Damselflies chase water striders

Water striders

I could sit for hours on the dock at the cottage, just watching the water glide and nature unfold. On calm days (and most mornings are calm) water striders spark across the surface of the bay. Dragonflies hover, defend their airspace, hunt and mate. But this summer for the first time I noticed these two insect group interacting in a particular way.

Water striders have always entranced me. Their community looks like a negative image reflection of the multitude of stars in the same still water at night. Each one moves like a jerking vector with quick strokes of its little legs. They seem to bounce off each other like molecules in Brownian motion. They vanish when wind whips up waves, but sometimes they pile onto lily pads. Here are some interesting facts about water striders:

  1. Their hydrophobic legs allow them to walk on surface tension. The middle pair of legs is specially adapted with extra hairs for rowing.
  2. Their front legs detect water vibrations, allowing them to locate prey and declare individual territory.
  3. Water striders prey on other insects, preferring live victims that fall onto the surface.
  4. They are true bugs (Hemiptera), so they have sucking mouth parts. They feed by puncturing prey with their claws, then sucking out their meal.
  5. Water strider nymphs are left to fend for themselves and often cannibalized by adults. North American species do not recognize their own kin, so they may eat their own young.
  6. Outside of mating season, they seldom cannibalize. Large cooperative groups may form to feed on large insects.
  7. Some generations of the same species do not have wings and others do, allowing them to disperse to other areas or bodies of water. Most individuals inhabiting stable bodies, such as lakes, do not have wings.
  8. Cannibalism and flight dispersal control population size.
  9. Scientists have identified more than 1,700 species worldwide, with 10 percent living on seas and oceans.

Water strider movements

This summer we had a big, healthy crop of water striders on the bay. This caught the attention of little bluets. These damselflies are one of nine Enallagma species found in Ontario, all difficult to distinguish. They’re no bigger than a darning needle and make no sound as they fly.

Bluet (damselfly)

Bluets hovered over the crowd, causing a commotion. Wherever they flew, a path cleared like the waters of the Red Sea as water striders scrambled to escape. However, the damselflies spent less time over the large groups than scattered individuals. They looked like sharks confused by a swirling school of fish, waiting to pick off stragglers. Occasionally one would duck down to the surface for a catch.

Water striders can also fly to escape predators. I couldn’t tell whether these ones were wingless. But even in the air, presumably they would have a hard time evading such an agile hunter.

 


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