Water Scorpions

by Ade Foster

ranatraIn September, 1995, Claire McCormick and I visited Inverleigh Common in search of coconut ant nests and frogs. While scooping in the dam on the adjacent golf-course we found two very unusual insects. At first I thought they were Phasmids,stick insects that had fallen into the dam and were drowning. Then we noticed the front legs... raptorial like a praying mantis. We put them back into the water, still confined in the net, and they swam immediately to the bottom. We placed them carefully into a jar and carried them home proudly, where Dennis identified them as Ranatra or water scorpions. I put them in an aquarium and kept them until they died recently, sixteen months later!

Water scorpions are true bugs, Hemiptera, and belong to the family Nepidae, which has about two hundred species world-wide. I could find very little written about Australian Ranatra but it seems there are three species. Ours were probably R. australiensis, called quite descriptively, Needle-bugs. They inhabit shallow swamps, ponds and slow-moving streams, where they conceal themselves among under-water plants. They are seldom found in water more than a metre deep.

Ranatra measure about 100mm when fully grown, half of this being the siphon or breathing tube which extends from the end of the abdomen, and is pushed up through the surface film to allow the insects to breathe. They are able to dive to the bottom of their habitat and remain there, immobile for very long periods. Though able to swim, the long, thin, hind legs are not fringed with hairs as are many of the aquatic bugs and beetles.

The front legs are raptorial ... grasping legs armed with sharp barbs for seizing and holding prey. Ranatra eat aquatic insects and their larvae, tadpoles and even small fish. These are impaled on the short beak, while the fluids are sucked out. They are voracious feeders, and I have seen them happily eating a small beetle with a wriggling fish held firm in one claw, waiting for the entree to be finished.

Eggs are laid in a slit in the stems of water-weed, with long filaments leading from each egg to the surface of the water. They young are tiny replicas of the adult. I scooped some from Reedy Lakes in January 1996 which were about 15mm long and so finely built that they were unable to support their own weight out of water. Most species of Ranatra are able to fly, unfolding their wings with the aid of their hind legs before taking off in search of other waters.