The Water Stick Insect - Ranatra linearis in Cornwall
Many of us are aware of Stick Insects with their inherent ability of merging twig like into their surroundings, indeed a number of species are now naturalized in Cornwall. However few people are aware of our aquatic resident the Water Stick Insect - Ranatra linearis, a spectacular, if elusive opportunist lurking in our freshwater pools, ready to pounce on their unsuspecting prey.
At first glance the Water Stick Insect does look remarkably like it's terrestrial namesakes but despite this they are not closely related. Much more closely related is the Water Scorpion - Nepa cinerea with which Ranatra linearis shares certain features such as the head structure, the beak, & the breathing tube. Taxonomically both the genera Nepa and Ranatra belong to the family Nepidae or Water Scorpions. Indeed the Water Stick Insect looks a little like a Water Scorpion that has been physically stretched to the limit!
Adults can reach 6 to 7cm in length and are a reddy brown to tan brown colour. The long thin body is tipped by a breathing tube of a similar length which unfortunately looks a little like a sting but in reality is completely harmless. These insects can remain underwater for around 30 minutes before needing to break the surface film to take in air through their "snorkel". However they have sensory organs which allow the insect to maintain the correct depth for the breathing tubes to be effective most of the time.
Adults are difficult to spot amongst the aquatic vegetation, hence perhaps the reason they are under recorded, where they sit motionless waiting for their prey to come within striking distance of their formidable front legs. The legs are generally long and thin but the front pair end in hook shaped tarsus while the lengthy tibia has a notch to help impale prey items such as Mayfly larvae & Water Fleas in a pincer movement. If two prey items present themselves at the same time the Water Stick Insect can successfully strike at both simultaneously, each foreleg acting independently during the capture. They will tackle prey as large as tadpoles or very small fish fry.
The head has a prominent 3 segmented beak with which the insect pierces the body of it’s prey through a needle-like structure that firstly delivers saliva to sedate, and then digest internals, before sucking out the resulting fluids.
I have never witnessed an airborne individual, however the fully winged Water Stick Insect is known to fly on warm days, allowing it to escape from pools that are drying up or simply to colonize new sites. When not in use, (most of the time!), the wings are folded neatly over the back, flattened against the abdomen , running from roughly the middle pair of legs to almost as far back as the base of the breathing tube.
Occasionally Water Stick Insects can be found walking across dry land.
Although the adults are active all year long, except perhaps during extremely cold weather, mating usually occurs in the spring and the eggs are laid shortly after. The eggs are easy to spot once you get your eye in and are often laid in batches of 10 to 20 together along the floating leaves or stems of aquatic plants such as potamogeton.
Each individual egg appears as a white cigar shape, spearing the host aquatic vegetation in neat rows. At one end of the 3mm long egg there are two thread like structures which are thought to draw air down to the egg providing it with oxygen. These thread like structure provide a useful indication of the presence of Water Stick Insects at a site given that they will often appear as a line of “white shoe laces” along the surface of floating leaves.
After 3 or 4 weeks the eggs hatch and at first the young nymphs are a greeny yellow colour with contrasting brick red eyes. The nymphs breathing tubes will remain relatively short until the final moult around two months after hatching.
Many texts suggest that this is a creature of deep water with plenty of vegetation, however while this may certainly be the case for overwintering adults, during our cornish summers my own encounters with the insect have frequently been around the shallow margins of pools, amongst relatively sparse vegetation, where the water is perhaps less than 30cm (1ft) deep.
In Cornwall I have found Ranatra linearis at Tregoss (SW963601) on Goss Moor, St Gothian Sands (SW582416) at Gwithian, Rospannel Farm (SW391258) near Crows-an-wra, and Hayle Kimbro Pool (SW694169) on the Lizard peninsula , but perhaps the best place to see them is at the dragonfly scrapes at Windmill Farm (SW689155), also on the Lizard peninsula. Given that all these sightings have occurred during 2009 & 2010 after a gap of almost 20 years since my first encounter with the species, it has to be said that the Water Stick Insect, although not rare, is far from easy to find until you become atuned to the macro scale of it's aquatic world.
The NBN Gateway shows four further records from the Bude Canal (SS207058), Bell Lake near Camborne (SW624420), Central Goss Moor Pools (SW950600), and a 10km record for an unspecified site on the Lizard peninsula (SW71).
Photos from the top:
Water Stick Insect showing the beak detail
Water Stick Insect showing the long front legs
Water Stick Insect eggs inserted through a plant stem showing the rudimentary breathing tubes
Water Stick Insect eggs in potamogeton leaves with recently emerged nymphs (note how short the nymph's breathing tubes are)
Water Stick Insect nymph with breathing tube starting to grow
Close up of Water Stick Insect eggs
Fully adult Water Stick Insect