Hunters in the Pond Part 2 – Terry Thormin

The dragonfly patrols the south shore of the pond, searching for a mate. It is the largest of the dragonflies in the pond, a Common Green Darner, and so pays little attention to the other dragonflies even when the diminutive Blue Dasher comes darting out at it in an effort to protect its territory. From the top of a pine tree at the east end of the pond a Merlin surveys the pond, waiting for the right moment. There are several Common Green Darners flying and surely one will be careless this time. Now! It launches itself from the pine and plummets down, picking up speed until it levels out low over the water. Distracted by the dasher, the darner doesn’t see the Merlin until it is almost too late. Frantically it twists and picks up speed, but the Merlin is too close and rakes its left side, almost tearing both wings off. The Merlin continues down the pond without its prey, but the darner, badly injured, spirals to the water’s surface where it flaps in a futile effort to take flight.

The rippling water sends out a circle of vibrations and very quickly a water strider responds, skating across the water’s surface towards the injured dragonfly. It approaches and when the struggles subside it moves in. The initial jab from its needle-like mouthparts starts the dragonfly struggling again and the strider quickly backs away. A few minutes later the struggles quiet down again as the life energies seep from the body of the dragonfly. Again the strider moves in and pierces the dragonfly’s body with its mouthparts. Now the dragonfly is only struggling weakly and the strider persists, sending digestive enzymes into the dragonfly and sucking up the resulting dissolved fluids. Within a few minutes there are four striders feeding on the dragonfly, then five and finally six. Two species share the dragonfly carcass. There are three larger water striders, wingless even as adults, and three smaller ones, all with wings. Within a couple of hours the water striders have abandoned the darner’s body and there is little left but an empty husk. Even then a couple of snails move in, gleaning whatever is left. Nothing goes to waste in the pond.

I did not actually observe this event, it is fictional, but one that could easily happen. I have observed water striders feeding on other insects trapped on the surface of the water, and just recently photographed a group of water striders feeding on a dragonfly, in this case a Hudsonian Whiteface.

Water striders are both predators and scavengers, feeding on any insect that falls onto the surface of the water. They are also known as pond skaters and Jesus bugs because of their ability to walk on the surface of the water. They are found on quiet water like marshy lake edges ponds, marshes and backwaters of rivers and creeks. They do not like fast flowing water. There is one exception to this though, and quite a remarkable exception. Water striders in the genus Halobates are the most marine of all insects. While most of the 46 species in this genus are found in coastal areas, 7 species are truly marine in distribution and can be found hundreds of miles from any land. Unlike other members of the genus, and other water striders, these 7 species feed on plankton.

Water striders are true bugs and as such they do not go through complete metamorphosis. Instead the young water striders look very similar to the adults, just smaller and without wings. The young, like the adults, live strictly on the surface of the water. There are about 750 species worldwide, the largest getting up to more than 36 mm in length.

There is another pond insect that deserves attention, the water scorpion. It is not a true scorpion, but rather an insect, and is a true bug just like the water strider. Unlike the water strider though, it spends most of its time below the surface of the water. It is an air breather though, and has to come to the surface to breath. Water Scorpions, however, have breathing tubes at the back end that are often almost as long as the body. For this reason they are often found sitting on vegetation below the surface with just the tip of the breathing tube sticking out of the water.

Water scorpions are ambush predators that look like aquatic walking sticks, but with front legs that look more like those of a praying mantis. Some species though are much broader through the body and look something like a skinny giant water bug.

Earlier this summer I observed a rather interesting episode with a pair of damselflies and a water scorpion. As usual I was sitting in my camp chair beside Little River Pond waiting for opportunities to photograph dragonflies when I noticed some activity in the water right at my feet. It was a female damselfly below the surface of the water. Female damselflies will often go completely underwater to lay eggs and the theory is that by so doing they will not be harassed by other males attempting to mate with them. Usually they remain attached to the male they mated with, but on this occasion she was by herself and she seemed to be struggling to climb to the surface on an emergent plant stem. I looked more carefully and noticed a water scorpion that had grabbed her by the wings. Most likely while she was ovipositing she got too close to the water scorpion which made a grab for her and only got the wings.

With great effort she managed to drag herself partly out of the water. Then a male flew in, most likely the one she had mated with, and grabbed her with his appendages by the front of the thorax. Anyone who is familiar with dragonflies and damselflies will recognize this as the tandem position. Between the two of them they managed to eventually drag the water scorpion completely out of the water. Shortly after the female was clear of the water the water scorpion dropped back into the water, either having let go or perhaps tearing through the wings of the female damselflies, and the damselflies flew away. That the male came to “help” her I seriously doubt, but my guess is that the female would not have made it without his help.

Water scorpions range in size up to about 44 mm not including the breathing tube. There are about 270 species distributed worldwide. All are ambush predators and will feed on things up to the size of tadpoles and small fish. They are found in quiet, shallow lakeshores, ponds and slow moving streams.

There are many other predators found in ponds, marshes, lakes, streams and other freshwater habitats, and perhaps at sometime in the future I will write about some of them. Anyone who takes time to sit, and observe at the edge of a pond will undoubtedly see and learn something about many of these fascinating creatures. Unfortunately, most of us leave our fascination with pond life behind when we reach out teens. I think that this is unfortunate as it is an amazing habitat with many amazing creatures, and one that is a good indicator of the health of the planet.

Advertisements

About annieandterry

This is a blog shared by two friends who have never met in person, Annie Pang and Terry Thormin. We both live on Vancouver Island, Annie in Victoria and Terry in Comox. All communication to date has been either by email or telephone. We are both passionate about nature and conservation and we are both nature photographers. Annie is also a very fine poet and was a concert violinist, while Terry worked as an entomologist for the Royal Alberta Museum until he retired in 2005. We hope you enjoy this joint effort to share our nature musings with anyone who is interested.

Posted on August 18, 2012, in Nature, Photography and tagged , , , , , , , , . Bookmark the permalink. Leave a comment.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s