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Saturday, June 2, 2012

Upside down

Backswimmers were also common at the shallow pool in the dunes where we saw the clam shrimp (see yesterday's post).

Although it might be hard to tell because the water was so clear, in all of these pictures the backswimmers are completely submerged, floating upside down at the water's surface.  (All of these individuals were between 1-1.5 cm long.)

Note the extremely long oar-like hindlegs that are fringed with hairs, an adaptation for swimming or rowing.  In the view below you can see the raptorial forelegs tipped with claws.  They use them for capturing and holding prey.

Backswimmers are active predators, detecting their prey visually and through vibrations.  They have two large compound eyes that can focus both horizontally through the water and at an angle towards and above the water surface (see photo and diagram below).

Adapted from Schwind, R.  1983.  Zonation of the optical environment and zonation in the rhabdom structure within the eye of the backswimmer, Notonecta glauca.  Cell & Tissue Research 232: 53-63.

Backswimmers eat a variety of prey (invertebrates and vertebrates).  We observed one with a small tadpole and another with a damselfly nymph.

In the photo above, the tadpole's head is close to the backswimmer's eyes.  The backswimmer is also holding a small snail (between the second pair of legs).  I'm not sure, but I'm guessing the snail had been floating and that the backswimmer was just using it as a place to rest.  (They often grasp vegetation or other floating objects.)

It was great fun watching them swim and following the ripples created by their powerful strokes.

P.S.  Although a different species, here is some amazing video footage of backswimmers swimming, laying eggs, and hatching.

1 comment:

Claudia said...

Cool link! After watching the backswimmers hatch, I watched the dragonfly larvae eating the backswimmers!