EPOW - Ecology Picture of the Week

Water Scorpion Down Under

Water Scorpion (Ranatra dispar or R. diminuta), Family Nepidae, Order Hemiptera
Otway Range, Victoria, Australia

Explanation:  Not a true scorpion, this large aquatic insect, known as a Water Scorpion, nonetheless can exact a painful bite.  

This week we are exploring the remote Otway Range of southern Victoria, southeast Australia.  These are native eucalypt forests protected in the large Otway National Park that extends to the southern coast, and in various disjunct segments of Otway Forest Park of Victoria.  Much of the national park is accessible only by footpaths ("tracks") or the few paved and dirt roads that cross these very steep parallel ridges.

We are at a small forest pond on one of the ridgelines in the park.  How did this water scorpion get to this isolated habitat?  Well, water scorpions not only crawl and swim, they also fly, which is how I once discovered another species on a footpath in northern Tanzania.

Water scorpions (sometimes also called water sticks) "breath underwater" through long tubules extending from the end of their bodies, seen in the main photo above.  They prey on other insects as well as tadpoles, frogs, and fish; this pond was alive with tadpoles, so this insect was well supplied.   

This individual belongs to family Nepidae which are largely ambush predators.  Their raptor-like forelegs have special sensors that detect movement through the water; sensing vibrations, along with visual cues, give them a predator's advantage.  It is of the genus Ranatra, and is of the species Ranatra dispar or Ranatra diminuta.  

One research study (Bailey 1986) found that the water scorpion Ranatra dispar chooses whether or not to strike at a prey based largely on prey size and less on degree of hunger, but the distance of the prey when it does strike is more influenced by hunger and not prey size.  And all this is influenced by the structure of the grasping forelegs.  

Regardless, be careful where you place your toes and fingers in this pond!

And check out this brief video I shot of this individual milling about in the tepid, algae-filled waters of this particular pond (the audio has cockatoos screeching in the trees overhead):

  
    
Information:
     Anderson, N.M. and T.A. Weir.  2004.  Australian water bugs: their biology and identification (Hemiptera-Heteroptera, Gerromorpha & Nepomorpha.  Apollo Books, Denmark, CISRO Publishing, Canberra, Queensland, Australia.
     Bailey, P.C.E.  1986.  The feeding behaviour of a sit-and-wait predator, Ranatra dispar (Heteroptera: Nepidae): the combined effect of food deprivation and prey size on the behavioural components of prey capture.  Ethology 71(4):315-332.

Acknowledgments:
     My thanks to proto-Dr. Sandra Penman, University of Melbourne, for letting me join her on her field research outing where we discovered this pond and its inhabitant.