Walking on water – taking it in their stride

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Joseph Brider, Natural Heritage Trust
First published CI News (19 Sept 2020)

Previously recorded Riffle-bug (Microvelia oceanica) for the Cook Islands

The Cook Islands Biodiversity and Ethnobiology Database lists nearly 650 species of insect and it is estimated we have around 1,300 species. We have found about 300 of the unrecorded insects, and are working to find and record the rest. Although most our insects are already known somewhere in the world, at present we have only a dozen that are unique to the Cook Islands, known as endemics.

Over the last few weeks, Natural Heritage Trust has been looking at our freshwater insect life, specifically the bugs that live on top of the water surface.

Walking on water for humans is a miraculous feat reserved for religious deities and superhuman beings but for some insects, it is as easy as falling off a log. Exploiting the high surface tension of water, these tiny insects have very fine, wax-coated, hairs on their feet, that trap air and keep them buoyant as they glide around on the water surface as if they were a skater on ice.

The as-yet unidentified Water-striders for the Cook Islands

Previously we had recorded three water-walking insects in the Cook Islands, the largest, a Water-strider (Family: Gerridae), being 20mm long and the other two, a Water-treader (Mesoveliidae) and a Riffle-bug (Veliidae), being only a few millimetres long. Water-striders, the most well-known type of water-walkers, use their front and back pair of legs to keep themselves upright and their middle pair to row themselves across the water surface. Water-treaders and Riffle-bugs scamper about with at least 3 legs touching the water surface at a time, similar to a cockroach on land. Interestingly, when a Riffle-bug needs to escape, it releases a chemical into the water that reduces the water surface-tension and jet-propels it forward.

As a result of our recent investigations, we can add another three water-walkers: a second Water-strider, a second Water-treader and a first Velvet Water-bug (Hebridae) – a mere 1mm in length. At present, we cannot judge which species arrived naturally and which were brought by people. It is likely they are all native, that is, species that arrived naturally.

The as-yet unidentified Water-treader, with a body length of less than a millimetre, compared to much-larger Water-striders

These insects are predators, feeding on insects that have fallen into the water, which they find by sensing vibrations in the water. They also grab insects from underneath the water, Water-striders are known to grab insects from beneath the surface. water, they for using this technique to hunt mosquito larvae. They in turn, are preyed upon by fish.

Surprisingly, we did not find any Diving-beetles (Dytiscidae), despite two of them being recorded in the database. We would be very interested to hear from people who may have come across these beetles in their ponds or creeks.

The more we look at the world around us, the more pieces of natural history we uncover. The micro-world of insects, spiders and mites is a trove waiting to be discovered. The next time you are by a pond or a creek, take a look, you may be surprised by what you see.

To learn more about our insects, visit: http://cookislands.pacificbiodiversity.net/ and type “insect” in the search bar.


Brider, Joseph. (2021). Walking on water – taking it in their stride. Cook Islands Natural Heritage Trust. https://cinature.org/2021/01/06/walking-on-water-taking-it-in-their-stride/

Joseph Brider

Posted by Joseph

Joseph has worked on Cook Island environmental management since 2000 with the National Environment Service. Concluding his term there as Director in 2019, he began working for the Trust. He is particularly interested in the plants of the Cook Islands and produced the "Takitumu Conservation Area Field Handbook" in 2019.

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