We used a kitchen sieve to search around in a slow moving drain and a pond, at one of the parks in town (Mona Vale). We were looking for backswimmers again, which we found.
They are quite active, although only 5-6 mm long.
We plan to keep them for a while to observe them. We put a lid on the aquarium as some of them can fly – which is how they get between ponds.
Backswimmers have 3 pairs of legs. They ‘row’ about with their third pair, which are flattened to make them more paddle-like. The front 2 sets of legs catch and hold their prey.
The ones we found are probably the common backswimmer (Anisops assimilis) but they don’t stay still long enough to be able to look at the fine detail we would need to be sure. NZ has only two species, both of them are native to this country.
They actually breathe by taking a bubble of air down with them. The air also gives them buoyancy.
Some research done on an Australian species found that they could use oxygen stored in haemoglobin in their bodies, and then released slowly into the bubble as required, to help keep them buoyant. Perhaps our species do that too?
Back swimmers do indeed swim on their back. Their colouring takes advantage of this – many aquatic species are light underneath and dark on top as this makes it harder for them to be seen from above (looking down in water is usually darker) or from below (looking up towards the sky is usually lighter). Back swimmers have the light colour on their back instead.
The last time we kept some backswimmers we recorded them calling, which they mostly do at night. This is called stridulating.
While fossicking about with our sieve we also found a water boatman (probably Sigara arguta). We have kept it separate from the backswimmers, since the backswimmers eat other insects and we want to observe the boatman for a while – apparently they stridulate too.
The boatmen are described as “predacious, saprophagous and bottom-foraging” so they should be ok with plenty of sediment from the pond and some duckweed we have growing as well.
A couple of years ago a guide about backswimmers and water boatmen was published by Emeritus Professor Young from Auckland, and we are hoping to track a copy down. And we will see what our current residents get up to – we haven’t kept a water boatman before.
We also want to put a longer handle on our seive so we can sample in the river.