Water-measurers are part of the insect ‘boat’ community - whirligig beetles, water crickets, pond skaters and water-measurers all live on top of the water surface. All these animals have hydrophobic (water fearing) hairs on their undersides or on their legs. The repulsive force between the water and these hair are sufficient to support the weight of the insects on the water surface.
Common water-measurer (Hydrometra
Stagnorum) © Entomart
These insects are scavengers or carnivores which feed on bodies of small animals which land on or rise up to the water’s surface - dead or alive. Water-measurers are very sensitive to vibrations in the surface film of the water and use these to locate prey or dead bodies. They will also sit on the surface and catch water fleas, mosquito larvae and other small animals moving underneath them by impaling them on its needle like mouthparts. Most of the ‘boat’ insects are fast moving zipping in to deal with any new food source, but the gangly water-measurers are unhurried and cautious. If there was an ‘insect boat’ Olympics the water-measurers would lose every race!
Water-measurers under threat
There are two species of water-measurer in the UK, the Common water-measurer (Hydrometra stagnorum) and the Lesser water-measurer (Hydrometra gracilenta). The Common water-measurer is indeed very common on ponds and ditches in England and Wales, although it is more scattered in Scotland, Ireland and North East England. The Lesser water-measurer is a very rare animal, only found on the Broadland fens around the River Ant in Norfolk and a single record on the Pevensey Levels. There are very few records of the Lesser water-measurer and there is concern that the remaining populations are small and fragile. As the Ant Valley and Pevensey Levels are both very low lying the future of the Lesser water-measurer is under great threat from human-induced climate change which is likely to cause enough sea level rise to flood all the animal’s homes with sea water in the next 150 years. As a result the insect is listed on the UK Biodiversity Action Plan list and it is hoped that the animal can be saved through a combination of sea defences, habitat creation and translocation, as needed.
The two species look similar, but the Lesser water-measurer is slightly smaller reaching a maximum body length of 8.5 mm compared with the 9-11.5 mm of the Common water-measurer. While the Common water-measurer is matt black all over, the centre of the top surface of the Lesser water-measurer’s abdomen is shiny, resembling the rough surface of raw anthracite. This glittery reflectivity can be seen at a distance of several feet or even metres!
Tell us about your sightings
Water-measurers are well worth watching in their natural habitats. If you think you have seen a Lesser water-measurer please let us know at Buglife, and send us a photo if you can.