface of the coxa becomes lateral and the dis- 
tal portion of the leg is raised (Fig. 5c). Muscles 50, and 46 plus 
47 act on each side of the pivotal axis formed by the dicondylic 
articulation of the trochanter on the coxa (Fig. 6a). The pivotal 
axis is a dorso/ ventral one (Fig. 6a) so the plane of movement 
of the leg is mediolateral (Fig. 6b). The trochanteral bend trans- 
lates this into a forward and backward movement of the extremi- 
ties of the leg. Contraction of muscle 50 swings the extremities 
forward (promotion, protraction; Fig. 6d) and contraction of 
muscles 46 and 47 swings them backwards (remotion, retrac- 
tion; Fig. 6 c). 
Thus coxal rotation can move the leg only in a dorso/ventral 
plane, the proximal part of the trochanter can move only in a 
medio/ lateral plane and there are only five muscles which are 
important. It would seem as though the range of movement of 
the mesothoracic leg of Gerris would be very limited. In fact, 
however, the leg is extraordinarily versatile. It can move in a 
complete vertical circle, a horizontal semicircle, and any com- 
bination of the two. 
Discussion 
Comparison with Gelastocoris and Belostoma 
In gerrids the mesothoracic legs and their musculature have 
become so highly modified for rowing that it is of interest to 
compare them with those of Heteroptera with a different mode 
Figure 4: Hemisection of the mesothorax of G. remigis, to show the major 
muscles and their innervation. 
c r coxal rim 
gang fused ganglionic mass 
f p first phragma 
met metathorax 
pro prothorax 
trn trochantin 
40 muscle 40 M. nototrochantinalis 
41 muscle 41 M. notocoxalis 
44 muscle 44 M. furca coxalis 
46 muscle 46 M. nototrochanteralis 
47 muscle 47 M. pleurotrochanteralis 
1,2,3 main mesothoractic nerve branches 
