- Last updated:
- 23 Apr 2019
If you look carefully where there are areas of dry sand – but not on the beach – you might see what look like little craters about 40mm across and up to 20mm deep. The sides of the craters are smooth and in any type of sand are all at the same angle sloping inwards. This is known as the Angle of Rest, it just balances gravity and friction. If the slopes were any steeper they would collapse, if less steep, they would not work as the intended trap.
What sets the trap and lives in the little hole at the base of the cone is a most vicious creature, only about 5mm long, the size of a match head.
After hatching from an egg laid in the sand it has dug the crater by backing into the surface, throwing excess and out with its head. Then it sits in ambush and waits. Any unlucky insect, usually an ant, that crawls onto the edge or side of the slope is unable to climb out again and slides down into the cone of death where it is immediately seized by the monster's ferocious pincers. In quick time it has been sucked dry and its exoskeleton thrown out of the trap. Another victim of the Antlion.
Our studies (in our terrarium) have shown that the traps are more successful when made in fine sand and the denser the population of antlions the smaller are the traps. Also, the bigger the trap, the better it works, letting fewer ants escape. Nothing works perfectly and some ants do manage to climb back out of the crater. But not many.
The antlion has a strategy to deal with that. As the ant starts to climb up the steep side the antlion flicks sand on and above the ant. The sand rolls down the slope again bringing the ant with it. We have noticed too, that when a wandering ant comes to the rim of a crater it usually does not sense danger and back off, but seems to have a death wish, intent on running down the slope. When two or more ants fall into the crater, usually all except the first one get away. We swear we have seen an ant push another one in. We have also seen an antlion flick sand into another one's trap.
Surprisingly, the blood thirsty little killers in the sand are the larvae of a beautiful winged creature, the Antlion Lacewing, somewhat like a delicate little brown dragonfly. The veined, clear wings of about a 50mm span are longer than the body (unlike dragonflies) and when folded cover the insect's body like a tent. Mainly nocturnal and attracted to light, the lacewing's flight is slow and of short duraction. Like other insects of that group they go through a larval stage, then pupate, before metamorphosing into the flying adult. The species described here is the Common Brown Lacewing, Myrmeleon acer. Antlions, beside their ferocity, have another claim to fame, the larvae, unlike other creatures, do not have an... um... bottom. They store their waste matter until at the end of the pupa stage, when it is cast off with the other pupa remnants. It took over a year but finally we saw the pupae. The casings are small balls of silk with a protective coating of adhering grains of sand. The fully-fledged Myrmeleon emerge from these sperical cases, usually at night.
There are many species in the Myrmeleon family. They are all carnivorous, some actively stalk other insects while only a few build sand-traps. We think they are the most interesting ones and as usual when we study ferocious little creatures we are so glad that they are not bigger.
The observation of antlions has some historical context. In 1836 a young naturalist sat on the bank of Cox's River on Wallerawang Station west of the Blue Mountains in New South Wales. He was there to observe that (then) almost mythical creature, the platypus. He became distracted when he noticed the traps and behaviour of some antlions and the similarity between these and European species. He started musing about the then held theories about creators and design... his name was Charles Darwin.
Footnote: Most of this article is based on personal observation of the antlions. Several ants were sacrificed in the cause of science.
Article: Kon Hepers