Ant lions

Ants are a necessity we fail to appreciate.  They clean up our gardens, disposing of dead things keeping the garden sweet-smelling.  Ants are generally hard to see, so people often ignore their usefulness.  When they show up around picnics, we resent them, if they get into the house, we hate them, and when they sting us, we loathe them.  Small wonder, then, that most Australian children love the ant lion.

The ant lions are not a tall tale.  They are small predatory insects, the larval stages of lacewings, which prey on ants.  In their adult stages, some lacewings are said to eat aphids, so they are highly regarded by even the most ant-loving adults, or at least those few who know them.

There are ant lions all around the world, but we seem to have more of them in Australia.  The larval ant-lion digs a pit of great cunning, and lurks at the bottom, waiting for an ant to tumble down to its lair.  Once there, the ant will be seized in the ant lion's terrible pincer jaws, and sucked dry.  To do this effectively, the ant lion uses a rather handy little principle of engineering.

Pile some dry sand upon a table, and it will form a cone of a very predictable shape.  Dry sand has a typical ‘angle of rest’, about 30.5 degrees from the horizontal, if the grains are perfectly spherical.  If you shovel dry sand back over your head, you will dig a hole that will slowly get deeper.  If the slope of the sides becomes steeper than the angle of rest, some sand will roll down, making your hole broader and shallower, until the angle of rest is achieved, all around the edges.

If the slope is less than the angle of rest, you can go on deepening your hole until the angle of rest is exceeded.  In the end, either way, you get a beautifully symmetrical conical depression with dry sandy sides, all poised to fall away at the lightest tread.  The ant lion uses its large head like a flicking shovel.

It burrows backwards into the sand, and then flicks the dry grains up and away.  Some fall back into the hole, but soon there is a conical depression which grows until the trap is some 2 cm across.  Then the ant lion digs in at the bottom, with only its pincers in view, waiting for dinner to slide into reach.

An ant, blundering over the edge, starts to slip down the side, dislodging a small avalanche of sand grains as it goes.  This is no problem for the ant, this sort of thing happens all the time in sandy soil.  It starts to scramble laboriously up the slope.  As it goes, it casts more sand down, but slowly it creeps up towards the rim and safety.

But at the bottom of the pit, the ant lion feels the patter of the falling grains, and begins furiously casting sand out of the hole in all directions.  This deepens the hole and undermines the struggling ant from below, and some of the falling grains help to tumble it back down from above.  Occasionally, an ant escapes, but mostly, they fall to the bottom where they are seized and reduced to a husk.  This dried shell is later cast out of the pit with a single toss of the ant lion's powerful head.

Children who have been bitten by ants delight in dropping their tormentors' relatives into the pits of wild or captive ant lions.  In the past, I have collected a wide range of other small insects, and found that ant lions are eclectic diners.  They only eat ants because ants are so much more freely available.  A small weevil, a small sand hopper, even a spider will be dragged down and consumed if it strays within reach.

Ant lions are found wherever there is sandy soil, especially where it is protected from direct rain: under rock ledges, in hollow trees, beside fallen logs, close to buildings, and under any buildings which are on raised piers, a common Australian architectural form.  In really sandy soil, they may even cover open ground with neatly spaced traps.  

All you need to keep ant lions.

The ant populations around our house are increasing now to the highs of high summer, and the ant lion numbers are building up as well.  In a few weeks, I shall catch half a dozen, and take them to school in an ice cream container of sandy soil.  I can think of few better animals to use when introducing students to the hidden world of small insects.  After all, a system is a system, wherever you find it . . .