Pond bugs - the long and short of it

As you may have noticed if you're a regular, since building my own garden pond, I have been pretty enthusiastic about all things lentic (I like that word) and this post is no exception. During a recent Southampton Natural History Society field trip (hosted by Naomi, an ecologist from Pond Conservation) to look at invertebrates associated with New Forest ponds, numerous fine creatures were found, including a couple of spindly beasts that are probably unfamiliar to most people who don't regularly wade around in the mud and reeds, pond net in hand. The first of these is the Water Stick-insect Ranatra linearis.

Water stick-insect Ranatra linearis

The first thing to say is that this isn't a stick-insect, it's a true bug (Hemiptera) in the same family (Nepidae) as water-scorpions, and as such has piercing/sucking mouthparts. The long 'tail' is its breathing tube 15-20mm long at the end of a 30-35mm long body (not including the legs) - quite a striking creature, and though locally common in southern England and wales, rarely seen as it tends to lurk in dense pond vegetation, where it is an ambush predator (like a mantis for example) grasping prey with its raptorial front legs. If caught in a pond net, it tends to look like a bit of twiggy detritus until it moves - the occasional human has been known to jump in surprise at pond-dipping events and the like...

The raptorial front legs of Ranatra linearis

Here you can clearly see how the legs can shoot out mantis-like and grab prey (often small newts and fish) which will then be pierced by the pointed mouthparts (also visible). It is able to fly - useful if a pond dries up, and they also hunt on land - and although I have never seen one doing so, apparently it looks similar to a damselfly when in flight. The eggs are a peculiar sight too - when laid on leaves of water plants such as reedmaces and bur-reeds, breathing tube emerge into the air, and the eggs look like little rows of surgical stitches; there are good photos in Denton (2007).

Moving onto something smaller, at 9-12mm long (excluding legs and antennae), the Water-measurer (Hydrometra stagnorum) is another true bug common in much of lowland England and Wales (less so in Scotland and northern England) and is especially elongate, having the longest length-to-width ratio of any British insect - even the head is elongate, protruding well in front of the eyes which can be seen about half way between the antennae and front legs.

The Water-measurer Hydrometra stagnorum

It tends to be found around the edges of ponds and streams, including bankside vegetation as well as the water surface, and are also known from ephemeral waters where they can be found under debris during dry periods. Although they are generally unwinged, individuals with wings do occasionally occur. Attracted by ripples of movement (Savage 1989), they feed on smaller invertebrates such as water-fleas (Cladocera e.g. Daphnia) and mosquito larvae, piercing them through the water surface with their mouthparts, and sometimes pulling them up through it - Fitter & Manuel (1994) includes a photo of prey being consumed. Unlike R. linearis above, the front legs are simply used for walking and are not involved in catching prey - their very regular tip-toe gait gives them their common name as they appear to be pacing out distances. There is a second slightly smaller species, H. gracilenta, but this is very rare in Britain, known only from a handful of locations in southern and eastern England. Back in the 1950s, it was apparently found in the New Forest, but the account was never published and so the alleged location remains a mystery (Huxley 2003) - to find one there would be an entomological highlight!

Although both of these species are fairly common, this is not true of all our aquatic bugs - many (most) ponds in lowland Britain are polluted to some extent, especially by nutrients from our intensive agricultural system, and this has seriously impacted the biodiversity of pond ecosystems. With both point and diffuse sources being a problem, it can be difficult to ensure good ecologicsl water quality, but advice (on managing existing ponds or creating new ones) can be sought from organisations such as Pond Conservation. Garden pond ponds are a little different as few of us have enough room for the metres of shallow muddy margins that a really good pond needs, but some of the same advice applies and they are really important oases - and excellent fun to watch as the wildlife colonises it and does its thing!

Naomi from Pond Conservation hard at work looking for aquatic invertebrates in a shallow pond in the New Forest.

References

Denton, J. (2007). Water Bugs and Water Beetles of Surrey. Surrey Wildlife Trust, Woking. [Covers a good range of species; lots of useful info and excellent photos even if you're not in Surrey].
Fitter, R. & Manuel, R. (1994). Lakes, Rivers, Streams & Ponds of Britain & North-West Europe. HarperCollins, London. Also published in 1986 as Field Guide to the Freshwater Life of Britain and North-West Europe. [Both versions are out of print, but worth tracking either down online as they can be quite cheap.]
Huxley, T. (2003). Provisional Atlas of the British Aquatic Bugs (Hemiptera, Heteroptera). BRC, Huntingdon.
Savage, A.A. (1989). Adults of the British Aquatic Hemiptera Heteroptera: A Key with Ecological Notes. FBA, Ambleside. [The standard identification guide].