Tuesday, 4 July 2017

what beasts lurk within

The talk at the garden club this month was about the invertebrates that might live in our garden ponds.  The speaker, who is the Suffolk County Recorder for freshwater invertebrates, opened with the depressing statistics about natural ponds, half of all those in the UK lost in the last century and four fifths of the remaining ones in a poor state, not quite as depressing as the statistics for meadows but not very good.  He warned us to make sure that our garden ponds offered an escape route to any hedgehogs that might fall in.  Painted Jaguar's rhyme, Curls up but can't swim, Stickly-Prickly that's him, is wrong: hedgehogs can swim very well, but they can't scale vertical pond sides to get out again.  Then with a final warning not to release fish or invasive pond plants into the wild we moved on to the substance of the evening, the weird and marvellous mini-beasts that inhabit our ponds, or might do so.

Insects are truly weird and marvellous, and freshwater invertebrates are every bit as fabulous as bees in their own ways.  Pond skaters do not merely rest on the surface of the water because they are small: they have feet covered in greasy hairs that actively repel water.  The water cricket goes one better and propels itself along by excreting a surfactant which breaks down the surface tension at one end of its body, dragging it forwards across the water surface.  Water boatmen skate on the underside of the surface, looking downwards into the pond for prey while drawing air from above the surface via their protruding bottoms.

Whirligig beetles find their prey using the same echo location principles as bats, but based on returning ripples of water rather than sound.  They skitter around the surface of the pond, their four eyes looking up into the air and down into the water for predators that might eat them, and their skittering motion sending out tiny ripples.  They can detect the returning waves and distinguish between plants, other beetles, and potential prey which they rush towards propelled by flattened hind legs that they are capable of feathering like an oar for the return stroke.

The lesser water boatman dives enfolding an air bubble with its body.  It breathes using the air in the bubble, and as the oxygen concentration starts to drop and the carbon dioxide levels rise, the air in the bubble absorbs more oxygen from that dissolved in the pond, while carbon dioxide diffuses out into the surrounding water.

Most beetles can fly so can move between ponds.  They detect water surfaces from the polarised light reflecting off them, sometimes leading to unfortunate accidents with car windscreeens which are also polarised.  Water fleas cannot fly, but in times of environmental stress produce resting eggs which can remain viable for decades, if not centuries, waiting for conditions to improve. Water flea eggs have been found on the feet of frozen Siberian mammoths, and since water fleas predate the dinosaurs they probably hitched a lift with them too.  Some species of leech spend the winter up the noses of wild ducks, feeding modestly on blood before emerging in the spring to breed in a different pond, or perhaps a different country.

Some leeches carry their young attached to the undersides of their bodies.  If in a laboratory tank you deliberately disturb the water so that the babies are washed off, the parent leech will go around the tank until it has carefully retrieved all of them.  Once the young are old enough to leave for real the parent leech creeps under a stone and dies.  No UK wild leech is likely to attach itself to your legs if you go in a pond, since most species' mouth parts are not strong enough to pierce human skin.  The medicinal leech, once widespread, was collected almost to extinction, and while still used in medicine nowadays they are hygienically bred in captivity.

One species of caddis fly larva makes a case for itself out of bitten off sections of plant roots, arranged in a stack of hexagons.  How it knows how long to make each side of the hexagon to end up with a case that is neither too small nor too loose is a mystery, nor how it is able to arrange the pieces into hexagons.

None of this has anything directly to do with gardening, but it is all very wonderful and strange. The best time to go and look in your pond is after dark, with a powerful torch.

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