322 
Psyche 
[September-December 
Methods 
The apparatus consisted of a plexiglass trough (7 X 32cm) with- 
in which was placed a second, “U” shaped trough with screen 
baffles on each end. Water was fed into the inner trough by means 
of a 2.4cm hose coming from a 20 1 reservoir positioned 35cm 
above the trough. A hose from a second distilled water source 
kept the reservoir filled and an overflow kept the water level con- 
stant. In this way water within the hose entering the trough was 
maintained at a constant pressure. A clamp between the reservoir 
and the trough regulated the current flow. Water flow within 
the confined area of the U-shaped inner trough appeared to be 
uniform when checked with methylene blue dye. Current velocity 
was measured with a Gurley No. 625 Pygmy Current Meter. 
Water temperature was maintained between 18 and 22° C. 
Insects were placed singly in the inner trough and water ve- 
locity increased until they appeared to be unable to make head- 
way against the current . A total of 45 trials were run for each 
species. Trial results are given in Table 1. 
Results and Discussion 
As predicted, insulata is best able to maintain its position 
against a current. But the ability of backswimmers to swim 
does not seem related to the mechanical advantage of swim- 
ming legs alone. Presumably, larger backswimmers have greater 
muscle volumes per unit of frictional surface, and as in Corixidae 
(Young 1969) swimming ability seems improved by large body 
size. 
Notonecta insulata and irrorata are similar in body size but 
differ in hind-leg mechanical advantage and accordingly in 
swimming ability. The lower mechanical advantage of insulata 
favors its ability to swim against a current. Laboratory obser- 
vations (Gittelman, in prep.) on the ability of these species to 
capture highly mobile prey (Corixidae) indicate that irrorata, 
presumably in part because of its higher mechanical advantage, 
is better able to make the short, quick attacks, necessary for 
capturing these prey. 
When compared to irrorata and undulata, lunata has a low 
mechanical advantage. But due to its small size it swims only 
as well as irrorata. N . undulata is intermediate in body size and 
mechanical advantage and is intermediate in swimming ability. 
