On the 19^th of May I received an e-mail from Andrew Green of Harleston in Norfolk with a fascinating observation of the behaviour of Ranatra linearis, the Water Stick Insect. Andrew’s message contained an attached photograph which, as you can see shows the insect walking amongst stones and well out of water.

Andrew says that the insect was spotted at the Hen Reedbeds Reserve of Suffolk Wildlife Trust. It was initially seen by reserve's voluntary warden and was walking relatively briskly and purposefully along a gravel path, near the car park. Then whilst the reserve's voluntary warden, Peter Vincent and Andrew looked on it continued for another 30 yards along the path. The observation was made at 12.45pm on Sunday 16 May 2004. The weather was sunny and the temperature about 20ºC. The water scorpion made no attempt to enter the damp vegetation on either side of the path. Parts of the path contained larger stones and the insect had no problem in climbing over these, as can be seen in the photo.

Andrew, I suspect in common with most people, had never seen Ranatra before, and asked if its behaviour was unusual. Some general information will be useful in order to answer his question.

Ranatra linearis belongs to the Hemiptera, Heteroptera - the aquatic bugs. It is of  the family Nepidae. The only other British species in this family is Nepa cinerea, the Water Scorpion. Both species are similar in that they have a breathing tube enabling air to be drawn from the surface whilst submerged. However, whilst Nepa is flat and boat shaped, only superficially like a scorpion due to the breathing tube, Ranatra is very much reminiscent of a stick insect, as can be seen from Andrew’s photograph. However its aquatic habitat makes his observations seem very unusual.

Water Stick Insects are locally common in parts of South East England and the current known northern distribution seems to end in Nottinghamshire although there are old records from parts of Wales and also Yorkshire (Huxley, 2003). It's certainly not rare in Suffolk, Essex and Norfolk and although usually found in ponds, gravel pits and canals I have also recorded it from the sides of flowing rivers here in Suffolk, particularly in float grass. Several other heteropterists have also occasionally recorded it from rivers.

Most people would, I suspect, never have seen one unless they had been actively pond dipping in the right location since it seemingly spends most of its life in water. However the insect has occasionally been observed in flight, especially on hot days. Joan Hardingham, the SNS chairperson, saw a specimen of Ranatra fly in and land on black plastic, laid for strawberries, on one of the hottest days of summer in 1996 (SNS Transactions 32, 1996). Moreover there are also other reports of it on land, 'readily leaving the water to hunt' (Kirby, 1990). Quite what activity this insect was engaged in is hard to ascertain, however 30 yards does seem to be a very long way for such an insect to walk either before flight or whilst hunting. Flight might be a response to falling water levels or to breeding requirements. Hunting, for an aquatic insect, might be assumed to be rather more likely in the cool of the evening, which is when I have always observed the Water Scorpion emerging to hunt around the stream in my own garden in Boxford.

So was this behaviour unusual? I think we cannot say until we have many more observations to base an opinion on. I am sure that Andrew was lucky to be in the right place at the right time and looking in the right direction as relatively few reports of Ranatra walking on land have been published and none to my knowledge have mentioned terrain or weather conditions that seem so hostile, nor a distance so large for a creature of merely 50mm or so in length. All such observations add to our knowledge of the behaviour of our less common fauna and I should like to thank Andrew for sending it in.  It shows how all naturalists are able to add to our knowledge. I hope that his example will encourage others to also write to the SNS recorders or to White Admiral. A copy of his report has also been sent to Het News, the newsletter of the Heteroptera Recording Schemes.

References

Huxley, T.  (2003).        Provisional atlas of the British aquatic bugs (Hemiptera-Heteroptera). Huntingdon: Biological Records Centre.

Kirby, P.  (1990).          Recorder 3 – Species accounts for Heteroptera. Peterborough:: Joint Nature Conservation Committee.

© 2004   Suffolk Naturalists' Society

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