320 
Psyche 
[September-December 
sor muscle tendon, were measured for each species of backswim- 
mer studied. As an example of these measurements a ventral view 
of the proximal leg segments of the metathoracic leg of Noto- 
necta undulata is included in Figure 1. In this earlier study, the 
mechanical advantage differed among species of different genera, 
with Martarega having the lowest and Buenoa the highest Ri/R .2 
ratios. It was concluded that the low mechanical advantage of 
the former is an adaptation to living in a current. These back- 
swimmers live in rivers and swim slowly against the water flow, 
maintaining their position, while feeding on immobile prey trapped 
in the water surface. The larger mechanical advantages found in 
Buenoa seem associated with a diet of highly mobile prey re- 
quiring the predator to accelerate rapidly in pursuit. It was con- 
cluded that the intermediate mechanical advantage exhibited 
by Notonecta represents an ecological compromise between 
Buenoa and Martarega. Buenoa, which swim below the water 
surface, prey almost exclusively on free-moving prey. Notonecta 
spend much of their time at the water surface and probably eat 
.5mm 
dep. 
msc. 
Fig. 1. Ventral view of the proximal leg segments of the right metathoracic leg of 
Notonecta undulata. tr, trochanter; cx, coxa;f, femur; dep. msc., depres- 
sor muscle; Rj , the lever arm; R 2 , the lever (see text for explanation). 
